The drug atlas,
read as behavior.
Read the drug as behavior—channel, rate, substrate, and the signal that reveals failure first.
Read the drug, not the class label
Class labels are a filing system, not a mechanism. What changes your bedside move is how a drug behaves: which channel and state it binds, whether it strengthens at fast or slow rates, the substrate that turns benefit into harm, how the patient clears or stores it, and the first monitored sign that it is failing. Dosing lives in Lexicomp and the corrected EHRA compendium; the judgment lives here.
- What tissue and channel-state does it alter?
- What rate or substrate makes it work?
- What substrate makes that same effect unsafe?
- How will this patient clear, store, or interact with it?
- What monitored signal reveals failure first?
One idea · two opposite failure rates
Read the rate before you trust the class.
Sodium-channel block and repolarization block are both rate-dependent—in opposite directions. Class Ic tightens its grip as the heart speeds up; Class III tightens as it slows down. The agent that looks quiet at 70 is the one that can hurt your patient at 200—or at 40.
- Class Ic — use-dependence. More Na⁺ block per beat as rate rises. Flecainide, propafenone.
- Class III — reverse-use-dependence. More QT/APD prolongation as rate falls. Sotalol, dofetilide.
- The curves meet at moderate rates—both agents are quiet there. Each turns dangerous at the opposite extreme.
QRS widens, atrial flutter can conduct 1:1, and ischemia plus tachycardia is the exact CAST signal. Pill-in-the-pocket flecainide is paired with AV-nodal cover for precisely this reason.
Long diastoles and pauses stretch the QT into pause-dependent torsades—why sotalol and dofetilide are initiated on telemetry with renal dosing, and why a new bradycardia is a red flag, not a bystander.
The kinetics behind the curves
Use-dependence (Class I). These agents bind open or inactivated Na⁺ channels and release during diastole. Faster rate → less time to release → block accumulates. The subclasses differ only in release speed: Ib lets go in well under a second (little block at normal rates—lidocaine's quiet niche in ischemic VT), Ia in seconds, Ic slowest of all—block is already meaningful at rest and climbs steeply with rate. That steep climb is why flecainide widens the QRS, and why CAST found excess mortality when it was pushed into post-infarct hearts.
Reverse-use-dependence (Class III). IKr blockers prolong repolarization most at slow rates and least at fast rates—so they threaten torsades exactly when the heart pauses, and lose potency at the fast rates you most want to convert. Amiodarone is the informative exception: it shows little reverse-use-dependence, part of why its torsades rate stays low despite a visibly long QT.
A teaching schematic—the shapes are relative, not measured values. Confirm agent-specific kinetics, monitoring, and dosing in Lexicomp and the EHRA compendium.
Slows conduction and, via active metabolites, prolongs repolarization.
Ia ProcainamideNa+ block (intermediate) + IKr block via NAPA A long-runway drug—useful when you have a stable patient and time.
Blocks fast sodium channels (slows conduction, widens QRS) and, through its active metabolite NAPA, blocks IKr (prolongs repolarization and QT). It slows conduction in accessory-pathway as well as normal tissue.
Stable, tolerated, monomorphic wide-complex tachycardia (probable VT) and stable pre-excited AF—where slowing accessory-pathway conduction is exactly the goal.
Hypotension (it vasodilates and is negatively inotropic), decompensated heart failure, a prolonged QT, and renal impairment (NAPA accumulates). Instability means the shock comes first, not the infusion.
Parent drug plus a renally-cleared active metabolite, NAPA; renal impairment lets NAPA build and pushes the QT. The runway shortens as pressure, HF, QRS, QT, and cumulative dose evolve.
Additive QT effect with other QT-prolonging drugs; other sodium-channel or AV-nodal agents stack the conduction slowing.
Explicit stop rules: hypotension, QRS widening beyond ~50%, QT prolongation, arrhythmia termination, or the maximum cumulative dose—whichever comes first. Watch pressure and ECG continuously.
Conduction block and QRS widening from sodium block; QT prolongation and torsades from NAPA, especially with renal accumulation.
Selected stable WCT; set the stop rules before the infusion and keep pads on.
Track NAPA-driven QT as renal function and cumulative dose change—a tolerated dose yesterday can be proarrhythmic today.
Long-term use raises drug-induced lupus and cytopenias; usually a bridge to a definitive rhythm strategy.
PROCAMIO favored it over amiodarone in tolerated WCT, but mechanism (VT vs SVT with aberrancy) and structural substrate still drive the definitive plan.
PROCAMIO (2017): fewer major cardiac adverse events and more termination than amiodarone in tolerated WCT—small, open-label, with exclusions.
Loading infusion to a clinical endpoint, then maintenance; renal dose adjustment matters. Confirm the exact regimen and stop rules in Lexicomp and the corrected EHRA compendium.
Selective for depolarized/ischemic ventricular tissue; little effect on normal atrium.
Ib LidocaineNa+ block (fast on/off) The ventricular agent for the sick, ischemic myocardium.
Blocks fast inward sodium channels with rapid on/off kinetics, binding preferentially in the open and inactivated states, so it targets depolarized, ischemic, or rapidly firing ventricular tissue while sparing normal atrium and healthy myocardium. Use-dependent block deepens at fast rates and in acidotic, partially depolarized cells; as a class Ib agent it shortens action-potential duration and does minimal harm to the QT.
Ventricular arrhythmias driven by acute ischemia or infarction, including shock-refractory VF or pulseless VT and ischemic electrical storm, where the substrate is depolarized ventricle. It is not an atrial drug and adds little in structurally normal hearts.
Low hepatic blood flow, heart failure, shock, or hepatic impairment, where flow-dependent clearance falls and the same infusion accumulates toward CNS toxicity and seizures. Advanced conduction disease and bradycardia are also poorly tolerated; failure to convert a wide-complex tachycardia argues the substrate is not ischemic and lidocaine is the wrong tool.
Hepatic metabolism with clearance that tracks liver blood flow; active metabolites MEGX and GX accumulate in hepatic, renal, or low-output states. Extensive first-pass metabolism makes it an IV-only agent, and its effect follows perfusion more than any fixed rate.
Anything that lowers hepatic blood flow or inhibits its metabolism, notably beta-blockers, cimetidine, and amiodarone, raises levels and toxicity risk. Effects on conduction and the CNS are additive with other sodium-channel blockers.
The earliest signal of trouble is CNS, not ECG: perioral numbness, tinnitus, tremor, and confusion precede seizures, so track mental status alongside the rhythm and QRS. In sustained infusions the CNS complaint is the toxicity threshold made visible.
Low torsadogenic risk, since it shortens rather than prolongs the action potential; its signature harm is CNS excitation and seizures, and at high levels myocardial depression with bradyasystole, rather than QT-driven torsades.
Execution: the ready antiarrhythmic in the ischemic arrest, given as an alternative to amiodarone in shock-refractory VF or pulseless VT, and reached for in ischemic VT when procainamide is not the play.
Dynamic physiology: clearance collapses as cardiac output and hepatic perfusion fall, so in shock or HF the standing infusion silently accumulates. Titrate down and let CNS signs, not a target rate, set the ceiling.
Longitudinal coherence: lidocaine is an acute IV bridge, not a maintenance drug; its oral class Ib cousin mexiletine carries the outpatient role, so reconcile the plan before discharge rather than continuing lidocaine thinking.
Diagnostic and procedural counter-lens: its efficacy in ischemic ventricle and near-inertness in structurally normal or scar-based non-ischemic VT is itself informative, so a non-response should redirect the differential and the ablation or reperfusion strategy rather than prompt dose escalation.
ALS guidance positions lidocaine as an accepted alternative to amiodarone in shock-refractory VF or pulseless VT; for stable monomorphic VT it is not first-line, where trial evidence favored procainamide over IV amiodarone (PROCAMIO) and lidocaine remains a situational, ischemia-directed choice.
IV bolus followed by a titratable maintenance infusion, reduced in heart failure, hepatic impairment, and the elderly; confirm in Lexicomp.
Ib MexiletineNa+ block (fast on/off), oral The oral lidocaine: quiet on healthy tissue, sharper on sick, fast, and refractory myocardium.
Blocks fast cardiac sodium channels with rapid on-off kinetics, binding preferentially to the inactivated and open states. This gives it use-dependence that is amplified at fast rates, in depolarized or ischemic tissue, and where action-potential duration is long, so it shortens the action potential and refractory period most where the substrate is abnormal while barely touching normal myocardium.
Chronic oral suppression of ventricular arrhythmias, especially the monomorphic VT of scarred or ischemic myocardium and, distinctively, the long-QT3 and other sodium-channel or repolarization phenotypes where shortening a pathologically prolonged action potential is the therapeutic goal. It is an adjunct that lowers arrhythmia burden and ICD shocks, not a standalone cure.
Structural heart disease with reduced ejection fraction, where a class I sodium blocker carries the CAST-era shadow of mortality; also advanced conduction disease and high-grade block, since it further depresses conduction. The benefit in scar-related VT does not license first-line use as monotherapy in a failing ventricle.
Hepatic clearance, largely CYP2D6 with a CYP1A2 contribution, so metabolism varies with genotype and smoking status; renal handling is pH-dependent and minor. The therapeutic window is narrow and the same molecule is neurotoxic before it is cardiotoxic, so gut, hepatic, and CNS handling all shape tolerability.
A CYP2D6 substrate whose levels rise with strong inhibitors (paroxetine, fluoxetine, quinidine) and whose CYP1A2 pathway is slowed by smoking cessation, caffeine load, and fluvoxamine. It also inhibits CYP1A2, raising theophylline and caffeine, and its effect is additive with other sodium-channel and conduction-slowing agents.
Neurologic and gastrointestinal tolerance is the first signal to move, not the QT: tremor, dysarthria, dizziness, ataxia, diplopia, and nausea flag rising levels well before cardiac toxicity. Track arrhythmia burden and, where the indication is LQT3, the QT/QTc response; reserve levels for suspected toxicity or interaction.
Class Ib agents are the least proarrhythmic of the sodium blockers because they shorten rather than prolong repolarization, so torsades is not the signature. The real hazard is conduction slowing and arrhythmia aggravation in diseased, low-EF myocardium, plus CNS toxicity that arrives before the heart complains.
Rarely an acute-room drug: it is oral, slow to load, and not a termination agent, so in the ED it is the home medication to reconcile and continue, not the tool reached for during active VT. Recognize its toxicity syndrome, tremor and ataxia in a patient on it, as a level problem, not a stroke.
When ventricular arrhythmias smolder despite amiodarone and a beta-blocker, mexiletine is the oral add-on that pushes burden down and buys the ICD fewer shocks. Its dynamic weakness is the failing ventricle and the narrow window, so titrate against neurologic tolerance and the underlying pump, not a target number.
The longitudinal job is keeping the level in range across CYP2D6 genotype, smoking changes, and interacting SSRIs, and revisiting the EF and conduction system that make the drug safe or not over time. Sustained suppression, tolerability, and adherence, not a single clinic reading, define success here.
As a counter-lens it forces the mechanistic question: is the arrhythmia one that shortening the action potential should help, as in LQT3 or fast-tissue scar VT, or is this a low-EF substrate where any class I agent is the wrong bet. It is adjunctive and phenotype-directed, so the diagnosis of the substrate, not the ambition of monotherapy, decides whether it belongs.
No mortality-benefit trial supports mexiletine; the evidence base is arrhythmia-burden and ICD-shock reduction as an adjunct in refractory VT, plus mechanistic and small-cohort data for QT shortening in LQT3, all read against the CAST caution that class I agents raise mortality in post-infarct low-EF hearts.
Oral only, divided dosing titrated to tolerance with no acute load; confirm in Lexicomp or the corrected EHRA compendium.
Strong use dependence; potent in a structurally normal heart, dangerous with scar or ischemia.
Ic FlecainideNa+ block (slow dissociation, strong use dependence) Potent in a structurally normal heart—dangerous the moment there is scar.
Potently blocks fast sodium channels and dissociates slowly, so block accumulates at faster rates (marked use dependence). It slows conduction and terminates AF by blocking the activation front, with little repolarization effect.
Rhythm control of AF and some SVTs in patients WITHOUT structural or ischemic heart disease—including the outpatient 'pill-in-the-pocket' strategy in selected patients.
Structural heart disease, prior MI, or significant CAD—CAST showed increased mortality here. In atrial flutter it can slow the atrial rate enough to permit 1:1 AV conduction; always pair it with an AV-nodal blocker.
Renal and hepatic elimination; accumulation raises QRS and proarrhythmia risk. Use dependence means the danger surfaces during exertion or tachycardia, not necessarily at rest.
Additive with other sodium-channel blockers and negative inotropes; levels rise with CYP2D6 inhibitors.
QRS at rest and, ideally, with exertion or faster rates (use-dependent widening is the tell). Confirm the absence of structural or ischemic disease before starting.
Rate-related QRS widening, incessant VT or flutter with 1:1 conduction, and increased mortality in structural or ischemic hearts.
Recognize the pill-in-the-pocket patient; beware flecainide flutter with 1:1 conduction presenting as a fast wide tachycardia.
Rarely an ICU rhythm drug—if a patient on it develops ischemia or a wide tachycardia, think proarrhythmia or toxicity.
Confirm a structurally normal heart (echo, ischemic evaluation) before initiation, and cover the 1:1 flutter risk with rate control.
Candidacy hinges on imaging and ischemic risk—this is a diagnosis-gated drug.
CAST (1989): class Ic agents increased mortality after MI—the enduring boundary that defines who cannot receive it.
Oral and IV regimens differ; the pill-in-the-pocket dose is protocolized. Confirm candidacy and dosing in Lexicomp.
Ic PropafenoneNa+ block (slow dissociation) + weak beta-block Restores sinus rhythm in a structurally normal heart, and punishes one that isn't.
Blocks fast Na+ channels with slow dissociation kinetics, so conduction slows and QRS widens progressively at faster rates (use-dependence). It carries intrinsic weak beta-blockade and mild L-type Ca2+ block, which add rate-slowing and negative inotropy on top of the sodium effect.
Pharmacologic cardioversion of recent-onset AF and maintenance of sinus rhythm, including patient-administered pill-in-the-pocket, in patients with a structurally and functionally normal heart.
Structural heart disease flips the benefit: coronary disease or prior MI, HFrEF, or significant LV hypertrophy convert Na+ block into lethal ventricular proarrhythmia (the CAST lesson). Screen for and exclude these before it is ever given.
Hepatic clearance via CYP2D6 with a genetically bimodal population; poor metabolizers (or those on 2D6 inhibitors) accumulate parent drug and unmask its beta-blocking effect, while the active 5-hydroxy metabolite contributes in extensive metabolizers. Effect tracks heart rate through use-dependence.
CYP2D6 and CYP3A4 substrate; 2D6 inhibitors, and its own inhibition of digoxin and warfarin clearance, raise those levels. Additive AV-nodal and sinus depression with beta-blockers, and additive QRS effects with other Na+ blockers.
The ECG is the first tell: watch QRS duration (especially the rate-dependent widening on exertion) and PR interval, and confirm the atrial rhythm has not organized into flutter. New QRS prolongation beyond baseline is the early signal of toxicity.
Its signature is organizing AF into a slower atrial flutter that the AV node conducts 1:1, producing a wide, fast, hemodynamically dangerous tachycardia; pair it with an AV-nodal blocker to pre-empt this. In diseased myocardium it also drives incessant monomorphic VT.
Execution point: it can convert recent-onset AF in the right patient, but only after structural disease and pre-excitation are excluded and an AV-nodal blocker covers the 1:1-flutter risk; a wide-complex tachycardia after a dose is proarrhythmia until proven otherwise.
Dynamic physiology: the negative inotropy and weak beta-block matter in the marginal ventricle, and rate-dependent conduction slowing worsens under catecholamines or tachycardia; sodium-channel toxicity is managed by shifting toward hypertonic sodium bicarbonate.
Longitudinal coherence: confirm the structurally normal heart holds over time, verify metabolizer status and interacting drugs at each reconciliation, and pair chronic therapy with a durable AV-nodal blocker rather than assuming one-time safety.
Diagnostic counter-lens: a wide-complex tachycardia in a patient on propafenone is not automatically VT, it may be 1:1 flutter with use-dependent aberrancy; the eligibility decision is really a decision about the substrate, and the substrate can change.
AF guidelines endorse class Ic agents for cardioversion and rhythm maintenance strictly in the absence of structural heart disease, and pair pill-in-the-pocket dosing with AV-nodal blockade to prevent 1:1 flutter conduction.
Single oral loading dose for pill-in-the-pocket cardioversion, or divided oral maintenance (immediate- or sustained-release), always over an AV-nodal blocker. Confirm in Lexicomp.
Slows the sinus and AV node; blunts the sympathetic drive behind many arrhythmias.
II EsmololUltra-short-acting beta-1 block A beta-blocker you can take back in minutes.
Selective beta-1 adrenergic antagonism that lowers cyclic-AMP-driven calcium entry, slowing sinus rate and AV-nodal conduction. It is a negative chronotrope and dromotrope; because its block competes with catecholamine tone, its rate-slowing is greatest exactly when sympathetic drive is highest.
Acute ventricular rate control in AF or flutter, and blunting of catecholamine-driven sinus tachycardia or SVT, when you want the option to reverse the effect quickly, as in the hemodynamically borderline or peri-procedural patient.
Decompensated HF or fixed low-output states, symptomatic bradycardia or high-degree AV block without a pacer, and bronchospastic disease at the top of the dose range where beta-1 selectivity erodes. In pre-excited AF, AV-nodal block can favor accessory-pathway conduction, so it is not the rate-control tool there.
Hydrolyzed by red-blood-cell esterases, not the liver or kidney, giving a roughly 9-minute half-life independent of organ failure; effect is gone within minutes of stopping the infusion. The acid metabolite is inert; the vehicle carries a meaningful volume load over prolonged high-rate infusions.
Additive AV-nodal depression and bradycardia with calcium-channel blockers, digoxin, and other beta-blockers; hypotension compounds with other vasodilators and anesthetics. It can mask the adrenergic warning signs of hypoglycemia.
Blood pressure is the first thing to fail; hypotension typically arrives before the intended rate control and is the signal to down-titrate or stop. Track heart rate, AV conduction, and, in reactive airways, work of breathing.
Not proarrhythmic through repolarization; the hazards are hemodynamic, bradyasystolic pauses and hypotension when stacked on other AV-nodal blockers, and dangerous rate acceleration if given for pre-excited AF.
The titratable, reversible rate-control choice when you are unsure the patient will tolerate beta-blockade, since a wrong guess washes out in minutes rather than committing you for hours.
A continuous infusion whose short half-life lets you steer heart rate against shifting shock physiology and sedation, but the pressure and the volume of vehicle both need watching during sustained high-rate delivery.
A bridge, not a maintenance drug; use it to prove tolerance, then reconcile with a durable oral beta-blocker or rate strategy and confirm the EF before committing longitudinally.
Its reversibility makes it the diagnostic probe: transient AV-nodal slowing can unmask atrial flutter or confirm a suspected diagnosis, and if the rhythm is pre-excited the whole rate-control plan is wrong.
AF and ACLS guidance position IV beta-blockers, esmolol among them, as first-line acute rate control in preserved EF, favored when titratability matters and avoided in decompensated HF and pre-excited AF.
Optional loading bolus followed by a titratable continuous infusion for acute rate control. Confirm in Lexicomp or the corrected EHRA compendium.
II MetoprololBeta-1 selective block Rate control that works hardest exactly where the sympathetic drive is highest.
Competitively blocks beta-1 adrenergic receptors, lowering cAMP, funny-current (If) pacemaker drive, and calcium loading, which slows sinus automaticity and AV-nodal conduction. The block is adrenergic-dependent, so the effect is largest under high catecholamine tone and modest at rest; it is a negative chronotrope, dromotrope, and inotrope.
Ventricular rate control in AF or flutter, particularly adrenergically driven rates (thyrotoxicosis, post-operative, ischemia, sepsis recovery), and slowing or terminating AV-node-dependent SVT. Also suppression of catecholamine- and ischemia-driven ventricular arrhythmia, including as an adjunct in electrical storm.
Decompensated heart failure or cardiogenic shock, high-grade AV block without pacing, severe bradycardia, and pre-excited AF, where AV-nodal block can shunt conduction down the accessory pathway. Do not stack IV metoprolol with IV non-dihydropyridine calcium blockers, and avoid it in cocaine or stimulant toxicity, where unopposed alpha tone can worsen ischemia.
Lipophilic, hepatically cleared through CYP2D6, and BBB-penetrant; the immediate-release tartrate is short-acting while the succinate salt is an extended-release formulation. No clinically important active metabolite, and the antiarrhythmic effect tracks prevailing sympathetic tone rather than a fixed serum level.
A CYP2D6 substrate: paroxetine, fluoxetine, bupropion, and quinidine raise exposure. Additive AV-nodal depression with diltiazem, verapamil, and digoxin, and it blunts the response to epinephrine used for anaphylaxis.
Heart rate, blood pressure, PR interval, and signs of decompensation or bronchospasm. The first sign the drug has overshot is a falling rate and pressure with worsening perfusion rather than smooth rate control.
Not QT-prolonging and not torsadogenic; its signature harms are bradycardia, AV block, and sinus arrest, dangerous rate acceleration in pre-excited AF, and rebound tachyarrhythmia or ischemia after abrupt withdrawal.
First-line for adrenergically driven rapid AF with preserved EF, after excluding pre-excitation and overt heart failure; the IV form is titratable but commits you to sustained beta blockade.
Beta blockade strips away the compensatory tachycardia a shocked patient may depend on, so weigh the inotropic and chronotropic reserve; it earns its place in ischemic electrical storm and catecholamine-fueled arrhythmia, not in undifferentiated low-output states.
Succinate is the durable-rate-control and post-MI or HFrEF mortality agent; uptitrate slowly, reconcile it against other AV-nodal blockers, and never stop it abruptly.
Beta blockade can mask the tachycardia of thyrotoxicosis, pheochromocytoma, or hypoglycemia and can convert a pre-excited-AF plan into harm; confirm the rhythm mechanism and the EF before committing.
AF guidelines place beta-blockers as first-line rate control across preserved and reduced EF, and metoprolol succinate carries a post-MI and HFrEF mortality role (MERIT-HF); the boundary is decompensated HF, high-grade block, and pre-excitation.
IV boluses for acute rate control; oral tartrate (immediate-release) or succinate (extended-release) for maintenance. Confirm in Lexicomp.
Prolongs refractoriness with reverse use dependence—the QT/torsades class.
III AmiodaroneMulti-channel (K+, Na+, Ca2+) + antiadrenergic The electrophysiology Swiss Army knife—with a storage-unit lease.
Blocks potassium (repolarization), sodium, and calcium channels and has antiadrenergic activity—broad coverage. IV and chronic pharmacology differ: acute effects lean sodium/calcium/β, while the class III/QT effect builds over time.
Broad utility: stable VT, AF (rate and rhythm), and arrest pathways; a reasonable choice when the substrate is uncertain or the heart is structurally abnormal—the setting where Ic agents are barred.
IV boluses drop the pressure; the deeper cost is chronic multi-organ toxicity and an enormous volume of distribution—effects and interactions persist for weeks after stopping.
Very large volume of distribution and tissue storage; half-life of weeks. What you load does not leave quickly—offset is far slower than the infusion suggests.
CYP and P-glycoprotein inhibition raises warfarin, digoxin, and many others; additive QT and bradycardia with other agents. Anticipate proactive dose reductions.
Acute: pressure and rhythm. Chronic: thyroid, liver, pulmonary, ophthalmic, and skin surveillance, plus QT—a real monitoring commitment.
Lower torsades risk than pure IKr blockers despite QT prolongation, but bradycardia and conduction block occur, and IV boluses cause hypotension.
A workhorse for stable VT and rate/rhythm control; give IV boluses slowly and watch the pressure.
Cumulative infusion plus evolving organ function shifts exposure; anticipate interactions with everything else running.
Chronic therapy is a surveillance commitment (thyroid, liver, lung), and the drug lingers for weeks after it is stopped.
Breadth buys time, not a diagnosis—define the substrate and the definitive plan rather than defaulting to indefinite amiodarone.
Embedded across ACLS and VT/AF guidance; PROCAMIO tempered its first-line role in tolerated monomorphic WCT.
Acute loading differs sharply from oral maintenance (the EHRA correction touched the oral-maintenance footnote). Confirm both in Lexicomp and the corrected compendium.
III SotalolIKr block + non-selective beta-block Rhythm control you have to earn at the bedside, one QT at a time.
A racemate: both enantiomers block the rapid delayed-rectifier potassium current (IKr) to prolong repolarization and refractoriness (class III), while the l-isomer adds non-selective beta-blockade (class II). The IKr block shows reverse use dependence, so QT prolongation and torsades risk are worst at slow rates and after pauses, not during tachycardia.
Maintenance rhythm control of AF or flutter and of ventricular arrhythmias, including in structural heart disease and ICD patients where class Ic agents are barred. It is a chronic oral suppressant, not an acute cardioversion drug.
Renal impairment (it accumulates), hypokalemia or hypomagnesemia, baseline QT prolongation, and bradycardia together set up torsades. Decompensated HFrEF adds beta-blocker intolerance to the proarrhythmic risk; a falling GFR or a new diuretic can turn a stable regimen dangerous.
Predominantly renal elimination with no meaningful hepatic metabolism and no active metabolites, so exposure tracks GFR directly. This is why initiation is dose-adjusted to renal function and why AKI is a proarrhythmic event, not just a lab change.
Additive QT prolongation with other antiarrhythmics and QT-active drugs (macrolides, azoles, ondansetron, and the like), additive bradycardia and AV block with other beta-blockers and AV-nodal agents, and amplified torsades risk from potassium-wasting diuretics.
QTc on telemetry through the monitored initiation and after any dose increase, with renal function, potassium, and magnesium alongside. A QTc crossing the protocol threshold is the first signal of failure and triggers dose reduction or stopping, before any arrhythmia appears.
Torsades de pointes is the signature: reverse-use-dependent, clustered early after starting or up-titrating, and potentiated by bradycardia, hypokalemia, hypomagnesemia, renal accumulation, and female sex. Sinus bradycardia and AV block come from the beta-block.
Recognize sotalol torsades: syncope or polymorphic VT after a recent start, dose change, or worsening renal function. Correct potassium and magnesium, and treat the bradycardia-dependent pauses that sustain it.
Exposure and QT move with renal function and electrolytes in real time; AKI or brisk diuresis can convert a tolerated regimen into a torsades setup within a day. The beta-block also blunts the compensatory tachycardia a shocked patient needs.
This is a stewardship drug, not set-and-forget: monitored inpatient initiation, renal dose adjustment, interval QTc checks, and reconciliation of every other QT-active drug and potassium-wasting diuretic across the medication list.
Candidacy is gated by renal reserve and QT headroom, and weighed against ablation and amiodarone; its beta-block lets it work where structural disease bars class Ic, but that same physiology and renal dependence define who can safely carry it.
AF and ventricular-arrhythmia guidelines position sotalol as a maintenance rhythm-control agent requiring monitored, renally-dosed initiation for QT and torsades, the boundary that defines who can start it.
Renally dosed with QTc-guided, telemetry-monitored initiation; oral maintenance, with an IV form available. Confirm in Lexicomp and the corrected EHRA compendium.
III DofetilidePure IKr block Sinus rhythm for the structurally sick heart, bought with a monitored bed.
Selectively blocks the rapid delayed-rectifier potassium current (IKr) in its open and inactivated state, prolonging phase 3 repolarization, action potential duration, and atrial and ventricular refractoriness without slowing conduction. It shows reverse use-dependence: block is greatest at slow rates, which weakens its atrial effect at the fast rates of AF while concentrating its torsadogenic liability at slow ones.
Pharmacologic conversion and, more often, maintenance of sinus rhythm in AF or atrial flutter. It is one of the few rhythm-control agents acceptable in structural heart disease and HFrEF, where class IC drugs are contraindicated.
Renal impairment, hypokalemia or hypomagnesemia, baseline QT prolongation, and bradycardia, each of which amplifies IKr block into torsades. It must be started in hospital on continuous ECG, and it is contraindicated below a floor of renal function or above a QTc ceiling.
Predominantly renal elimination, largely as unchanged drug with minor CYP3A4 metabolism and no clinically important active metabolites. Dose is set by creatinine clearance and then adjusted by QTc, so every renal shift changes the exposure.
Contraindicated with inhibitors of renal cationic secretion that raise its level: verapamil, cimetidine, trimethoprim, ketoconazole, hydrochlorothiazide, prochlorperazine, megestrol, dolutegravir. Any additional QT-prolonging drug is additive and dangerous.
QTc is the first signal of trouble: continuous telemetry through initiation, with dose reduction or discontinuation if QTc crosses its threshold or rises sharply from baseline. Track renal function and potassium and magnesium alongside it.
Torsades de pointes, characteristically early and clustered in the first few doses, driven by concentration, bradycardia (reverse use-dependence), hypokalemia, and renal accumulation. This is why initiation is a monitored, protocolized event rather than an outpatient start.
Rarely an ED-initiated drug; the ED job is recognizing the patient already on it. Check QTc, potassium, magnesium, and renal function, avoid stacking a QT-prolonger or a secretion inhibitor, and treat any torsades with magnesium and rate support.
Physiology moves under the drug: an AKI, a diuretic-driven hypokalemia, or a bradycardic drift each raises effective exposure. Re-verify creatinine clearance, electrolytes, and QTc before every dose and hold when they cross.
Longitudinal medication reconciliation is the safety net; a single new outpatient prescription such as trimethoprim, an azole, or verapamil can turn a stable regimen torsadogenic. Maintain sinus rhythm while re-checking renal function and interactions at each encounter.
The work is patient selection: rhythm versus rate, and which rhythm agent the substrate allows. Dofetilide, sotalol, or amiodarone where structural disease bars flecainide, weighed against catheter ablation and the cost of a monitored initiation.
DIAMOND (CHF and MI): in patients with LV dysfunction and heart failure, dofetilide was mortality-neutral while restoring and maintaining sinus rhythm. The claim is safety through neutrality, not survival benefit, and only with monitored, renally-dosed initiation.
Renally dosed by creatinine clearance with mandatory in-hospital, telemetry-monitored initiation and QTc-driven adjustment. Confirm in Lexicomp / the corrected EHRA compendium.
III DronedaroneMulti-channel (amiodarone analogue, no iodine) Amiodarone's cleaner-organ cousin, until the heart is failing or the AF is permanent.
A true multichannel blocker like its parent: it inhibits delayed-rectifier potassium currents (IKr, IKs, IKACh), peak and late sodium current, and L-type calcium current, with non-competitive beta-adrenergic antagonism. Net effect is prolonged action potential and refractoriness plus AV-nodal slowing, so it spans Vaughan-Williams I through IV. Removing the iodine and adding a methanesulfonyl group cuts lipophilicity and tissue accumulation, giving a far shorter half-life than amiodarone.
Maintenance of sinus rhythm and reduction of hospitalization for AF in paroxysmal or persistent atrial fibrillation, in patients who are in, or will be restored to, sinus rhythm. It is the agent for the low-risk rhythm-control candidate who cannot accept amiodarone's cumulative organ toxicity.
Two substrates flip it from benefit to harm: decompensated or NYHA III-IV heart failure or low EF (ANDROMEDA, excess mortality), and permanent AF (PALLAS, excess cardiovascular death, stroke, and HF hospitalization). It is not a rate-control or amiodarone substitute in the failing or permanently fibrillating heart.
Oral only, hepatic CYP3A4 metabolism to an active metabolite, half-life near a day, and no iodine, so it sidesteps amiodarone's thyroid toxicity and carries far less—but not zero—pulmonary risk, with rare interstitial lung disease still reported. Absorption depends on food, and it inhibits tubular creatinine secretion, raising serum creatinine without lowering true GFR. Rare but severe hepatocellular injury, including cases requiring transplant, is its distinctive organ signal.
A CYP3A4 substrate and inhibitor plus a P-glycoprotein and CYP2D6 inhibitor: strong CYP3A4 inhibitors and inducers are avoided, and it raises digoxin, simvastatin, and dabigatran levels. It roughly doubles dabigatran exposure, so avoid or dose-reduce that pairing—especially in renal impairment—and stack cautiously with other QT-prolonging or AV-nodal-blocking drugs.
Watch transaminases early and any weight gain, edema, or dyspnea, because new or worsening heart failure is the first sign the substrate was wrong. Track the QT interval and note the expected benign creatinine rise so it is not mistaken for renal injury.
Modest QT prolongation with a low torsades rate relative to dofetilide and sotalol, so its danger is not primarily electrical. The real proarrhythmic and mortality hazard emerges as HF decompensation in the wrong substrate, along with bradycardia when layered on other AV-nodal blockers.
Not an acute drug: it neither converts nor rate-controls in the emergency setting. When a patient arrives already on it, the job is to check for HF decompensation and interacting drugs, not to give or up-titrate it.
A poor fit for the unstable or critically ill: oral-only, contraindicated in decompensated HF and low EF, and not titratable for dynamic control. In shock or acute pulmonary edema it is the wrong tool, and an inherited dose warrants reassessment.
The longitudinal home for this drug: outpatient rhythm maintenance in the confirmed non-permanent, preserved-EF patient. Reconcile CYP3A4 and P-gp interactions, monitor liver function, anticipate the creatinine shift, and periodically confirm the AF has not become permanent.
The counter-lens question is whether this is genuinely the right patient, not just an amiodarone dodge: verify the AF is truly non-permanent and the EF preserved before committing, and accept that it maintains sinus rhythm less effectively than amiodarone. The substrate, not the toxicity profile, decides.
ATHENA showed reduced cardiovascular hospitalization or death in non-permanent AF without severe HF, but the benefit is tightly bounded by ANDROMEDA (harm in HF) and PALLAS (harm in permanent AF), which define who must not receive it.
Oral, twice daily with meals, no loading. Confirm in Lexicomp or the corrected EHRA compendium.
III IbutilideIKr block (pure class III) + slow inward Na current A fast pharmacologic cardioversion for flutter—rented, then watched.
Blocks the rapid delayed-rectifier potassium current (IKr) and enhances a slow inward sodium current, prolonging repolarization and refractoriness—converting recent-onset atrial flutter and AF.
Pharmacologic cardioversion of recent-onset atrial flutter (its best indication) and AF in appropriate patients; also a pathway-active option in stable pre-excited AF.
A prolonged QT, hypokalemia or hypomagnesemia, significant LV dysfunction, and bradycardia—all amplify torsades. Reverse use dependence means the danger is greatest at slow rates and just after conversion.
Given as a short infusion; the QT and torsades risk is front-loaded in the hours afterward, which is exactly why the monitoring window is defined.
Additive with other QT-prolonging drugs; correct potassium and magnesium first.
Replete potassium and magnesium before giving; continuous ECG for at least ~4 hours after (longer if the QT prolongs), with a defibrillator and magnesium ready. Torsades is the event you are watching for.
Torsades in a few percent—usually within the monitoring window—hence the mandatory post-dose observation.
Excellent for recent-onset flutter, but only if you can commit to the monitored post-dose window with magnesium and pads ready.
The monitored environment suits it; pre-correct electrolytes and watch the QT trend after conversion.
A conversion tool, not maintenance; the longitudinal plan is anticoagulation, recurrence risk, and a definitive rhythm strategy.
Flutter that converts easily still merits an ablation conversation—cavotricuspid-isthmus flutter is highly curable.
Guideline-supported pharmacologic cardioversion, with higher efficacy for flutter than AF; the torsades risk defines its monitoring.
Weight-based short infusion, repeatable once; the post-dose monitoring window is the point. Confirm in Lexicomp.
Rate control; hazardous in pre-excitation and reduced ejection fraction.
IV DiltiazemL-type Ca2+ channel block (AV node) Rate control that also drops the pressure and the squeeze.
Blocks L-type calcium channels in the AV node (and vasculature), slowing AV conduction to control the ventricular rate. It is a negative chronotrope, dromotrope, and inotrope.
Ventricular rate control in AF or flutter, and termination of AV-node-dependent SVT, in patients with preserved LV function.
Reduced ejection fraction or decompensated HF (negative inotropy), hypotension, and—critically—pre-excited AF, where AV-nodal block can accelerate accessory-pathway conduction toward VF. Do not stack IV diltiazem with IV beta-blockers.
Hepatic metabolism via CYP3A4; infusions are titratable for rate control. The effect tracks vascular tone and LV reserve.
A CYP3A4 substrate and inhibitor—raises levels of many drugs; additive AV-nodal block with beta-blockers and digoxin; raises some statin and immunosuppressant levels.
Heart rate, blood pressure, and signs of heart failure; in AF, confirm it is not pre-excited before giving.
Not proarrhythmic in the usual sense, but dangerous rate acceleration in pre-excited AF and bradyasystole when stacked with other AV-nodal blockers.
First-line rate control in preserved-EF AF with rapid ventricular response—after ruling out pre-excitation and overt HF.
Titratable infusion, but watch the pressure and inotropic state; often not the choice in shock or low EF.
Chronic oral rate control; reconcile with beta-blockers and check the EF before committing.
Rate versus rhythm and the EF drive the choice; pre-excitation flips the entire plan.
AF guidelines: non-dihydropyridine calcium blockers for rate control in preserved EF, avoided in HFrEF and pre-excited AF.
Bolus then titratable infusion for acute rate control; oral for maintenance. Confirm in Lexicomp.
IV VerapamilL-type Ca2+ channel block (AV node) A heavier hand on the AV node and the squeeze, with one VT it uniquely owns.
Blocks L-type calcium channels in the AV node in a use-dependent way, binding the open and inactivated states so block deepens at faster rates, which slows AV conduction and prolongs nodal refractoriness. A phenylalkylamine that is more myocardium-selective and more negatively inotropic than the dihydropyridines, acting as a negative chronotrope, dromotrope, and inotrope.
Ventricular rate control in AF or flutter and termination or suppression of AV-node-dependent SVT (AVNRT, orthodromic AVRT) in preserved LV function; also the treatment of choice for idiopathic verapamil-sensitive fascicular VT.
Reduced ejection fraction or decompensated HF (its inotropic drop is steeper than diltiazem), hypotension, and pre-excited AF, where AV-nodal block can accelerate accessory-pathway conduction toward VF. Do not stack IV verapamil with IV beta-blockers, and do not reach for it in undifferentiated wide-complex tachycardia, where it can precipitate collapse.
Extensive hepatic metabolism via CYP3A4 with high first-pass, so oral bioavailability is low and variable; the active metabolite norverapamil contributes. It is also a P-glycoprotein inhibitor, and its effect tracks LV reserve and vascular tone.
A CYP3A4 substrate and inhibitor plus a P-gp inhibitor, so it raises digoxin and other substrate levels, with additive AV-nodal block when combined with beta-blockers or digoxin.
Heart rate, blood pressure, PR interval, and signs of heart failure; in AF confirm it is not pre-excited before giving, and follow digoxin levels when co-administered.
Not proarrhythmic in the QT or repolarization sense; the signature harms are bradyasystole and high-grade AV block (worst when stacked with other AV-nodal blockers), profound hypotension, and lethal ventricular-rate acceleration in pre-excited AF.
A first-line rate-control alternative to diltiazem in preserved-EF AF with rapid ventricular response once pre-excitation and overt HF are excluded, and an option for AVNRT after vagal maneuvers and adenosine; never give it to undifferentiated wide-complex tachycardia.
More negatively inotropic than diltiazem and often the wrong choice in shock or low EF; be ready to rescue hypotension with calcium or a pressor, and prefer a more titratable agent when hemodynamics are fragile.
Chronic oral rate control and SVT prophylaxis; reconcile against beta-blockers, verify the EF before committing, and watch the digoxin level and constipation on maintenance.
Verapamil-sensitive fascicular VT is the narrow, ECG-defined exception where this drug is both diagnostic and therapeutic, but that entity must be distinguished from every other wide-complex tachycardia, where giving verapamil is the classic fatal error.
AF guidelines position non-dihydropyridine calcium blockers for rate control in preserved EF, avoided in HFrEF and pre-excited AF; verapamil sensitivity separately defines idiopathic fascicular VT as a treatable entity.
IV bolus, which may be repeated, for acute SVT termination or rate control, and oral for maintenance. Confirm in Lexicomp.
Agents outside Vaughan-Williams: the AV-node circuit breaker, the membrane stabilizer, and others.
other AdenosineA1-receptor agonist (AV-nodal) The AV node's circuit breaker—not a universal reset button.
Activates cardiac A1 receptors, opening the GIRK potassium current and inhibiting adenylate cyclase—hyperpolarizing and transiently blocking AV-nodal conduction for a few seconds. It acts on the AV node, not directly on atrium or ventricle.
A stable, regular, narrow-complex tachycardia that is AV-node-dependent (AVNRT, orthodromic AVRT), after a vagal maneuver. It can also be diagnostic—unmasking flutter or atrial tachycardia by transiently blocking the AV node.
Irregular, polymorphic, or pre-excited wide-complex tachycardia—in pre-excited AF the AV-nodal block can favor accessory-pathway conduction. Severe bronchospastic disease. It will not fix a rhythm the AV node is not part of.
Half-life in seconds—cleared by red cells and endothelium. Give as a rapid push with an immediate flush; the effect is gone before you can titrate it, so the plan matters more than the dose.
Dipyridamole and carbamazepine potentiate it (reduce the dose); methylxanthines—caffeine, theophylline—antagonize it (may blunt the effect). Transplanted hearts are hypersensitive.
Continuous rhythm strip through the push with a defibrillator at hand; expect a dramatic pause, then reversion or revealed atrial activity. Warn the patient about brief chest tightness, flushing, and a sense of doom.
Transient AF at conversion in roughly one in eight; rare VF, most dangerous with an occult accessory pathway. Brief bradycardia or asystole is expected and self-limited.
Own the vagal-first, then-adenosine sequence: flush, monitor, and shock plan ready before the push.
Useful diagnostically in a monitored patient; start lower in transplant and dipyridamole patients.
Not maintenance therapy—its job ends in the acute moment; the longitudinal plan is recurrence, triggers, and EP referral.
A terminated SVT still needs a mechanism—AVNRT vs AVRT vs atrial tachycardia changes the ablation conversation.
Current SVT guidance places adenosine after a correctly performed modified Valsalva (REVERT) in the stable, regular, narrow pathway.
Rapid IV push with immediate flush; escalate and adjust (dipyridamole, transplant) per Lexicomp and local protocol.
other MagnesiumMembrane stabilizer; afterdepolarization suppressant The trigger suppressant that quiets torsades before the QT ever moves.
A physiologic calcium antagonist that dampens the L-type calcium current driving early afterdepolarizations, abolishing the triggered beats that initiate torsades. It stabilizes the myocyte membrane and supports the Na/K-ATPase; at higher serum levels it also slows sinus and AV nodal conduction. It suppresses the trigger without shortening the action potential or the QT interval.
Polymorphic VT with a long QT (torsades), given empirically as the first move even when serum magnesium is normal. Also digoxin-toxic arrhythmias and repletion where a low magnesium is itself the substrate; a documented deficit, not a reflex, is what justifies it in AF.
Renal impairment, where it accumulates into hypermagnesemia: hypotension, loss of deep tendon reflexes, respiratory depression, then bradycardia, heart block, and asystole. Myasthenia gravis and concurrent neuromuscular blockade amplify the same paralysis; calcium is the antidote.
Not metabolized; cleared entirely by the kidney, so the safety margin tracks renal function. Given IV for acute effect, with the serum level and the clinical exam, not a fixed dose, defining toxicity.
Potentiates non-depolarizing neuromuscular blockers and additive hypotension with calcium-channel blockers. Hypokalemia will not correct until the magnesium is replaced, so the two are repleted together.
In torsades, the disappearance of triggered ectopy signals it is working. For toxicity, the loss of the patellar reflex is the first bedside warning and precedes respiratory depression; follow blood pressure and the serum level in renal disease.
Essentially non-proarrhythmic; it does not provoke tachyarrhythmia. The signature harm runs the other way, toward bradyarrhythmia, high-grade AV block, and asystole from hypermagnesemia.
Reach for it immediately in polymorphic VT with a long QT, before chasing the level; it is not the drug for monomorphic VT or routine shockable arrest.
Titratable by infusion, but in renal impairment watch reflexes, pressure, and respiration as the toxicity ladder; keep calcium ready to reverse.
Find why the magnesium is low, diuretics, PPIs, GI losses, alcohol, and co-replete potassium, or the deficit and its arrhythmia recur.
It buys time by suppressing the trigger but does not shorten the QT or remove the driver; withdraw the offending agent, and consider pacing or isoproterenol for bradycardia-dependent torsades.
ALS guidance: magnesium for torsades and polymorphic VT associated with a long QT, not recommended for routine cardiac arrest or monomorphic VT.
IV bolus for torsades, followed by infusion where triggers persist; confirm in Lexicomp or the corrected EHRA compendium.
other DigoxinNa/K-ATPase inhibitor; vagotonic AV-nodal effect Resting rate control that leans on the vagus and forgives nothing renal.
Inhibits the sarcolemmal Na/K-ATPase, raising intracellular sodium and, via the Na/Ca exchanger, intracellular calcium, which gives a modest positive inotropy. Its rate control is largely indirect and autonomic: enhanced central vagal tone slows AV-nodal conduction and lengthens its refractoriness, an effect that fades when sympathetic drive rises.
Resting ventricular rate control in AF or flutter as an add-on or when beta-blockers and non-dihydropyridine calcium blockers are limited by hypotension or reduced ejection fraction, and in sedentary patients. It slows the node but does not terminate the arrhythmia and does not restore sinus rhythm.
A narrow therapeutic index that collapses with declining renal function, hypokalemia, and hypomagnesemia, each of which potentiates digitalis toxicity and its arrhythmias. Like any AV-nodal agent it is contraindicated in pre-excited AF, and its vagal mechanism is overridden by the sympathetic surge of exercise, sepsis, or critical illness, leaving exertional rate uncontrolled.
Predominantly renal clearance with a large volume of distribution, a long half-life, and delayed onset, so steady state and any loading strategy lag the clinical need. It is a P-glycoprotein substrate with minimal hepatic metabolism, which is why renal decline and transporter interactions drive accumulation.
Amiodarone, dronedarone, verapamil, and quinidine raise levels through P-glycoprotein inhibition and demand a dose reduction. Potassium-wasting diuretics amplify toxicity by lowering potassium, and beta-blockers or calcium blockers add to AV-nodal slowing.
Serum level interpreted alongside potassium, magnesium, and renal function, plus the ECG, with a low target trough. The first sign of failure is either exertional rate escaping control despite an adequate resting rate, or the early toxicity triad of nausea, visual disturbance, and new ectopy.
Its signature is digitalis toxicity: the paradoxical combination of enhanced automaticity from calcium-overload triggered activity with AV-nodal block. Watch for paroxysmal atrial tachycardia with block, bidirectional ventricular tachycardia, accelerated junctional rhythms, and a regularized ventricular response in AF.
A rate-control adjunct in acute AF with heart failure or hypotension where beta-blockers and calcium blockers are unsafe, accepting that slow onset makes it a poor sole agent for rapid control. Recognize toxic rhythms and reach for digoxin-specific antibody fragments when instability or hyperkalemia signals overdose.
The dynamic problem is that catecholamine drive in shock and sepsis defeats its vagal mechanism, so resting numbers mislead, while shifting potassium, evolving renal failure, and interacting drugs make the level a moving target. Calcium-overload physiology is what turns a therapeutic drug proarrhythmic here.
The longitudinal job is to track renal function and electrolytes over time, because a stable regimen becomes toxic when the kidney or the diuretic changes. Reconcile interacting agents at every visit and confirm steady-state levels rather than trusting a single reading.
The counter-lens is diagnostic: name digitalis toxicity when you see PAT with block, bidirectional VT, or an AF response that has gone regular, and do not mistake resting control for adequate control across the day. The rate-versus-rhythm decision and the ejection fraction still frame whether digoxin belongs in the plan at all.
AF guidelines position digoxin as second-line or adjunctive rate control, favored in HFrEF or sedentary patients, not as a rhythm agent; the DIG trial in heart failure showed no mortality benefit with reduced HF hospitalizations, and observational AF cohorts raise a confounded mortality signal that argues for restraint and low serum targets.
Renally adjusted maintenance with optional loading, targeting a low serum concentration; confirm in Lexicomp or the corrected EHRA compendium.
other IsoproterenolNon-selective beta-agonist (chronotrope) Race the heart past the pause—a bridge, never the destination, and wrong when catecholamines are the trigger.
A non-selective beta-1/beta-2 agonist that raises cAMP, augmenting the funny current (If) and L-type calcium current to increase rate, enhance AV conduction, and boost contractility. The rate rise shortens ventricular repolarization and the QT, and the larger calcium current restores the epicardial action-potential dome—the two effects that make it useful in bradycardia-dependent torsades and in Brugada storm.
Chronotropic bridge in symptomatic bradycardia or high-grade AV block refractory to atropine while transvenous pacing is arranged; pause- or bradycardia-dependent torsades de pointes, to raise the rate and shorten the QT when overdrive pacing is unavailable; and electrical storm in Brugada or early-repolarization syndrome, where the added calcium current suppresses recurrent VF.
Catecholamine-driven arrhythmia—CPVT and adrenergically triggered congenital LQT1/LQT2—where beta stimulation is the trigger, not the fix; and myocardial ischemia or fixed coronary disease, where the rate and inotropy raise oxygen demand into supply-dependent VT/VF. The same drug that outruns a pause-dependent torsades will feed a catecholamine-dependent one.
A catecholamine given only as a titratable continuous IV infusion, with onset and offset in minutes and no oral role; cleared by COMT and hepatic/tissue uptake. The effect tracks the infusion, so it is titrated to the rate, not dosed to a target.
Additive with other sympathomimetics and blunted by beta-blockers—though a beta-blocker is the correct antidote when the arrhythmia is catecholamine-driven. Halogenated inhalational anesthetics sensitize the myocardium to its arrhythmic effect, and hypokalemia compounds the proarrhythmic risk.
Continuous rhythm and rate are the first read—titrate to the target heart rate and watch the QT shorten in torsades or the ST dome normalize in Brugada. New angina, ischemic ST change, or emergent ventricular ectopy is the signal it is now doing harm rather than good.
Its signature is demand ischemia and provoked ventricular ectopy in coronary disease, and precipitation or acceleration of catecholamine-sensitive VT (CPVT, adrenergic LQTS). Sinus tachycardia and rising myocardial oxygen demand are expected, not incidental.
Own the execution: a chronotropic infusion to hold rate and perfusion in refractory bradycardia or pause-dependent torsades while the pacing wire is placed—it buys minutes, it does not fix the conduction system.
Titratable and fast, but the physiology cuts both ways—balance the rate needed to bury a pause against the demand ischemia and catecholamine sensitivity it can provoke, and wean it as pacing or definitive control takes over.
Not a maintenance therapy—the durable answer is a pacemaker for bradycardia, removing QT-prolonging drugs and correcting electrolytes in acquired long QT, or a beta-blocker/quinidine/ICD plan for CPVT and Brugada. Reconcile the mechanism before the rhythm recurs.
The diagnostic counter-lens: in the EP lab it is a provocation agent—unmasking latent conduction, facilitating VT induction, and revealing the Brugada or CPVT phenotype. The catecholamine that diagnoses one substrate is contraindicated as therapy in another, so name the substrate before you infuse.
No randomized trial supports it. The chronotropic-bridge use is anchored in the 2018 ACC/AHA/HRS bradycardia guideline, which lists isoproterenol among agents that may be considered to raise the rate in symptomatic sinus-node disease at low likelihood of ischemia—while the current ACLS bradycardia algorithm itself has moved to dopamine and epinephrine as its named chronotropes. Brugada electrical storm and pause- or bradycardia-dependent torsades rest on expert consensus and ventricular-arrhythmia guidance. The evidence is mechanistic and observational, and its boundary is the catecholamine-driven arrhythmia it must not treat.
Continuous titratable IV infusion adjusted to heart rate; no oral form. Confirm in Lexicomp and the corrected EHRA compendium.
other AtropineAntimuscarinic (vagolytic, AV-nodal) Buys minutes by cutting the vagus, not by pacing the heart.
Competitively blocks cardiac M2 muscarinic receptors, withdrawing vagal tone from the sinus node and AV node; this raises the sinus rate and speeds AV conduction. The effect is entirely dependent on there being vagal tone to remove, so it works upstream of the SA and AV nodes, not on the conducting tissue itself.
Symptomatic bradycardia that is vagally or nodally mediated, including sinus bradycardia and AV block at the level of the AV node, as the first temporizing move while pacing or the reversible cause is addressed.
Infranodal block, Mobitz II or complete heart block with a wide-complex escape, where the lesion is in the His-Purkinje system: speeding the atrial rate without capturing the block can worsen the AV ratio and paradoxically slow the ventricles. It also does nothing for a denervated transplanted heart, where it can instead precipitate complete AV block or sinus arrest, and it can provoke ischemia by raising rate in the setting of coronary disease.
A tertiary amine that crosses membranes and the blood-brain barrier; rapid onset after IV push, with hepatic and plasma esterase clearance and renal elimination of drug and metabolites. Effect is brief, so it is a bridge, not a maintenance therapy.
Additive antimuscarinic burden with other anticholinergics, antihistamines, and tricyclics; low or slowly titrated doses can produce a paradoxical bradycardia. It will not overcome bradycardia driven by beta-blocker or calcium-channel-blocker toxicity, where the receptor being blocked is not the muscarinic one.
The heart rate and, on the monitor, whether the P-P interval accelerates while the AV ratio stays fixed or worsens, which reveals an infranodal lesion that will not respond. Watch for the antimuscarinic cost: dry mouth, urinary retention, mydriasis, delirium in the elderly.
Not a repolarization proarrhythmic, but it can provoke sinus tachycardia, precipitate or worsen ischemia by raising myocardial demand, and, in infranodal block, drop the ventricular rate rather than raise it.
First-line drug for unstable symptomatic bradycardia, given while transcutaneous pacing is set up; if the rhythm is high-grade or wide-complex block, move to pacing early rather than chasing it with repeat doses.
A short-acting bridge whose failure signals the need for a definitive chronotrope infusion or transvenous pacing; reserve judgment in ischemic or post-arrest physiology, where the tachycardia it produces is not free.
Rarely the durable answer: a positive response points to a vagal or nodal mechanism and a reversible cause worth hunting, while a poor response reframes the problem as conduction-system disease needing a pacemaker workup.
The response is a diagnostic test of where the block sits: nodal lesions accelerate, infranodal lesions do not, so a failed atropine trial localizes the disease and changes the plan from pharmacology to a pacing wire.
ACLS bradycardia guidance: atropine is first-line for symptomatic bradycardia but is expected to be ineffective in Mobitz II and third-degree block, where pacing or chronotropic infusion is preferred.
Repeated IV boluses to an effect ceiling, as a bridge to pacing; confirm dose and interval in Lexicomp.
Decision drills · 8 cases
Choose the agent, then see why.
One rule: choose from the patient plus the substrate, never the drug name alone. Commit before you reveal—each option then explains itself.
Assessment · 12 items
Explain the answer you choose.
Single best answer. Every distractor gets a reason it fails and when it would fit.