Cardiology

Cardiac Myocyte Action Potential

Connect cardiac ion currents to contraction, ECG intervals, antiarrhythmic effects and arrhythmia mechanisms through visible diagrams and clinical cases.

A patient starts sotalol and develops a long QT. Another starts flecainide and develops a wide QRS. Both drugs alter electrical activity, but they disturb different parts of the cycle. First identify the tissue, then identify whether activation or recovery has changed.

Phase 0 does not always mean sodium. Ventricular muscle uses a sodium-dominant upstroke; central nodal tissue uses a calcium-dominant upstroke.

Read the ventricular curve as current balance

A membrane potential describes voltage inside one cell relative to its surroundings. Sodium and calcium entry usually supply depolarizing inward current. Potassium exit usually supplies repolarizing outward current. Voltage depends on the balance of currents, so a flat line does not mean all channels are closed. Neighboring cells pass current through gap junctions; conduction also depends on tissue architecture and cell coupling. [1]

The plateau contains opposing currents. The surface ECG sums electrical differences across many cells.

Phase 0. Activate: Voltage-gated Nav1.5 sodium channels open. Sodium entry produces the steep upstroke. Availability of these channels affects excitability and conduction; their inactivation follows opening. A sodium blocker can slow ventricular activation and widen QRS.
Phase 1. Form the notch: Sodium current inactivates while transient outward potassium current, Ito, contributes to the brief initial decline. The notch is not a separate surface ECG wave.
Phase 2. Sustain the plateau: L-type calcium entry opposes outward potassium currents. This balance supports a prolonged action potential and provides trigger calcium for contraction. Several currents coexist throughout the plateau.
Phase 3. Recover: Calcium current declines and outward potassium currents predominate. IKr and IKs contribute to repolarization; IK1 contributes strongly toward terminal recovery. Reduced repolarization reserve increases vulnerability to long QT and triggered activity.
Phase 4. Maintain readiness: Working ventricular muscle usually has a stable resting potential near -90 mV, supported largely by IK1. The Na+/K+ ATPase maintains gradients using ATP, exporting three sodium ions per two potassium ions imported. It is not the principal current producing phase 3.

The ST segment roughly corresponds to a period when much ventricular tissue is depolarized. The T wave reflects distributed ventricular repolarization. QT spans the beginning of QRS through the end of T, so it includes activation as well as recovery. Avoid assigning one surface interval to exactly one cell phase. [1] [13]

Try it here · Checkpoint 1 of 3

Make your prediction before reading the choices. A first attempt is just a starting point.

Case 2

Show answer and explanations

A. Outward IK1 current dominates the interval (Why this does not fit)
IK1 is especially important near resting voltage and terminal repolarization; it does not explain the sustained plateau near 0 mV.
B. The transient outward potassium current sustains the interval (Why this does not fit)
Ito helps generate the early notch; sustained outward dominance would repolarize the cell rather than maintain the plateau.
C. Fast sodium current supplies the sustained inward current (Why this does not fit)
The large fast sodium current inactivates soon after the upstroke. L-type calcium current supplies the principal sustained inward plateau current.
D. Inward calcium current opposes outward potassium current (Best answer)
The relatively stable voltage reflects a balance of active inward and outward currents during phase 2.

Takeaway: A flat portion of an action potential can contain substantial opposing currents.

Case sources: [1] [3]

Try a question on this topic

A 69-year-old woman is admitted to a telemetry unit to begin oral dofetilide for recurrent episodes of a fast, irregularly irregular rhythm that persisted despite rate control. On the second day of therapy, she reports sudden lightheadedness and briefly loses consciousness while seated in bed. Telemetry captures a 20-second run of a wide-complex rhythm at 240/min whose QRS axis repeatedly rotates around the baseline, followed by spontaneous return to an organized rhythm. Blood pressure is now 118/72 mm Hg. Serum potassium is 3.4 mEq/L, and magnesium is 1.6 mg/dL. Her baseline ECG showed a corrected QT interval of 430 msec; the current ECG shows sinus rhythm at 58/min with a corrected QT interval of 590 msec. She is alert and oriented, and cardiac examination shows a regular rhythm without murmurs.

Blockade of which of the following currents is most directly responsible for this event?

Choose an answer before revealing the reasoning.

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Distinguish a pacemaker from working muscle

The central sinoatrial node gradually depolarizes during diastole. Its phase 4 is sloped, its upstroke is calcium-dominant, and it lacks the prominent notch and plateau of ventricular muscle. The maximum diastolic potential is less negative, often around -60 mV. Sodium-channel expression and contribution vary across nodal regions; saying every nodal cell lacks sodium channels is too absolute. [2]

Start the next impulse

HCN channels carry the funny current, If. Hyperpolarization activates this mixed sodium/potassium current, which is net inward at diastolic voltages. Declining outward potassium current and calcium currents also contribute. T-type calcium current can support diastolic depolarization, whereas L-type current dominates the central nodal upstroke. Spontaneous SR calcium release activates inward NCX current, coupling the calcium clock to membrane currents. [1] [2]

Conduct through the node

L-type calcium channels supply the main phase 0 current in central nodal tissue. Verapamil and diltiazem slow AV nodal conduction. For acute AF rate control, their use requires hemodynamic stability and EF above 40%; avoid IV use with moderate or severe LV systolic dysfunction. In sinus rhythm this can lengthen PR. During atrial fibrillation, fewer atrial impulses reach the ventricles, but there is no organized P wave from which to measure a PR interval. [12]

Autonomic signals change the interval between impulses

Beta-1 stimulation activates Gs and increases cAMP. cAMP modulates HCN channels and promotes calcium-channel activity, increasing sinus rate and AV conduction. M2 muscarinic signaling lowers cAMP through Gi, while G-protein subunits activate GIRK potassium channels. Increased outward current hyperpolarizes nodal cells. Chronotropy means rate, dromotropy means conduction, and inotropy means force. [1] [2] [12]

Atropine blocks muscarinic receptors and reduces vagal restraint. It does not directly stimulate beta receptors. Resting sinus rates are often below the intrinsic denervated rate, which is approximately 100/min. Predominant right-vagal effects on the SA node and left-vagal effects on AV conduction are useful tendencies with overlapping innervation, not exclusive wiring.

Usual rhythm ranges provide orientation rather than diagnostic cutoffs. Resting sinus rhythm is conventionally 60 to 100/min, junctional escape about 40 to 60/min, and ventricular escape about 20 to 40/min. Purkinje tissue has latent automaticity despite a sodium-dependent upstroke. Overdrive suppression includes electrogenic pump effects. An escape rhythm appears late after failure of a higher pacemaker; an ectopic premature complex arrives early.

Ivabradine targets HCN/If to reduce sinus firing without a direct negative inotropic effect. It is not an AV nodal rescue drug for atrial fibrillation. [8]

Follow calcium from electrical signal to contraction

The calcium entering a ventricular cell during the plateau has two jobs. It carries inward electrical current and initiates calcium-induced calcium release. Depolarization reaches the T-tubules, opens Cav1.2 L-type channels, and admits trigger calcium. That calcium activates RyR2 channels on the sarcoplasmic reticulum, releasing a much larger store. [3]

Entry through Cav1.2
Extracellular calcium provides the trigger at the junction between T-tubule and SR.
Release through RyR2
The local signal recruits SR calcium release and increases cytosolic calcium.
Binding to troponin C
Tropomyosin changes position, allowing actin-myosin interaction and sarcomere shortening.
Recovery through SERCA and NCX
SERCA2a returns calcium to the SR. Forward NCX exports one calcium ion while importing three sodium ions. Plasma membrane calcium ATPase contributes a smaller amount of extrusion.

Phospholamban inhibits SERCA when dephosphorylated. Beta-adrenergic signaling promotes its phosphorylation, relieving that inhibition. Calcium returns to the SR sooner, improving relaxation, called positive lusitropy, and increasing calcium available for subsequent contractions. Mitochondria buffer calcium and couple calcium signaling to energy production; they are not the main route for clearing each cytosolic transient. [3]

Digoxin inhibits Na+/K+ ATPase. Intracellular sodium rises, reducing the sodium gradient that normally supports calcium extrusion by NCX. Greater cellular and SR calcium loading increases contractile force. It is unnecessary to claim that NCX must reverse direction in every treated cell. Digoxin also has separate vagal effects that slow AV conduction. Therapeutic exposure can cause scooped ST depression and PR prolongation without proving toxicity. Suspected serious toxicity requires clinical, rhythm, electrolyte and renal assessment; digoxin-specific antibody fragments treat life-threatening poisoning. Renal impairment and low potassium or magnesium increase toxicity risk. [6]

Contrast skeletal muscle. Cav1.1 serves as a voltage sensor coupled to RyR1, so immediate SR release does not require the same extracellular calcium trigger. This does not mean extracellular calcium is irrelevant to long-term skeletal muscle health. Cardiac force varies with calcium handling and loading conditions; skeletal force also varies with stimulation frequency and motor-unit recruitment. RyR1 dysfunction can cause malignant hyperthermia, whereas RyR2-related disease can cause CPVT.

Separate failed recovery, extra triggers and reentry

During most of the ventricular action potential, sodium channels cannot yet support another normal propagated response. This prolonged refractoriness overlaps contraction and prevents ordinary tetanic summation. Action potential duration varies by cell type and rate; roughly 200 to 300 ms is an orientation for ventricular muscle, not a fixed constant. [1]

Absolute refractory period: A second action potential cannot be elicited.
Effective refractory period: A stimulus cannot produce a normally propagated response. This functional definition is closely related to, but not identical to, absolute refractoriness.
Relative refractory period: Partial sodium-channel recovery permits a stronger stimulus to produce a response with reduced upstroke and potentially slower conduction. Late repolarization is a vulnerable period.

A brief supernormal excitability period can be demonstrated in some tissues and conditions. Do not use the end of a surface T wave as an exact map of one cell's excitability. Spatially unequal recovery explains why a premature ventricular complex overlapping the T wave can initiate a dangerous rhythm.

Early afterdepolarization

An EAD interrupts phase 2 or 3 before repolarization is complete. Prolonged inward current or reduced outward current permits reactivation of depolarizing currents. Acquired long QT, pauses, hypokalemia and QT-prolonging drugs create a typical setting. Torsades is polymorphic VT associated with prolonged QT between episodes.

Delayed afterdepolarization

A DAD follows completed repolarization. Spontaneous SR calcium release stimulates forward NCX, whose net inward current can trigger another action potential. Digoxin toxicity and catecholamine-sensitive calcium-handling disorders are useful examples. Increased calcium loading during higher rates can favor DADs.

Reentry is circulating excitation, not a new calcium-driven impulse each cycle. A premature impulse may encounter unidirectional block, traverse an alternative route, and return after tissue recovers. For a fixed path, circuit transit time must exceed local refractoriness. The approximate wavelength is conduction velocity multiplied by refractory period. Conduction slowing can shorten wavelength and favor reentry; increasing ERP can make the returning impulse encounter refractory tissue. For the same path length, a higher conduction velocity brings the impulse back sooner and allows less recovery time. [1]

Sustained polymorphic VT requires immediate unsynchronized shock. For recurrent torsades associated with long QT, IV magnesium may be considered; correct electrolyte deficits and withdraw QT-prolonging drugs. Magnesium need not normalize QT. Specialist-directed pacing or isoproterenol can help selected pause-dependent acquired cases; adrenergic stimulation is not a general treatment for congenital long QT or CPVT. [9] [11]

Try it here · Checkpoint 2 of 3

Make your prediction before reading the choices. A first attempt is just a starting point.

Case 13

Show answer and explanations

A. It increases from 0.10 m to 0.15 m (Best answer)
Wavelength equals conduction velocity times ERP. Greater wavelength can prevent the returning impulse from finding recovered tissue.
B. It decreases from 0.10 m to 0.067 m (Why this does not fit)
This incorrectly divides by refractoriness. The wavelength relationship is a product.
C. It remains 0.10 m because only conduction velocity matters (Why this does not fit)
ERP is one of the two factors in the simplified wavelength expression.
D. It increases from 0.10 m to 0.30 m (Why this does not fit)
The new ERP must still be multiplied by 0.5 m/s. Treating its numerical value as a distance omits the conduction-velocity factor.

Takeaway: For a fixed circuit, prolonged refractoriness can interrupt reentry by increasing the distance occupied by refractory tissue.

Case sources: [1]

Predict drug effects without forcing one drug into one interval

Vaughan Williams classes and their useful discriminators
Class	Target and examples	Expected pattern
IA	Sodium block plus repolarizing potassium block. Quinidine, procainamide, disopyramide.	Conduction slowing and prolonged repolarization. QRS and QT may both increase.
IB	Sodium-channel block with relatively rapid recovery. Lidocaine, mexiletine.	Preference for depolarized tissue; AP duration may shorten. Lidocaine is not a potassium-channel opener.
IC	Pronounced sodium-channel block. Flecainide, propafenone.	QRS widening, often more evident at higher rates. Measured QT may lengthen because QRS is part of QT.
II	Beta blockade. Metoprolol, esmolol, propranolol.	Less adrenergic stimulation of sinus automaticity and AV conduction.
III	Repolarization prolongation. Sotalol, dofetilide, ibutilide, amiodarone.	Longer AP duration and QT. Effects and torsades risk differ among agents.
IV	L-type calcium block. Verapamil and diltiazem.	AV nodal slowing and negative inotropy. Amlodipine is not a substitute for nodal rate control.

Flecainide can increase PR, QRS and QT; most of its QT increase reflects wider QRS rather than a comparable JT increase. Avoid it in prior MI or significant structural heart disease. CAST demonstrated excess mortality, not merely more abnormal tracings, in post-MI patients receiving encainide or flecainide. Sodium-channel effects in heterogeneous tissue help explain proarrhythmia, although that trial did not establish a single mechanism for every death. [4] [14] [16]

Sotalol combines beta blockade with class III effects. The cited oral sotalol label calls for initiation or reinitiation in hospital for at least three days or until steady state, with continuous ECG monitoring and resuscitation capability. Correct potassium and magnesium deficits and use creatinine clearance to select the dosing interval. Amiodarone affects several channels and receptors. Its torsades risk is lower than that of some other QT-prolonging agents, but it still causes proarrhythmia and clinically important pulmonary, hepatic and thyroid toxicity. Do not equate a longer QT with equal risk across drugs. [5] [15]

Adenosine activates A1 receptors, increases nodal potassium conductance and reduces calcium-dependent conduction through Gi signaling. Transient AV block can terminate AV node-dependent reentry. This is rhythm termination, not chronic rate control. Rapid uptake by red cells and vascular endothelium gives adenosine a blood half-life under ten seconds. Caffeine and theophylline antagonize its effects; dipyridamole potentiates them. Failure to terminate a tachycardia does not establish antidromic AVRT. [7] [9]

In preexcited AF, avoid AV nodal blockers, including adenosine, beta blockers, verapamil, diltiazem, digoxin and IV amiodarone. Stable patients may receive specialist-directed procainamide or ibutilide; instability requires cardioversion. An antegrade accessory pathway bypasses AV nodal delay. Early ventricular activation through ordinary myocardium produces a slurred initial QRS, the delta wave, before fusion with activation through the normal conduction system. A short PR and delta wave in sinus rhythm identify preexcitation, while the dangerous acute rhythm is irregular with very rapid, variably wide ventricular complexes. [12]

Class IA drugs also have distinct extracardiac toxicities: quinidine can cause tinnitus, visual disturbance and gastrointestinal symptoms of cinchonism; procainamide can cause a lupus-like syndrome; disopyramide has antimuscarinic effects such as urinary retention and can worsen contractile function. Lidocaine's rapid channel recovery and greater action in depolarized tissue explain its different profile from class IC drugs. [18]

Use the resting tracing and trigger together

Hyperkalemia makes the resting membrane less negative and can impair sodium-channel availability. Peaked T waves, PR prolongation, loss of P waves, QRS widening and a sine-wave appearance are possible, but their order and potassium thresholds are unreliable. A reassuring tracing does not exclude severe hyperkalemia. With toxic ECG changes, IV calcium protects cardiac excitability without lowering potassium. Insulin with glucose shifts potassium intracellularly; glucose monitoring is essential. Dialysis and other elimination strategies address total-body excess. IV bicarbonate is not routine acute therapy. [10]

Hypokalemia can reduce repolarization reserve despite a larger potassium concentration gradient. Flattened T waves, ST depression and prominent U waves are useful findings. T-U fusion can mimic a long QT by extending the apparent QU interval. U-wave generation is not definitively explained by one cell population. Low magnesium increases arrhythmia susceptibility and can hinder potassium correction. Hypocalcemia typically lengthens the ST portion of QT; hypercalcemia tends to shorten it. [19] [13]

LQT1: KCNQ1 loss reduces IKs. Exertion, especially swimming, is a characteristic trigger.
LQT2: KCNH2 loss reduces IKr. Sudden auditory stimuli and emotion are characteristic triggers.
LQT3: SCN5A variants increase persistent late sodium current. Events may occur at rest or during sleep. Selected patients benefit from genotype-directed sodium-current inhibition in specialist care.
Brugada syndrome: A type 1 coved right-precordial ST pattern and arrhythmic history warrant evaluation. Fever may reveal the pattern. SCN5A loss is an established cause, but many patients have no identified SCN5A variant. Both abnormal right ventricular outflow tract conduction and unequal repolarization have supporting evidence; one universal cellular explanation is not established.
CPVT: Exercise or emotional stress provokes bidirectional or polymorphic VT despite a typically normal resting ECG. RYR2 and CASQ2 are important calcium-handling genes.

Romano-Ward describes inherited long QT without deafness; Jervell and Lange-Nielsen is typically recessive long QT with congenital sensorineural deafness, often involving KCNQ1 or KCNE1. One normal QT does not exclude a pathogenic long-QT genotype. Beta blockers are central to long-QT and CPVT care, with nadolol or propranolol preferred in the 2022 ESC guidance. ICD decisions depend on events and risk, rather than the syndrome name alone. [17] [11]

Use this sequence. Identify the tissue. Separate QRS activation from QT/JT recovery. Check electrolytes, renal function and drugs. Distinguish EADs during recovery from DADs after recovery and reentry through recovered tissue. Then let pulse, perfusion and rhythm morphology determine the immediate response.

Try it here · Checkpoint 3 of 3

Make your prediction before reading the choices. A first attempt is just a starting point.

Case 26

Show answer and explanations

A. Late sodium current through Nav1.5 (Why this does not fit)
Increased persistent late sodium current is associated with LQT3, usually involving SCN5A.
B. IKr, the rapid delayed rectifier potassium current (Why this does not fit)
Reduced IKr is characteristic of KCNH2-associated LQT2; the specified KCNQ1 variant identifies IKs.
C. HCN-mediated funny current (Why this does not fit)
HCN current participates in automaticity and does not define KCNQ1-associated long QT.
D. IKs, a delayed rectifier potassium current (Best answer)
KCNQ1-associated LQT1 reduces repolarizing IKs, and exertion or swimming is a characteristic trigger.

Takeaway: Combine genotype, current and trigger rather than using trigger alone to assign a syndrome.

Case sources: [11] [1]

Apply the physiology to patients

Case 1

Show answer and explanations

A. Reduced phase 0 sodium current in ventricular myocytes (Best answer)
Nav1.5 inhibition reduces the upstroke and slows propagation through sodium-dependent tissue, widening QRS.
B. Reduced phase 0 calcium current in the central SA node (Why this does not fit)
Central nodal calcium current affects pacemaker activation and nodal conduction, not the principal ventricular upstroke.
C. Increased outward potassium current during phase 3 (Why this does not fit)
Greater outward repolarizing current primarily changes recovery; it does not explain this sodium-blocker conduction pattern.
D. Reduced funny current during phase 4 depolarization (Why this does not fit)
If inhibition slows sinus firing. It does not directly account for marked ventricular QRS widening.

Takeaway: Use QRS widening to identify impaired ventricular activation before attributing a drug effect to repolarization.

Case sources: [1] [4]

Case 3

Show answer and explanations

A. Reduced ventricular RyR2 calcium release by direct receptor blockade (Why this does not fit)
Ivabradine targets HCN channels rather than directly blocking the SR calcium-release channel.
B. Inhibition of ventricular sodium channels during phase 0 (Why this does not fit)
This would predict conduction effects such as QRS widening rather than selective sinus-rate reduction.
C. Reduced sinus-node diastolic inward current through HCN channels (Best answer)
Ivabradine inhibits If and slows sinus firing without a direct negative inotropic effect.
D. Reduced L-type calcium current in the AV node (Why this does not fit)
Direct nodal calcium-channel inhibition describes verapamil or diltiazem; the intended ivabradine target is HCN-mediated If.

Takeaway: Identify If as a pacemaker current and keep its role separate from AV nodal conduction.

Case sources: [2] [8]

Case 4

Show answer and explanations

A. A1 signaling reduces calcium-dependent AV conduction (Why this does not fit)
Adenosine can slow nodal conduction through A1 receptors, but acetylcholine released by vagal nerves acts at muscarinic M2 receptors.
B. M2 signaling lowers cAMP and increases GIRK potassium conductance (Best answer)
Gi signaling reduces pacemaker stimulation while GIRK-mediated outward current makes diastolic depolarization less effective.
C. Beta-1 signaling increases cAMP and calcium entry (Why this does not fit)
This is the sympathetic pattern and ordinarily increases sinus rate rather than slowing it.
D. Beta-1 receptor blockade reduces cAMP (Why this does not fit)
A beta blocker reduces adrenergic signaling, but the stem describes increased parasympathetic activity rather than drug-mediated beta blockade.

Takeaway: Cardiac vagal effects combine reduced cAMP with increased potassium conductance.

Case sources: [1] [2]

Case 5

Show answer and explanations

A. Calcium-dependent AV nodal conduction is reduced (Best answer)
Diltiazem limits transmission of atrial impulses through the AV node. An organized PR interval cannot be measured in AF.
B. Fast ventricular sodium-channel inhibition slows activation (Why this does not fit)
This primarily slows ventricular conduction and can widen QRS; it does not explain selective reduction of AV transmission with a narrow QRS.
C. HCN inhibition reduces the rate of sinus-node discharge (Why this does not fit)
Sinus firing does not govern the ventricular response during continuing AF; transmission through the AV node matters here.
D. Repolarizing potassium-channel inhibition terminates the atrial arrhythmia (Why this does not fit)
Diltiazem controls AV transmission rather than acting as a potassium-channel rhythm-conversion drug. A slower ventricular response does not establish conversion.

Takeaway: Explain AF rate control using AV transmission, rather than a PR interval measured during AF.

Case sources: [1] [12]

Case 6

Show answer and explanations

A. Direct stimulation of beta-1 receptors (Why this does not fit)
A catecholamine can stimulate beta receptors, but atropine acts by muscarinic antagonism.
B. Direct inhibition of nodal HCN channels (Why this does not fit)
HCN inhibition ordinarily slows sinus firing, whereas atropine relieves vagal inhibition.
C. Activation of nodal adenosine A1 receptors (Why this does not fit)
A1 activation slows AV conduction; atropine instead antagonizes muscarinic signaling.
D. Blockade of cardiac muscarinic receptors (Best answer)
Atropine reduces parasympathetic restraint on nodal tissue. It does not directly activate adrenergic receptors.

Takeaway: A vagolytic response identifies removal of muscarinic restraint, not direct sympathetic receptor stimulation.

Case sources: [2] [9]

Case 7

Show answer and explanations

A. Increased calcium sensitivity of troponin C (Why this does not fit)
Myofilament sensitivity affects force at a given calcium concentration, but the immediate missing signal after L-type blockade is calcium-triggered SR release.
B. Phospholamban inhibition of SERCA (Why this does not fit)
This regulates SR reuptake and loading; it is not the immediate trigger for release from the preserved SR store.
C. Trigger calcium activation of RyR2 (Best answer)
Cardiac SR release normally depends on local calcium entry through Cav1.2, which activates RyR2.
D. Direct mechanical coupling of Cav1.1 to RyR1 (Why this does not fit)
This describes the principal skeletal-muscle coupling arrangement rather than the cardiac Cav1.2/RyR2 system.

Takeaway: Cardiac calcium entry is a trigger for larger SR release, not merely the entire contractile calcium supply.

Case sources: [3]

Case 8

Show answer and explanations

A. Reduced calcium extrusion through forward NCX (Why this does not fit)
Reducing calcium extrusion promotes calcium retention and does not explain the faster cytosolic calcium clearance required for relaxation.
B. Phospholamban phosphorylation increases SERCA activity (Best answer)
Relieving phospholamban inhibition accelerates calcium uptake into the SR and supports positive lusitropy.
C. Phospholamban dephosphorylation reduces SERCA activity (Why this does not fit)
Dephosphorylated phospholamban inhibits SR uptake; this would oppose the described acceleration of relaxation.
D. Increased myofilament calcium sensitivity (Why this does not fit)
Greater sensitivity can support force but can also prolong activation at falling calcium concentrations. Faster SR uptake better explains the stated lusitropic response.

Takeaway: Stronger contraction and more efficient relaxation can coexist when SR uptake increases.

Case sources: [3]

Case 9

Show answer and explanations

A. Na/K ATPase inhibition, higher intracellular sodium, reduced NCX calcium extrusion (Best answer)
The weaker inward sodium gradient limits forward calcium extrusion and promotes cellular and SR calcium loading.
B. Beta-1 stimulation, higher cAMP, enhanced L-type calcium entry (Why this does not fit)
This is a catecholamine-mediated route to positive inotropy. Digoxin begins with inhibition of Na/K ATPase.
C. Phospholamban phosphorylation, greater SERCA activity, faster SR uptake (Why this does not fit)
This describes an adrenergic calcium-handling pathway, not the initiating molecular action of digoxin.
D. M2 activation, lower cAMP, slower AV transmission (Why this does not fit)
Vagal effects help explain digoxin-related AV slowing, but they do not account for its direct positive inotropic action in ventricular muscle.

Takeaway: Reduced forward NCX calcium extrusion is sufficient to explain digoxin-related calcium loading.

Case sources: [3] [6]

Case 10

Show answer and explanations

A. Calcium-channel reactivation during a prolonged phase 2 plateau only (Why this does not fit)
That describes an EAD mechanism; digoxin-associated calcium overload is a classic DAD setting.
B. A circulating impulse traverses a scar-related reentrant circuit (Why this does not fit)
Scar-mediated reentry is a common VT mechanism, but glycoside toxicity with bidirectional VT and calcium loading favors triggered activity here.
C. An ectopic pacemaker develops faster spontaneous diastolic depolarization (Why this does not fit)
Enhanced automaticity differs from a calcium-dependent afterpotential caused by a preceding action potential; the toxicity pattern favors a DAD.
D. Spontaneous SR calcium release after recovery drives inward NCX current (Best answer)
Calcium overload can generate a phase 4 DAD. Forward NCX exchanges three sodium ions inward for one calcium outward, creating net inward current.

Takeaway: Digoxin toxicity links calcium overload to DADs; renal impairment and low potassium increase susceptibility.

Case sources: [1] [6]

Case 11

Show answer and explanations

A. A circulating wavefront travels through an infarct-scar circuit (Why this does not fit)
Reentry explains many monomorphic ventricular arrhythmias, but prolonged QT and pause-associated polymorphic VT favor an EAD trigger here.
B. An ectopic focus develops enhanced phase 4 automaticity (Why this does not fit)
Automaticity can cause ectopy, but the delayed-repolarization and pause-dependent pattern points to EAD-mediated torsades.
C. Depolarizing activity interrupts phases 2 or 3 before recovery is complete (Best answer)
Reduced repolarization reserve permits EADs, especially with a long QT and pauses. Renal function influences sotalol exposure.
D. Spontaneous SR release occurs only after a normal-duration action potential (Why this does not fit)
This describes a DAD pattern and does not best explain the prolonged-QT, pause-associated presentation.

Takeaway: A long-QT, pause-associated ventricular arrhythmia suggests EAD-mediated torsades.

Case sources: [1] [5] [9]

Case 12

Show answer and explanations

A. Supernormal excitability (Why this does not fit)
Supernormal excitability concerns a temporarily lower stimulus threshold; it does not define a local response that cannot propagate.
B. Effective refractoriness (Best answer)
ERP is defined by failure to produce a normally propagated response, even when some local responsiveness exists.
C. Absolute refractoriness with no local response possible (Why this does not fit)
The stem explicitly describes a local response, so a definition requiring no elicitable response is too restrictive.
D. Post-repolarization refractoriness (Why this does not fit)
That term requires persistence of refractoriness after repolarization is complete. The stem does not establish that timing; failure of propagation directly defines effective refractoriness.

Takeaway: Effective and absolute refractory periods answer related but different experimental questions.

Case sources: [1]

Case 14

Show answer and explanations

A. Enhance gastrointestinal potassium elimination (Why this does not fit)
A potassium binder can increase gastrointestinal elimination. Calcium instead acts rapidly on cardiac excitability.
B. Increase cellular potassium uptake through insulin signaling (Why this does not fit)
Insulin provides intracellular redistribution; IV calcium does not lower potassium through this mechanism.
C. Increase renal potassium excretion (Why this does not fit)
Renal elimination may be aided by selected diuretic strategies when kidney function permits, but it is not calcium-mediated membrane protection.
D. Protect cardiac excitability without lowering serum potassium (Best answer)
Calcium counters dangerous membrane effects. Intracellular redistribution and potassium elimination still require separate treatment.

Takeaway: Separate membrane stabilization, intracellular redistribution and elimination in hyperkalemia treatment.

Case sources: [10]

Case 15

Show answer and explanations

A. Defer active treatment while awaiting a second ECG in several hours (Why this does not fit)
A valid potassium of 6.9 mmol/L warrants urgent treatment and monitoring; ECG sensitivity is too limited to justify this delay.
B. Use IV bicarbonate as the sole acute potassium-lowering therapy (Why this does not fit)
Routine IV bicarbonate is not supported for acute hyperkalemia; it cannot replace indicated redistribution and elimination strategies.
C. Severe hyperkalemia still requires urgent assessment and treatment (Best answer)
ECG sensitivity is limited. The absence of a stereotyped pattern does not make a verified dangerous concentration safe.
D. Manage the result as pseudohyperkalemia and arrange outpatient retesting (Why this does not fit)
A repeated nonhemolyzed severe result in acute kidney injury supports real hyperkalemia. The unchanged ECG does not establish laboratory artifact.

Takeaway: A normal ECG cannot exclude clinically dangerous hyperkalemia.

Case sources: [10]

Case 16

Show answer and explanations

A. Whether tachycardia-related rate correction is responsible for the entire finding (Why this does not fit)
Rate correction matters, but no tachycardia is stated. The prominent second deflection specifically requires inspection of the T-U boundary.
B. Whether the algorithm included a prominent U wave in the QT measurement (Best answer)
T-U fusion may extend the apparent QU interval. Manual identification of the T-wave end is needed.
C. Whether a new bundle-branch block accounts for the interval increase (Why this does not fit)
A wide QRS can lengthen QT, but no QRS widening is described. The low potassium and second deflection more directly suggest U-wave inclusion.
D. Whether QT should start at the end of QRS (Why this does not fit)
QT starts at QRS onset. Starting at the end instead estimates JT.

Takeaway: A prominent U wave can distort automated QT measurements during hypokalemia.

Case sources: [13]

Case 17

Show answer and explanations

A. QT prolongation predominantly through a longer ST segment (Best answer)
Hypocalcemia generally prolongs the plateau-related ST portion of the QT interval.
B. PR prolongation with intermittent nonconducted P waves (Why this does not fit)
This suggests AV conduction disease; it is not the usual calcium-related ventricular recovery pattern.
C. Peaked T waves with progressive QRS widening (Why this does not fit)
This pattern raises concern for hyperkalemia, whereas the laboratory-confirmed disturbance here is hypocalcemia.
D. QT shortening predominantly through a shorter ST segment (Why this does not fit)
This is more typical of hypercalcemia, the opposite biochemical disturbance.

Takeaway: Use the ST portion of QT to connect calcium concentration with ventricular recovery.

Case sources: [13] [19]

Case 18

Show answer and explanations

A. Flecainide must have produced 40 ms of isolated repolarization prolongation (Why this does not fit)
QT includes depolarization. The entire measured change here lies within QRS.
B. JT has increased from 300 to 340 ms (Why this does not fit)
Subtract QRS from QT on each recording: both yield 300 ms. The proposed change counts the QRS increase as repolarization.
C. QT lengthening is primarily explained by heart-rate slowing (Why this does not fit)
The rate is unchanged. QRS widening, rather than rate-related adaptation, accounts numerically for the QT difference.
D. The QT increase can be explained by QRS widening without an increase in JT (Best answer)
JT is QT minus QRS. Both before and after treatment it is 300 ms, so these measurements do not show longer JT.

Takeaway: Subtract QRS from QT when asking whether a wider complex explains a longer QT.

Case sources: [4] [13]

Case 19

Show answer and explanations

A. CAST established improved survival whenever ventricular ectopy was suppressed (Why this does not fit)
CAST showed why successful suppression of this surrogate endpoint could coexist with increased mortality.
B. CAST evaluated symptom relief without measuring mortality (Why this does not fit)
CAST measured arrhythmic death or cardiac arrest and other clinical outcomes; it was not limited to symptom scores.
C. CAST found excess mortality with encainide or flecainide after MI (Best answer)
Suppressing a surrogate electrical finding did not establish clinical benefit and was associated with harm in this population.
D. CAST found equivalent survival with flecainide and placebo after MI (Why this does not fit)
The active-drug and placebo outcomes were not equivalent; excess deaths prompted discontinuation of encainide and flecainide.

Takeaway: A cleaner tracing is not proof of improved survival; prior MI changes antiarrhythmic selection.

Case sources: [4] [14]

Case 20

Show answer and explanations

A. Exclusive inhibition of AV nodal L-type calcium channels (Why this does not fit)
This describes the principal nodal action of verapamil or diltiazem rather than a class IB drug.
B. Sodium-channel inhibition with particular activity in depolarized tissue (Best answer)
Class IB behavior favors depolarized or inactivated channel states; lidocaine can shorten Purkinje action potential duration.
C. Direct opening of delayed rectifier potassium channels (Why this does not fit)
Lidocaine is not classified as a potassium-channel opener. Shorter AP duration does not establish that mechanism.
D. Selective blockade of SA-node HCN channels (Why this does not fit)
This describes ivabradine rather than lidocaine.

Takeaway: Do not infer potassium-channel opening solely from a shortened action potential.

Case sources: [1] [16] [18]

Case 21

Show answer and explanations

A. Thyroid and hepatic toxicity (Best answer)
Amiodarone labeling includes pulmonary, thyroid and hepatic effects. Its multiple channel actions do not eliminate extracardiac toxicity.
B. Drug-induced lupus with arthralgia (Why this does not fit)
This is particularly associated with procainamide; pulmonary, hepatic and thyroid toxicity characterize amiodarone monitoring.
C. Cinchonism with tinnitus and hearing disturbance (Why this does not fit)
Cinchonism is a characteristic quinidine toxicity rather than the organ-surveillance pattern asked about for amiodarone.
D. Antimuscarinic urinary retention and dry mouth (Why this does not fit)
These are characteristic disopyramide effects; they do not explain amiodarone-related pulmonary symptoms or its thyroid/hepatic surveillance.

Takeaway: Amiodarone requires organ-specific surveillance even when its arrhythmic effects appear controlled.

Case sources: [15] [18]

Case 22

Show answer and explanations

A. Enhanced renal clearance markedly shortens adenosine exposure (Why this does not fit)
Adenosine is chiefly removed through rapid cellular uptake and metabolism, rather than dependence on renal clearance. Caffeine antagonizes its receptors.
B. Caffeine prolongs adenosine action by blocking cellular uptake (Why this does not fit)
Dipyridamole potentiates adenosine through uptake inhibition; caffeine has an antagonistic receptor effect.
C. Caffeine increases adenosine A1 receptor sensitivity (Why this does not fit)
Greater receptor sensitivity would enhance nodal slowing. Methylxanthine receptor antagonism instead reduces the response.
D. Adenosine receptor antagonism reduces the response (Best answer)
Caffeine and other methylxanthines antagonize adenosine. Dose response also depends on delivery and the tachycardia mechanism.

Takeaway: An adenosine response depends on receptor exposure, delivery and whether the AV node is necessary to the circuit.

Case sources: [7] [9]

Case 23

Show answer and explanations

A. IV digoxin as first-line acute therapy (Why this does not fit)
Digoxin is contraindicated in preexcited AF because it may facilitate dangerous ventricular activation.
B. Monitored IV amiodarone infusion with specialist support (Why this does not fit)
The AF guideline lists amiodarone among contraindicated AV nodal blocking agents in preexcited AF.
C. Monitored procainamide with specialist support (Best answer)
For stable preexcited AF, procainamide or ibutilide can slow accessory-pathway conduction. Deterioration requires cardioversion.
D. IV diltiazem to selectively slow the AV node (Why this does not fit)
Nodal blockade in preexcited AF can favor dangerous accessory-pathway conduction.

Takeaway: Recognize the irregular, very rapid, variably wide pattern before selecting a nodal drug.

Case sources: [12]

Case 24

Show answer and explanations

A. Synchronized cardioversion with CPR after the shock (Why this does not fit)
Polymorphic VT cannot be reliably synchronized. In a pulseless patient, provide unsynchronized defibrillation and CPR.
B. Unsynchronized defibrillation and CPR per arrest protocol (Best answer)
Pulseless polymorphic VT is a shockable arrest rhythm. Shock should not be delayed for magnesium or laboratory results.
C. IV amiodarone before the first shock (Why this does not fit)
Antiarrhythmic administration does not precede the first indicated shock in a shockable arrest; amiodarone can also worsen a long-QT substrate.
D. Give magnesium and reassess for several minutes before shocking (Why this does not fit)
Magnesium can address recurrent long-QT torsades but is not a replacement for immediate defibrillation in arrest.

Takeaway: Treat sustained polymorphic VT with unsynchronized shock; treat long-QT triggers alongside resuscitation.

Case sources: [9]

Case 25

Show answer and explanations

A. Increase the rate with temporary pacing for acquired pause-dependent torsades (Best answer)
Preventing pauses can reduce recurrence in selected acquired long-QT cases. The strategy is not a universal treatment for inherited arrhythmias.
B. Slow the sinus rate further to lengthen diastolic recovery (Why this does not fit)
Additional bradycardia can worsen the pause-dependent substrate in this acquired setting.
C. Start sotalol to suppress further ventricular episodes (Why this does not fit)
Sotalol prolongs repolarization and can aggravate acquired long-QT torsades. Its beta-blocking effect can also worsen pauses.
D. Add IV amiodarone to prolong the ventricular refractory period (Why this does not fit)
Further QT prolongation can worsen recurrent torsades. This is distinct from treatment of polymorphic VT without a long QT.

Takeaway: Pause prevention is a selected acquired-torsades strategy, not a rule for all ventricular arrhythmias.

Case sources: [9]

Case 27

Show answer and explanations

A. Reduced IKr causes prolonged repolarization and EADs (Why this does not fit)
This is a long-QT mechanism; the RYR2 variant and stress-induced bidirectional VT point to abnormal calcium release.
B. Reduced IKs limits repolarization during exercise (Why this does not fit)
This explains KCNQ1-associated LQT1. The identified RYR2 variant affects SR calcium release instead.
C. Adrenergically provoked abnormal SR calcium release and triggered activity (Best answer)
The normal resting tracing with stress-induced bidirectional VT fits CPVT and calcium-related DAD mechanisms.
D. Reduced sodium current creates an RV outflow tract arrhythmic substrate (Why this does not fit)
This is associated with some Brugada phenotypes; the genotype and exercise-induced bidirectional VT favor CPVT.

Takeaway: A normal resting ECG can coexist with a dangerous stress-triggered calcium-handling disorder.

Case sources: [11] [1]

Case 28

Show answer and explanations

A. KCNH2-associated long-QT syndrome is the leading explanation (Why this does not fit)
LQT2 concerns prolonged repolarization; a fever-associated coved right-precordial pattern suggests a different electrical phenotype.
B. Evaluate for Brugada even if SCN5A testing is negative (Best answer)
A fever-associated type 1 pattern with a concerning history requires specialist assessment; the phenotype is not limited to patients with a detectable SCN5A variant.
C. The findings are adequately explained by benign early repolarization (Why this does not fit)
Unexplained syncope with a coved right-precordial pattern requires specialist assessment; benign reassurance would overlook this combination.
D. A negative SCN5A test would exclude the suspected syndrome (Why this does not fit)
Many people with a clinical Brugada phenotype have no detectable SCN5A variant; genetic testing cannot independently exclude it.

Takeaway: Interpret inherited electrical disease through phenotype and event history, with genetic testing as one component.

Case sources: [11] [17]

Sources

Ion Channels in the Heart (2015)
Ionic currents, automaticity, afterdepolarizations, conduction and reentry.
Function and Dysfunction of Human Sinoatrial Node (2015)
Human SAN regional heterogeneity and coupled membrane/calcium clocks. Detailed currents and receptor pathways are supported by source 1.
Calcium Signaling in Cardiomyocyte Function (2020)
Excitation-contraction coupling, RyR2, SERCA and phospholamban.
Flecainide acetate tablets prescribing information
Conduction effects, PR/QRS/QT and JT distinction, proarrhythmia.
Sotalol hydrochloride tablets prescribing information
Combined class II and III actions, QT risk, renal dosing and monitored initiation.
DIGOX digoxin tablets prescribing information
Na/K ATPase inhibition, inotropy, vagal effects, renal impairment and electrolyte-related toxicity.
Adenosine injection prescribing information
Transient AV nodal block, short duration, caffeine antagonism and dipyridamole potentiation.
CORLANOR ivabradine prescribing information
HCN/If inhibition, sinus-rate reduction and lack of direct negative inotropy.
2025 AHA Guidelines for CPR and ECC, Part 9, Adult Advanced Life Support (2025)
Pulse and stability decisions, synchronized versus unsynchronized shock, polymorphic VT, magnesium and bradycardia treatment.
UKKA Clinical Practice Guideline, Management of Hyperkalaemia in Adults, July 2026 update (2026-07)
ECG limitations, calcium stabilization, insulin/glucose, monitoring, dialysis and recommendation against routine IV bicarbonate.
HRS/EHRA/APHRS Expert Consensus Statement on the Diagnosis and Management of Patients with Inherited Primary Arrhythmia Syndromes (2013)
Inherited long-QT, Brugada and CPVT phenotypes, genotype and trigger distinctions, and risk-dependent management. Current preferred beta blockers are paired with the 2022 ESC source.
2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation (2023)
AF diagnosis, rate/rhythm decisions, cardioversion anticoagulation, mitral stenosis and preexcited AF contraindications.
AHA/ACCF/HRS ECG standardization, Part IV, ST segment, T and U waves, and QT interval (2009)
QT measurement and correction, QRS/JT distinction, repolarization and U-wave interpretation.
Mortality and morbidity in patients receiving encainide, flecainide, or placebo. The Cardiac Arrhythmia Suppression Trial (1991-03-21)
Primary randomized trial demonstrating excess mortality after MI despite ectopy suppression.
Amiodarone hydrochloride tablets prescribing information
Multiple channel actions and pulmonary, hepatic and thyroid adverse effects and monitoring.
Lidocaine hydrochloride injection prescribing information
Purkinje automaticity and action potential duration effects of lidocaine.
2022 ESC Ventricular Arrhythmias and Sudden Cardiac Death Guidelines, Essential Messages (2022)
Official guideline key messages for inherited electrical disease.
Practical compendium of antiarrhythmic drugs: a clinical consensus statement of the European Heart Rhythm Association of the European Society of Cardiology (2025)
Official EHRA consensus: sodium-channel blocker kinetics, lidocaine tissue preference, class IA adverse effects and drug mechanisms.
Prolonged ST segment and T-wave alternans with torsade de pointes secondary to hypocalcemia due to hypoparathyroidism: A case report (2022-02-11)
Primary clinical observation of ST/QT prolongation during hypocalcemia, paired with ECG standardization guidance. Not a treatment protocol or an estimate of torsades incidence.

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