CRAT Arrhythmia Recognition, Anatomy, and Physiology 2 — Questions and Answers

Question 1: The cardiac action potential plateau phase (Phase 2) is maintained primarily by which ion current?

Slow inward calcium current (ICa-L) (Correct answer)
Rapid sodium influx
Rapid potassium efflux
Chloride influx

Correct answer: Slow inward calcium current (ICa-L)

Phase 2 of the ventricular action potential is sustained by the slow inward L-type calcium current, which balances potassium efflux and maintains the membrane potential near zero for an extended period.

The five phases of the ventricular action potential: Phase 0 (rapid depolarization — Na+), Phase 1 (early repolarization — K+ Ito), Phase 2 (plateau — Ca2+ in, K+ out balanced), Phase 3 (rapid repolarization — K+ out dominates), Phase 4 (resting — K+ leak). The long plateau creates the prolonged refractory period unique to cardiac muscle, preventing tetanic contraction. Calcium channel blockers (verapamil, diltiazem) act on Phase 2, reducing automaticity and AV nodal conduction.

Question 2: Which ECG finding is expected during the absolute refractory period of the cardiac cycle?

No cardiac stimulus can produce a new action potential — corresponds to the QRS complex through early T wave (Correct answer)
Any stimulus can produce fibrillation
The SA node is firing rapidly
P waves are produced by atrial repolarization

Correct answer: No cardiac stimulus can produce a new action potential — corresponds to the QRS complex through early T wave

During the absolute refractory period (QRS through mid-T wave), cardiac cells are completely depolarized and cannot respond to any stimulus regardless of its strength.

The absolute refractory period corresponds to ventricular depolarization and early repolarization (Phases 0-2 and early Phase 3 of the action potential). During this period, sodium channels are inactivated and cells cannot depolarize again, preventing summation and tetanic contraction. This is followed by the relative refractory period (late Phase 3, peak of T wave) where only a strong stimulus can cause depolarization — the dangerous vulnerable period.

Question 3: Which ion primarily responsible for Phase 3 (rapid repolarization) of the ventricular action potential?

Potassium efflux through IKr and IKs channels (Correct answer)
Sodium influx
Calcium influx
Chloride efflux

Correct answer: Potassium efflux through IKr and IKs channels

Phase 3 rapid repolarization is driven by potassium efflux through the delayed rectifier potassium channels (IKr and IKs), restoring the negative resting membrane potential.

Phase 3 occurs when outward potassium current (IKr and IKs channels) overwhelms inward calcium current, causing rapid repolarization from 0 mV back to -90 mV. IKr (rapid delayed rectifier) is the target of Class III antiarrhythmic drugs (sotalol, dofetilide) and many drugs that cause drug-induced long QT syndrome. Blockade of IKr prolongs Phase 3, extending the QT interval and increasing risk of early afterdepolarizations and torsades de pointes.

Question 4: Automaticity in pacemaker cells is generated by which mechanism?

Spontaneous slow diastolic depolarization (Phase 4) driven by the If funny current (Correct answer)
Rapid sodium channel opening during rest
Passive potassium influx
Calcium pump activity

Correct answer: Spontaneous slow diastolic depolarization (Phase 4) driven by the If funny current

Pacemaker cell automaticity results from Phase 4 spontaneous diastolic depolarization — a slow inward current (If, the funny current) that progressively depolarizes the cell toward threshold during diastole.

Unlike working myocardial cells (which have a stable Phase 4 resting potential), pacemaker cells (SA node, AV node, Purkinje fibers) have an unstable resting potential that slowly drifts toward threshold via If (hyperpolarization-activated cyclic nucleotide-gated channels). When threshold is reached, the cell depolarizes spontaneously. If channels open when the cell is hyperpolarized and are activated by cAMP, explaining why sympathetic stimulation (increases cAMP) increases heart rate — ivabradine blocks If channels to reduce heart rate.

Question 5: A reentrant arrhythmia requires which three conditions to be established and maintained?

A closed circuit pathway, unidirectional block in one limb, and slow conduction in the other limb allowing recovery (Correct answer)
Two SA nodes and an accessory pathway
Complete AV block and ventricular escape rhythm
Three P waves for each QRS complex

Correct answer: A closed circuit pathway, unidirectional block in one limb, and slow conduction in the other limb allowing recovery

Reentry requires an anatomical or functional circuit with two pathways, unidirectional block in one pathway (impulse cannot go forward but can conduct backward), and slow enough conduction in the alternate pathway that the blocked pathway recovers and can be re-excited.

Reentry is the most common mechanism for sustained tachyarrhythmias including AVNRT, AVRT (WPW), atrial flutter, and many forms of VT. The three requirements: (1) a closed loop — anatomical (accessory pathway) or functional; (2) unidirectional block — impulse fails to conduct antegrade in one limb but can conduct retrograde; (3) slow conduction in the unblocked limb — giving the blocked limb time to recover and be re-excited. Breaking any one condition terminates reentry (adenosine blocks the AV node, ending AVNRT/AVRT).

Question 6: The atrioventricular node (AV node) is located in which part of the heart?

Posteroinferior right atrium at the junction with the interventricular septum (Koch's triangle) (Correct answer)
High right atrium near the superior vena cava
Left ventricular free wall
Interventricular septum at the cardiac apex

Correct answer: Posteroinferior right atrium at the junction with the interventricular septum (Koch's triangle)

The AV node lies in the posteroinferior right atrium, within Koch's triangle — bounded by the tricuspid annulus, tendon of Todaro, and the coronary sinus ostium.

Koch's triangle is the key surgical/electrophysiological landmark for the AV node: its apex points anterosuperiorly toward the membranous septum (where the bundle of His begins), its base is the coronary sinus ostium, its anterior boundary is the tricuspid annulus, and its superior boundary is the tendon of Todaro. The compact AV node sits at the apex of this triangle. EP ablationists target the slow pathway (posteroinferior) for AVNRT ablation.

CRAT Arrhythmia Recognition, Anatomy, and Physiology 2

The cardiac action potential plateau phase (Phase 2) is maintained primarily by which ion current?