Electrophysiology Basics Flashcards

What is the conduction velocity of the His-Purkinje system, and why is it important?

Answer: 4 m/s; enables near-simultaneous ventricular depolarization for effective contraction

The His-Purkinje system conducts at ~4 m/s, the fastest in the heart, ensuring both ventricles depolarize nearly simultaneously for coordinated ejection.

A patient's ECG shows a QRS duration of 160 ms with a left bundle branch block pattern. What does this indicate electrophysiologically?

Answer: Conduction fails in the left bundle branch; the left ventricle depolarizes slowly via cell-to-cell spread from the right

LBBB means the left bundle is blocked; the RV depolarizes normally then the LV depolarizes slowly via myocardial cell-to-cell conduction, producing a wide, bizarre QRS.

Which property describes a cell's ability to respond to a stimulus and generate an action potential?

Answer: Excitability (bathmotropy)

Excitability (bathmotropy) is the ability of cardiac cells to respond to an electrical stimulus and generate an action potential when threshold is reached.

On a standard 12-lead ECG recorded at 25 mm/s, each small box (1 mm) represents how much time?

Answer: 0.02 seconds (20 ms)

At standard speed of 25 mm/s, each 1 mm small box = 0.04 s/box ÷ 2 = 0.04 s per small box; actually each small box = 1mm / 25mm/s = 0.04 s.

Digoxin toxicity can cause which of the following arrhythmias due to its electrophysiological effects?

Answer: Accelerated junctional rhythm with AV block (PAT with block)

Digoxin toxicity classically causes PAT (paroxysmal atrial tachycardia) with AV block by increasing automaticity (via Ca2+ overload/DADs) while simultaneously slowing AV conduction.

The 'vulnerable period' of the cardiac cycle, when an R-on-T phenomenon can trigger ventricular fibrillation, corresponds to which ECG feature?

Answer: The ascending limb of the T wave (relative refractory period)

The ascending limb of the T wave represents the relative refractory period when cells have partially repolarized — a stimulus here can cause heterogeneous conduction and VF.

Which of the following best describes the electrophysiological difference between SA nodal cells and ventricular myocytes?

Answer: SA nodal cells lack a stable resting potential and spontaneously depolarize; ventricular cells have a stable -90 mV resting potential

SA nodal cells have an unstable 'maximum diastolic potential' of about -60 mV and rely on Phase 4 spontaneous depolarization, while ventricular myocytes maintain a stable -90 mV resting potential.