ECG Electrophysiology Basics 5 — Questions and Answers
Question 1: What is the conduction velocity of the His-Purkinje system, and why is it important?
- 0.05 m/s; ensures slow ventricular filling
- 4 m/s; enables near-simultaneous ventricular depolarization for effective contraction (Correct answer)
- 1 m/s; matches atrial conduction speed
- 0.5 m/s; prevents re-entry circuits
Correct 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.
Question 2: A patient's ECG shows a QRS duration of 160 ms with a left bundle branch block pattern. What does this indicate electrophysiologically?
- Conduction fails in the left bundle branch; the left ventricle depolarizes slowly via cell-to-cell spread from the right (Correct answer)
- The SA node is firing too rapidly for the AV node to keep up
- Accessory pathway pre-excitation is present
- The PR interval is prolonged due to AV nodal disease
Correct 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.
Question 3: Which property describes a cell's ability to respond to a stimulus and generate an action potential?
- Automaticity
- Conductivity
- Excitability (bathmotropy) (Correct answer)
- Contractility
Correct 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.
Question 4: On a standard 12-lead ECG recorded at 25 mm/s, each small box (1 mm) represents how much time?
- 0.02 seconds (20 ms) (Correct answer)
- 0.04 seconds (40 ms)
- 0.10 seconds (100 ms)
- 0.20 seconds (200 ms)
Correct 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.
Question 5: Digoxin toxicity can cause which of the following arrhythmias due to its electrophysiological effects?
- Accelerated junctional rhythm with AV block (PAT with block) (Correct answer)
- Prolonged QT and Torsades de Pointes
- Shortened PR interval with delta waves
- Right bundle branch block
Correct 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.
Question 6: The 'vulnerable period' of the cardiac cycle, when an R-on-T phenomenon can trigger ventricular fibrillation, corresponds to which ECG feature?
- The PR segment
- The ascending limb of the T wave (relative refractory period) (Correct answer)
- The ST segment
- The QRS complex
Correct 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.
Question 7: Which of the following best describes the electrophysiological difference between SA nodal cells and ventricular myocytes?
- SA nodal cells have a stable resting potential of -90 mV; ventricular cells spontaneously depolarize
- SA nodal cells lack a stable resting potential and spontaneously depolarize; ventricular cells have a stable -90 mV resting potential (Correct answer)
- Both cell types have identical action potential morphology
- Ventricular cells use Ca2+ for Phase 0; SA nodal cells use Na+
Correct 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.
ECG Electrophysiology Basics 5
What is the conduction velocity of the His-Purkinje system, and why is it important?