ECG Electrophysiology Basics 2 — Questions and Answers
Question 1: Which ion is primarily responsible for the rapid depolarization phase (Phase 0) of the ventricular action potential?
- Potassium (K+)
- Sodium (Na+) (Correct answer)
- Calcium (Ca2+)
- Chloride (Cl-)
Correct answer: Sodium (Na+)
Phase 0 rapid depolarization is caused by a fast inward sodium current through voltage-gated Na+ channels.
Question 2: The QT interval on an ECG represents which electrophysiological event?
- Atrial depolarization and repolarization
- Ventricular depolarization only
- Total ventricular electrical systole (depolarization + repolarization) (Correct answer)
- SA node firing time
Correct answer: Total ventricular electrical systole (depolarization + repolarization)
The QT interval encompasses the entire period of ventricular electrical activity from depolarization (QRS) through repolarization (T wave).
Question 3: What is the normal resting membrane potential of a cardiac myocyte?
- -40 mV
- -55 mV
- -70 mV
- -90 mV (Correct answer)
Correct answer: -90 mV
The resting membrane potential of a working cardiac myocyte is approximately -90 mV, maintained primarily by K+ conductance.
Question 4: Phase 4 spontaneous depolarization (automaticity) in SA nodal cells is primarily driven by which current?
- Inward funny current (If) (Correct answer)
- Outward K+ current (IK1)
- Inward Cl- current
- Outward Na+ current
Correct answer: Inward funny current (If)
The funny current (If), a mixed Na+/K+ inward current activated by hyperpolarization, drives slow diastolic depolarization in pacemaker cells.
Question 5: Which portion of the His-Purkinje system has the slowest intrinsic pacemaker rate?
- Bundle of His
- Right bundle branch
- Left bundle branch
- Purkinje fibers (Correct answer)
Correct answer: Purkinje fibers
Purkinje fibers have the slowest intrinsic rate of approximately 20-40 bpm, serving as a last-resort pacemaker.
Question 6: During the absolute refractory period of a cardiac cell, what is true?
- A supranormal stimulus can trigger an action potential
- No stimulus can trigger a new action potential (Correct answer)
- Only high-energy defibrillation can trigger depolarization
- Repolarization is complete
Correct answer: No stimulus can trigger a new action potential
During the absolute refractory period, Na+ channels are inactivated and no stimulus, regardless of strength, can generate a new action potential.
Question 7: The PR interval reflects conduction through which structures?
- SA node and atrial myocardium only
- AV node, bundle of His, and bundle branches
- Atrial myocardium, AV node, and His-Purkinje system (Correct answer)
- Bundle branches and Purkinje fibers only
Correct answer: Atrial myocardium, AV node, and His-Purkinje system
The PR interval represents the time from atrial depolarization onset through the AV node, His bundle, and bundle branches until ventricular depolarization begins.
ECG Electrophysiology Basics 2
Which ion is primarily responsible for the rapid depolarization phase (Phase 0) of the ventricular action potential?