ECG Electrophysiology Basics 3 — Questions and Answers
Question 1: Which electrolyte abnormality classically produces peaked T waves and widened QRS on ECG?
- Hypokalemia
- Hypercalcemia
- Hyperkalemia (Correct answer)
- Hypomagnesemia
Correct answer: Hyperkalemia
Hyperkalemia reduces the resting membrane potential, slows conduction (widened QRS), and causes tall peaked T waves due to accelerated repolarization.
Question 2: What does the term 'overdrive suppression' refer to in cardiac electrophysiology?
- Defibrillation terminating a dysrhythmia
- A faster pacemaker suppressing the automaticity of slower ones (Correct answer)
- Vagal stimulation slowing the SA node
- Beta-blockers reducing heart rate
Correct answer: A faster pacemaker suppressing the automaticity of slower ones
Overdrive suppression occurs when a faster pacemaker drives slower subsidiary pacemakers, accumulating intracellular Na+ and hyperpolarizing the slower cells via Na+/K+ ATPase.
Question 3: Re-entry arrhythmias require which of the following conditions?
- Two pathways with different conduction velocities and refractoriness (Correct answer)
- Enhanced automaticity in the SA node
- Complete block in both bundle branches
- Absence of the AV node
Correct answer: Two pathways with different conduction velocities and refractoriness
Re-entry requires a circuit with unidirectional block in one pathway and slow conduction in the other, allowing the impulse to re-excite previously depolarized tissue.
Question 4: Which phase of the cardiac action potential corresponds to the plateau and is maintained primarily by L-type calcium channels?
- Phase 0
- Phase 1
- Phase 2 (Correct answer)
- Phase 3
Correct answer: Phase 2
Phase 2 (the plateau) is maintained by a balance of slow inward Ca2+ current through L-type channels and outward K+ current.
Question 5: What is the approximate conduction velocity through the AV node?
- 0.05 m/s (Correct answer)
- 4 m/s
- 1 m/s
- 2.5 m/s
Correct answer: 0.05 m/s
The AV node conducts very slowly at approximately 0.05 m/s, creating the necessary delay between atrial and ventricular contraction.
Question 6: A patient has a QTc of 520 ms. Which condition does this most likely predispose them to?
- Atrial flutter
- Torsades de Pointes (Correct answer)
- Wolff-Parkinson-White syndrome
- First-degree AV block
Correct answer: Torsades de Pointes
Prolonged QTc (>500 ms) indicates delayed ventricular repolarization, creating a vulnerable window that predisposes to the polymorphic VT known as Torsades de Pointes.
Question 7: Which autonomic input slows SA node firing by increasing K+ conductance and hyperpolarizing the cell?
- Sympathetic stimulation via beta-1 receptors
- Parasympathetic stimulation via muscarinic (M2) receptors (Correct answer)
- Sympathetic stimulation via alpha-1 receptors
- Parasympathetic stimulation via nicotinic receptors
Correct answer: Parasympathetic stimulation via muscarinic (M2) receptors
Vagal (parasympathetic) stimulation activates M2 muscarinic receptors, opening IKACh channels and causing hyperpolarization that slows SA node automaticity.
ECG Electrophysiology Basics 3
Which electrolyte abnormality classically produces peaked T waves and widened QRS on ECG?