CET Cardiac Pharmacology Effects on ECG 3 — Questions and Answers
Question 1: A patient taking magnesium sulfate for eclampsia is monitored via ECG. Which change would indicate magnesium toxicity?
- Shortened PR interval and narrow QRS
- Prolonged PR and QRS with bradycardia (Correct answer)
- ST elevation in V1-V4
- Delta waves in lateral leads
Correct answer: Prolonged PR and QRS with bradycardia
Magnesium toxicity suppresses conduction, leading to prolonged PR and QRS intervals, bradycardia, and potentially complete heart block.
Question 2: Which antiarrhythmic drug class is most associated with causing a lupus-like syndrome AND QRS widening on ECG?
- Class IB (lidocaine)
- Class IA (procainamide) (Correct answer)
- Class III (amiodarone)
- Class IV (verapamil)
Correct answer: Class IA (procainamide)
Class IA drug procainamide can cause drug-induced lupus syndrome and, due to sodium channel blockade, prolongs the QRS complex.
Question 3: During an ECG, a patient on quinidine shows a markedly prolonged QT and polymorphic ventricular tachycardia. This is best described as:
- Quinidine syncope / Torsades de Pointes (Correct answer)
- Accelerated idioventricular rhythm
- Digitalis-induced ventricular bigeminy
- Re-entrant ventricular tachycardia
Correct answer: Quinidine syncope / Torsades de Pointes
Quinidine-induced QT prolongation can trigger Torsades de Pointes, historically called 'quinidine syncope,' a polymorphic VT that is life-threatening.
Question 4: Verapamil given for rate control in atrial fibrillation most directly produces which measurable ECG change?
- Lengthening of the QRS complex
- Slowing of the ventricular rate (Correct answer)
- Inversion of P waves
- Widening of T waves
Correct answer: Slowing of the ventricular rate
Verapamil blocks calcium channels in the AV node, slowing conduction and thereby reducing the ventricular rate in atrial fibrillation.
Question 5: A patient on chronic amiodarone therapy develops thyroid dysfunction. Which ECG finding might accompany amiodarone-induced hypothyroidism?
- Prolonged QT with bradycardia (Correct answer)
- Short PR interval with delta waves
- ST elevation in precordial leads
- Peaked T waves with narrow QRS
Correct answer: Prolonged QT with bradycardia
Hypothyroidism slows the heart rate and prolongs the QT interval; amiodarone-induced hypothyroidism can exaggerate these effects.
Question 6: Lidocaine (Class IB) is preferred for ventricular arrhythmias post-MI because it:
- Prolongs the QT interval to suppress ectopy
- Selectively suppresses depolarization in ischemic tissue with minimal effect on normal tissue (Correct answer)
- Blocks calcium channels in the SA node
- Increases automaticity of Purkinje fibers
Correct answer: Selectively suppresses depolarization in ischemic tissue with minimal effect on normal tissue
Lidocaine preferentially binds to inactivated sodium channels in ischemic myocardium, suppressing ventricular ectopy with little effect on normal conduction.
Question 7: Which drug given for rate control in atrial flutter is most likely to show a sudden conversion to sinus rhythm on the ECG monitor?
- Metoprolol
- Ibutilide (Correct answer)
- Verapamil
- Amiodarone
Correct answer: Ibutilide
Ibutilide is a Class III antiarrhythmic used for chemical cardioversion of atrial flutter and fibrillation, and may produce sudden conversion to sinus rhythm.
CET Cardiac Pharmacology Effects on ECG 3
A patient taking magnesium sulfate for eclampsia is monitored via ECG.
Which change would indicate magnesium toxicity?