, Question 1
A patient with acute decompensated heart failure has an irregularly irregular
narrow-complex tachycardia at 148 bpm. Which bedside maneuver would most
rapidly differentiate atrial fibrillation from multifocal atrial tachycardia?
A. Vagal stimulation with carotid sinus massage
B. Analysis of P-wave morphology and presence of multiple distinct
P-wave shapes
C. Administration of IV adenosine 6 mg rapid push
D. Measurement of QRS axis on a 12-lead ECG
Correct Answer: B - Analysis of P-wave morphology and
presence of multiple distinct P-wave shapes
RATIONALE
Multifocal atrial tachycardia is defined by at least three distinct
P-wave morphologies in the same lead, whereas atrial fibrillation lacks
organized P waves entirely. Vagal maneuvers and adenosine may slow
rates but do not reliably differentiate these rhythms. QRS axis is
irrelevant to atrial activity.
Question 2
Which ECG finding is most specific for identifying a premature ventricular
complex (PVC) rather than an aberrantly conducted premature atrial complex
(PAC)?
A. QRS duration > 120 ms
B. Absence of a preceding P wave and a full compensatory pause
C. Presence of a right bundle branch block pattern
D. Irregularity of the underlying rhythm
Correct Answer: B - Absence of a preceding P wave and a full
compensatory pause
Page 2
, RATIONALE
A PVC typically lacks a preceding P wave and is followed by a full
compensatory pause due to retrograde concealment into the AV node.
Aberrantly conducted PACs have a preceding premature P wave and
often a noncompensatory pause. QRS duration and bundle branch
block pattern can occur in both.
Question 3
A patient on a telemetry unit develops a sudden onset of regular
narrow-complex tachycardia at 186 bpm. The patient is alert and
hemodynamically stable. Which intervention is the most appropriate initial
action?
A. Administer IV adenosine 6 mg rapid IV push
B. Perform synchronized cardioversion at 50 J
C. Start an amiodarone infusion at 1 mg/min
D. Administer IV metoprolol 5 mg slow IV push
Correct Answer: A - Administer IV adenosine 6 mg rapid IV
push
RATIONALE
For stable, regular narrow-complex tachycardia (likely SVT), the 2026
ACLS guidelines recommend vagal maneuvers followed by adenosine
as first-line. Synchronized cardioversion is reserved for unstable
patients. Amiodarone and metoprolol are not first-line for acute
termination of SVT.
Question 4
Which characteristic best distinguishes a Mobitz type I (Wenckebach)
second-degree AV block from Mobitz type II?
A. Progressive PR interval lengthening before a dropped QRS in Mobitz I
B. Constant PR interval with intermittent dropped QRS in Mobitz I
C. Presence of a widened QRS in Mobitz I
Page 3
, D. Irregular atrial rate in Mobitz II
Correct Answer: A - Progressive PR interval lengthening before a
dropped QRS in Mobitz I
RATIONALE
Mobitz I is characterized by progressive PR prolongation until a QRS
is dropped, often due to AV nodal block. Mobitz II has a constant PR
interval with sudden dropped beats, typically infranodal and higher
risk. QRS width and atrial rate are not defining features.
Question 5
A patient with a permanent pacemaker set at 60 bpm has a rhythm strip
showing pacing spikes at 60 bpm but no subsequent QRS complexes. What is
the most likely cause?
A. Failure to capture
B. Failure to sense
C. Oversensing
D. Pacemaker-mediated tachycardia
Correct Answer: A - Failure to capture
RATIONALE
Failure to capture occurs when the pacemaker fires (spike) but does
not depolarize the myocardium, resulting in no QRS. Failure to sense
would show inappropriate spikes during intrinsic beats. Oversensing
inhibits pacing, and PMT is a rapid paced rhythm.
Question 6
Which electrolyte abnormality is most strongly associated with the
development of torsades de pointes in a patient receiving a QT-prolonging
antiarrhythmic?
A. Hyperkalemia
B. Hypomagnesemia
Page 4
A patient with acute decompensated heart failure has an irregularly irregular
narrow-complex tachycardia at 148 bpm. Which bedside maneuver would most
rapidly differentiate atrial fibrillation from multifocal atrial tachycardia?
A. Vagal stimulation with carotid sinus massage
B. Analysis of P-wave morphology and presence of multiple distinct
P-wave shapes
C. Administration of IV adenosine 6 mg rapid push
D. Measurement of QRS axis on a 12-lead ECG
Correct Answer: B - Analysis of P-wave morphology and
presence of multiple distinct P-wave shapes
RATIONALE
Multifocal atrial tachycardia is defined by at least three distinct
P-wave morphologies in the same lead, whereas atrial fibrillation lacks
organized P waves entirely. Vagal maneuvers and adenosine may slow
rates but do not reliably differentiate these rhythms. QRS axis is
irrelevant to atrial activity.
Question 2
Which ECG finding is most specific for identifying a premature ventricular
complex (PVC) rather than an aberrantly conducted premature atrial complex
(PAC)?
A. QRS duration > 120 ms
B. Absence of a preceding P wave and a full compensatory pause
C. Presence of a right bundle branch block pattern
D. Irregularity of the underlying rhythm
Correct Answer: B - Absence of a preceding P wave and a full
compensatory pause
Page 2
, RATIONALE
A PVC typically lacks a preceding P wave and is followed by a full
compensatory pause due to retrograde concealment into the AV node.
Aberrantly conducted PACs have a preceding premature P wave and
often a noncompensatory pause. QRS duration and bundle branch
block pattern can occur in both.
Question 3
A patient on a telemetry unit develops a sudden onset of regular
narrow-complex tachycardia at 186 bpm. The patient is alert and
hemodynamically stable. Which intervention is the most appropriate initial
action?
A. Administer IV adenosine 6 mg rapid IV push
B. Perform synchronized cardioversion at 50 J
C. Start an amiodarone infusion at 1 mg/min
D. Administer IV metoprolol 5 mg slow IV push
Correct Answer: A - Administer IV adenosine 6 mg rapid IV
push
RATIONALE
For stable, regular narrow-complex tachycardia (likely SVT), the 2026
ACLS guidelines recommend vagal maneuvers followed by adenosine
as first-line. Synchronized cardioversion is reserved for unstable
patients. Amiodarone and metoprolol are not first-line for acute
termination of SVT.
Question 4
Which characteristic best distinguishes a Mobitz type I (Wenckebach)
second-degree AV block from Mobitz type II?
A. Progressive PR interval lengthening before a dropped QRS in Mobitz I
B. Constant PR interval with intermittent dropped QRS in Mobitz I
C. Presence of a widened QRS in Mobitz I
Page 3
, D. Irregular atrial rate in Mobitz II
Correct Answer: A - Progressive PR interval lengthening before a
dropped QRS in Mobitz I
RATIONALE
Mobitz I is characterized by progressive PR prolongation until a QRS
is dropped, often due to AV nodal block. Mobitz II has a constant PR
interval with sudden dropped beats, typically infranodal and higher
risk. QRS width and atrial rate are not defining features.
Question 5
A patient with a permanent pacemaker set at 60 bpm has a rhythm strip
showing pacing spikes at 60 bpm but no subsequent QRS complexes. What is
the most likely cause?
A. Failure to capture
B. Failure to sense
C. Oversensing
D. Pacemaker-mediated tachycardia
Correct Answer: A - Failure to capture
RATIONALE
Failure to capture occurs when the pacemaker fires (spike) but does
not depolarize the myocardium, resulting in no QRS. Failure to sense
would show inappropriate spikes during intrinsic beats. Oversensing
inhibits pacing, and PMT is a rapid paced rhythm.
Question 6
Which electrolyte abnormality is most strongly associated with the
development of torsades de pointes in a patient receiving a QT-prolonging
antiarrhythmic?
A. Hyperkalemia
B. Hypomagnesemia
Page 4
