DYSRHYTHMIASEXAM QUESTIONS AND
DETAILED SOLUTIONS JUST RELEASED
SECTION 1: CARDIAC ELECTROPHYSIOLOGY & ACTION
POTENTIALS
1. The resting membrane potential of a cardiac pacemaker cell (e.g., SA node) is
approximately:
• A) -90 mV
• B) -70 mV
• C) -60 mV (unstable, gradually depolarizing)
• D) +30 mV
Correct Answer: C
*Rationale: Pacemaker cells (SA node, AV node) have an unstable resting
membrane potential of approximately -60 mV, which gradually depolarizes due to
the "funny" current (If) and decreasing potassium conductance. This spontaneous
diastolic depolarization is responsible for automaticity. Ventricular myocytes have
a stable resting potential of about -90 mV.*
2. Phase 0 of the cardiac action potential in ventricular myocytes is characterized
by:
• A) Rapid influx of sodium ions (Na+) through fast voltage-gated Na+
channels
• B) Calcium influx through L-type Ca2+ channels
• C) Potassium efflux
, • D) Chloride influx
Correct Answer: A
*Rationale: Phase 0 is the rapid depolarization phase caused by the opening of
fast voltage-gated sodium channels, leading to a massive influx of Na+ into the
cell. This is responsible for the upstroke of the action potential and is the target of
Class I antiarrhythmics (sodium channel blockers).*
3. Which phase of the cardiac action potential corresponds to the plateau phase
and is primarily due to calcium influx?
• A) Phase 0
• B) Phase 1
• C) Phase 2
• D) Phase 3
Correct Answer: C
*Rationale: Phase 2 is the plateau phase, maintained by a balance between
calcium influx (through L-type Ca2+ channels) and potassium efflux. This phase
prolongs the action potential and corresponds to the ST segment on the ECG. It is
the target of Class IV antiarrhythmics (calcium channel blockers).*
4. Phase 3 of the cardiac action potential is characterized by:
• A) Rapid sodium influx
• B) Calcium influx
• C) Potassium efflux (repolarization)
• D) Sodium-potassium pump activity
Correct Answer: C
Rationale: Phase 3 is the repolarization phase caused by potassium efflux through
,various potassium channels (e.g., the rapid and delayed rectifier currents, IKr and
IKs). This restores the negative resting membrane potential and corresponds to the
T wave on the ECG. This is the target of Class III antiarrhythmics (potassium
channel blockers).
5. The normal rate of spontaneous depolarization (automaticity) of the sinoatrial
(SA) node is approximately:
• A) 20–40 beats per minute
• B) 40–60 beats per minute
• C) 60–100 beats per minute
• D) 100–150 beats per minute
Correct Answer: C
Rationale: The SA node is the primary pacemaker of the heart and has an intrinsic
firing rate of 60–100 beats per minute. The AV node has a rate of 40–60 bpm, and
the Purkinje fibers have a rate of 20–40 bpm. The SA node normally suppresses
these lower pacemakers through overdrive suppression.
6. The intrinsic rate of the atrioventricular (AV) node is:
• A) 60–100 bpm
• B) 40–60 bpm
• C) 20–40 bpm
• D) < 20 bpm
Correct Answer: B
*Rationale: The AV node has an intrinsic firing rate of 40–60 beats per minute. It
becomes the primary pacemaker if the SA node fails or if there is high-grade AV
block. Purkinje fibers have the slowest intrinsic rate of 20–40 bpm and act as a
backup pacemaker.*
, 7. The P wave on the electrocardiogram (ECG) represents:
• A) Ventricular depolarization
• B) Atrial depolarization
• C) Ventricular repolarization
• D) AV nodal conduction
Correct Answer: B
Rationale: The P wave represents atrial depolarization, which originates in the SA
node and spreads through the atria. The PR interval represents the time from the
onset of atrial depolarization to the onset of ventricular depolarization (including
AV nodal conduction).
8. The QRS complex on the ECG represents:
• A) Atrial depolarization
• B) Ventricular depolarization
• C) Ventricular repolarization
• D) AV nodal conduction
Correct Answer: B
Rationale: The QRS complex represents rapid ventricular depolarization. Its
duration is normally < 0.12 seconds (3 small squares). Prolonged QRS duration
indicates intraventricular conduction delay (e.g., bundle branch block or
ventricular origin of the rhythm).
9. The T wave on the ECG represents:
• A) Atrial repolarization
• B) Ventricular repolarization
DETAILED SOLUTIONS JUST RELEASED
SECTION 1: CARDIAC ELECTROPHYSIOLOGY & ACTION
POTENTIALS
1. The resting membrane potential of a cardiac pacemaker cell (e.g., SA node) is
approximately:
• A) -90 mV
• B) -70 mV
• C) -60 mV (unstable, gradually depolarizing)
• D) +30 mV
Correct Answer: C
*Rationale: Pacemaker cells (SA node, AV node) have an unstable resting
membrane potential of approximately -60 mV, which gradually depolarizes due to
the "funny" current (If) and decreasing potassium conductance. This spontaneous
diastolic depolarization is responsible for automaticity. Ventricular myocytes have
a stable resting potential of about -90 mV.*
2. Phase 0 of the cardiac action potential in ventricular myocytes is characterized
by:
• A) Rapid influx of sodium ions (Na+) through fast voltage-gated Na+
channels
• B) Calcium influx through L-type Ca2+ channels
• C) Potassium efflux
, • D) Chloride influx
Correct Answer: A
*Rationale: Phase 0 is the rapid depolarization phase caused by the opening of
fast voltage-gated sodium channels, leading to a massive influx of Na+ into the
cell. This is responsible for the upstroke of the action potential and is the target of
Class I antiarrhythmics (sodium channel blockers).*
3. Which phase of the cardiac action potential corresponds to the plateau phase
and is primarily due to calcium influx?
• A) Phase 0
• B) Phase 1
• C) Phase 2
• D) Phase 3
Correct Answer: C
*Rationale: Phase 2 is the plateau phase, maintained by a balance between
calcium influx (through L-type Ca2+ channels) and potassium efflux. This phase
prolongs the action potential and corresponds to the ST segment on the ECG. It is
the target of Class IV antiarrhythmics (calcium channel blockers).*
4. Phase 3 of the cardiac action potential is characterized by:
• A) Rapid sodium influx
• B) Calcium influx
• C) Potassium efflux (repolarization)
• D) Sodium-potassium pump activity
Correct Answer: C
Rationale: Phase 3 is the repolarization phase caused by potassium efflux through
,various potassium channels (e.g., the rapid and delayed rectifier currents, IKr and
IKs). This restores the negative resting membrane potential and corresponds to the
T wave on the ECG. This is the target of Class III antiarrhythmics (potassium
channel blockers).
5. The normal rate of spontaneous depolarization (automaticity) of the sinoatrial
(SA) node is approximately:
• A) 20–40 beats per minute
• B) 40–60 beats per minute
• C) 60–100 beats per minute
• D) 100–150 beats per minute
Correct Answer: C
Rationale: The SA node is the primary pacemaker of the heart and has an intrinsic
firing rate of 60–100 beats per minute. The AV node has a rate of 40–60 bpm, and
the Purkinje fibers have a rate of 20–40 bpm. The SA node normally suppresses
these lower pacemakers through overdrive suppression.
6. The intrinsic rate of the atrioventricular (AV) node is:
• A) 60–100 bpm
• B) 40–60 bpm
• C) 20–40 bpm
• D) < 20 bpm
Correct Answer: B
*Rationale: The AV node has an intrinsic firing rate of 40–60 beats per minute. It
becomes the primary pacemaker if the SA node fails or if there is high-grade AV
block. Purkinje fibers have the slowest intrinsic rate of 20–40 bpm and act as a
backup pacemaker.*
, 7. The P wave on the electrocardiogram (ECG) represents:
• A) Ventricular depolarization
• B) Atrial depolarization
• C) Ventricular repolarization
• D) AV nodal conduction
Correct Answer: B
Rationale: The P wave represents atrial depolarization, which originates in the SA
node and spreads through the atria. The PR interval represents the time from the
onset of atrial depolarization to the onset of ventricular depolarization (including
AV nodal conduction).
8. The QRS complex on the ECG represents:
• A) Atrial depolarization
• B) Ventricular depolarization
• C) Ventricular repolarization
• D) AV nodal conduction
Correct Answer: B
Rationale: The QRS complex represents rapid ventricular depolarization. Its
duration is normally < 0.12 seconds (3 small squares). Prolonged QRS duration
indicates intraventricular conduction delay (e.g., bundle branch block or
ventricular origin of the rhythm).
9. The T wave on the ECG represents:
• A) Atrial repolarization
• B) Ventricular repolarization
