EKG RHYTHM INTERPRETATION ASSESSMENT (PDF) | 2026
EXAM QUESTIONS AND ANSWERS + RATIONALES | STUDY
GUIDE | 100% CORRECT
1. On standard ECG paper, what is the duration in seconds of one small box (1 mm)?
A) 0.20 seconds
B) 0.04 seconds
C) 0.12 seconds
D) 0.08 seconds
Correct Answer: B) 0.04 seconds
Rationale: On standard ECG paper running at 25 mm/s, each small 1 mm box represents 0.04
seconds . The larger boxes, composed of 5 small boxes, represent 0.20 seconds . This
measurement is fundamental for calculating heart rate and interval durations.
2. What is the normal duration range for the PR interval?
A) 0.08 – 0.11 seconds
B) 0.12 – 0.20 seconds
C) 0.20 – 0.30 seconds
D) 0.30 – 0.40 seconds
Correct Answer: B) 0.12 – 0.20 seconds
Rationale: The PR interval represents the time from the onset of atrial depolarization to the onset
of ventricular depolarization . A normal PR interval falls between 0.12 and 0.20 seconds . A
prolonged interval (>0.20 seconds) suggests an AV conduction delay.
3. Which waveform on the ECG represents ventricular repolarization?
A) P wave
B) QRS complex
C) ST segment
D) T wave
Correct Answer: D) T wave
Rationale: The T wave represents the repolarization of the ventricles, which is the recovery
phase of the cardiac cycle . The P wave represents atrial depolarization, while the QRS complex
represents ventricular depolarization.
4. Which lead is considered a standard limb lead that views the inferior surface of the heart?
A) Lead I
B) Lead aVR
C) Lead II
D) Lead V1
, Correct Answer: C) Lead II
Rationale: Lead II, along with Leads III and aVF, are the inferior leads that view the inferior
surface of the heart . Lead I is a lateral lead, and aVR is an augmented limb lead with a rightward
orientation. V1 is a precordial lead viewing the septal area.
5. The intrinsic firing rate of the sinoatrial (SA) node is:
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) 60-100 beats per minute
Rationale: The SA node is the primary pacemaker of the heart with an intrinsic firing rate of 60-
100 beats per minute . The AV node has a backup rate of 40-60 bpm, and the Purkinje fibers
have an escape rate of 20-40 bpm .
6. How many large boxes on an ECG strip represent 1 second at standard paper speed?
A) 2 large boxes
B) 3 large boxes
C) 4 large boxes
D) 5 large boxes
Correct Answer: D) 5 large boxes
Rationale: At the standard paper speed of 25 mm/s, each large box (5 mm) represents 0.20
seconds . Therefore, 5 large boxes equal 1 second, and 30 large boxes (or a 6-second strip) are
commonly used for heart rate calculations.
7. The nurse is analyzing a rhythm strip and notes an absent P wave with an irregularly irregular
ventricular response. Which rhythm should the nurse identify?
A) Atrial flutter
B) Atrial fibrillation
C) Sinus arrhythmia
D) Multifocal atrial tachycardia
Correct Answer: B) Atrial fibrillation
Rationale: Atrial fibrillation is characterized by the absence of organized P waves, an irregularly
irregular ventricular response, and a loss of the isoelectric line . The atria quiver instead of
contracting in a coordinated manner, leading to this chaotic rhythm.
8. What is the characteristic ECG finding in a patient with Wolff-Parkinson-White (WPW)
syndrome?
A) A prolonged PR interval and wide QRS
B) A short PR interval and a delta wave
, C) A prolonged QT interval
D) A shortened QT interval
Correct Answer: B) A short PR interval and a delta wave
Rationale: WPW syndrome is caused by an accessory pathway (bundle of Kent) that bypasses
the AV node, resulting in early ventricular activation . This produces a short PR interval and a
delta wave, a slurred upstroke on the QRS complex.
9. What is the hallmark ECG feature of a first-degree atrioventricular (AV) block?
A) Dropped QRS complexes
B) A PR interval greater than 0.20 seconds
C) Wide QRS complexes
D) A prolonged QT interval
Correct Answer: B) A PR interval greater than 0.20 seconds
Rationale: First-degree AV block is defined by a consistently prolonged PR interval (>0.20
seconds) . Unlike second- or third-degree blocks, all P waves are conducted to the ventricles;
there are no dropped beats.
10. Which step is the first priority when systematically interpreting an ECG rhythm?
A) Measure the QT interval
B) Determine the heart rate
C) Assess the rhythm for regularity
D) Identify the presence and morphology of P waves
Correct Answer: C) Assess the rhythm for regularity
Rationale: Systematic ECG interpretation requires a specific sequence to avoid errors . The first
step is to assess the rhythm for regularity . Determining the rate, identifying P waves, and
measuring intervals are subsequent steps.
11. A patient presents with a regular narrow-complex tachycardia at a rate of 160 bpm. The ECG
shows P waves that are hidden in the T waves of the preceding beats. This rhythm is most likely:
A) Sinus tachycardia
B) Atrial fibrillation
C) Paroxysmal supraventricular tachycardia (SVT)
D) Atrial flutter
Correct Answer: C) Paroxysmal supraventricular tachycardia (SVT)
Rationale: This description is classic for a reentrant SVT, often AV nodal reentrant tachycardia
(AVNRT) . It presents with a sudden onset/offset, a rapid regular rate around 150 bpm, narrow
QRS, and P waves that are difficult to see or appear retrograde.
EXAM QUESTIONS AND ANSWERS + RATIONALES | STUDY
GUIDE | 100% CORRECT
1. On standard ECG paper, what is the duration in seconds of one small box (1 mm)?
A) 0.20 seconds
B) 0.04 seconds
C) 0.12 seconds
D) 0.08 seconds
Correct Answer: B) 0.04 seconds
Rationale: On standard ECG paper running at 25 mm/s, each small 1 mm box represents 0.04
seconds . The larger boxes, composed of 5 small boxes, represent 0.20 seconds . This
measurement is fundamental for calculating heart rate and interval durations.
2. What is the normal duration range for the PR interval?
A) 0.08 – 0.11 seconds
B) 0.12 – 0.20 seconds
C) 0.20 – 0.30 seconds
D) 0.30 – 0.40 seconds
Correct Answer: B) 0.12 – 0.20 seconds
Rationale: The PR interval represents the time from the onset of atrial depolarization to the onset
of ventricular depolarization . A normal PR interval falls between 0.12 and 0.20 seconds . A
prolonged interval (>0.20 seconds) suggests an AV conduction delay.
3. Which waveform on the ECG represents ventricular repolarization?
A) P wave
B) QRS complex
C) ST segment
D) T wave
Correct Answer: D) T wave
Rationale: The T wave represents the repolarization of the ventricles, which is the recovery
phase of the cardiac cycle . The P wave represents atrial depolarization, while the QRS complex
represents ventricular depolarization.
4. Which lead is considered a standard limb lead that views the inferior surface of the heart?
A) Lead I
B) Lead aVR
C) Lead II
D) Lead V1
, Correct Answer: C) Lead II
Rationale: Lead II, along with Leads III and aVF, are the inferior leads that view the inferior
surface of the heart . Lead I is a lateral lead, and aVR is an augmented limb lead with a rightward
orientation. V1 is a precordial lead viewing the septal area.
5. The intrinsic firing rate of the sinoatrial (SA) node is:
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) 60-100 beats per minute
Rationale: The SA node is the primary pacemaker of the heart with an intrinsic firing rate of 60-
100 beats per minute . The AV node has a backup rate of 40-60 bpm, and the Purkinje fibers
have an escape rate of 20-40 bpm .
6. How many large boxes on an ECG strip represent 1 second at standard paper speed?
A) 2 large boxes
B) 3 large boxes
C) 4 large boxes
D) 5 large boxes
Correct Answer: D) 5 large boxes
Rationale: At the standard paper speed of 25 mm/s, each large box (5 mm) represents 0.20
seconds . Therefore, 5 large boxes equal 1 second, and 30 large boxes (or a 6-second strip) are
commonly used for heart rate calculations.
7. The nurse is analyzing a rhythm strip and notes an absent P wave with an irregularly irregular
ventricular response. Which rhythm should the nurse identify?
A) Atrial flutter
B) Atrial fibrillation
C) Sinus arrhythmia
D) Multifocal atrial tachycardia
Correct Answer: B) Atrial fibrillation
Rationale: Atrial fibrillation is characterized by the absence of organized P waves, an irregularly
irregular ventricular response, and a loss of the isoelectric line . The atria quiver instead of
contracting in a coordinated manner, leading to this chaotic rhythm.
8. What is the characteristic ECG finding in a patient with Wolff-Parkinson-White (WPW)
syndrome?
A) A prolonged PR interval and wide QRS
B) A short PR interval and a delta wave
, C) A prolonged QT interval
D) A shortened QT interval
Correct Answer: B) A short PR interval and a delta wave
Rationale: WPW syndrome is caused by an accessory pathway (bundle of Kent) that bypasses
the AV node, resulting in early ventricular activation . This produces a short PR interval and a
delta wave, a slurred upstroke on the QRS complex.
9. What is the hallmark ECG feature of a first-degree atrioventricular (AV) block?
A) Dropped QRS complexes
B) A PR interval greater than 0.20 seconds
C) Wide QRS complexes
D) A prolonged QT interval
Correct Answer: B) A PR interval greater than 0.20 seconds
Rationale: First-degree AV block is defined by a consistently prolonged PR interval (>0.20
seconds) . Unlike second- or third-degree blocks, all P waves are conducted to the ventricles;
there are no dropped beats.
10. Which step is the first priority when systematically interpreting an ECG rhythm?
A) Measure the QT interval
B) Determine the heart rate
C) Assess the rhythm for regularity
D) Identify the presence and morphology of P waves
Correct Answer: C) Assess the rhythm for regularity
Rationale: Systematic ECG interpretation requires a specific sequence to avoid errors . The first
step is to assess the rhythm for regularity . Determining the rate, identifying P waves, and
measuring intervals are subsequent steps.
11. A patient presents with a regular narrow-complex tachycardia at a rate of 160 bpm. The ECG
shows P waves that are hidden in the T waves of the preceding beats. This rhythm is most likely:
A) Sinus tachycardia
B) Atrial fibrillation
C) Paroxysmal supraventricular tachycardia (SVT)
D) Atrial flutter
Correct Answer: C) Paroxysmal supraventricular tachycardia (SVT)
Rationale: This description is classic for a reentrant SVT, often AV nodal reentrant tachycardia
(AVNRT) . It presents with a sudden onset/offset, a rapid regular rate around 150 bpm, narrow
QRS, and P waves that are difficult to see or appear retrograde.
