CRAT Sinus Rhythm Interpretation 2 — Questions and Answers

Question 1: Normal sinus rhythm requires which set of criteria?

Regular P waves upright in II and inverted in aVR, constant PR 120-200 ms, rate 60-100 bpm, narrow QRS (Correct answer)
P waves inverted in II, rate 60-100 bpm
PR interval greater than 200 ms, rate 60-100 bpm
Rate 60-100 bpm with no P waves

Correct answer: Regular P waves upright in II and inverted in aVR, constant PR 120-200 ms, rate 60-100 bpm, narrow QRS

Normal sinus rhythm requires an SA node origin (upright P in II, inverted in aVR), normal PR interval (120-200 ms), heart rate 60-100 bpm, and narrow QRS complexes.

The criteria for normal sinus rhythm: (1) P wave upright in leads I and II and inverted in aVR — confirming SA node origin, (2) constant PR interval 120-200 ms, (3) heart rate 60-100 bpm, (4) narrow QRS < 120 ms (unless bundle branch block is present), and (5) regular R-R intervals (slight variation with respiration — sinus arrhythmia — is acceptable).

Question 2: Sinus bradycardia is defined as sinus rhythm at a rate below which threshold?

60 bpm (Correct answer)
50 bpm
40 bpm
100 bpm

Correct answer: 60 bpm

Sinus bradycardia is defined as a normal sinus rhythm (SA node origin) at a rate less than 60 bpm. All other criteria for sinus rhythm remain intact.

Sinus bradycardia has all the features of normal sinus rhythm (upright P in II, constant PR interval, narrow QRS) but at a rate below 60 bpm. It may be physiological (athletes, sleep, vagal tone) or pathological (inferior MI, hypothyroidism, sick sinus syndrome, drug effect). Treatment is only needed for symptomatic bradycardia (syncope, hemodynamic compromise).

Question 3: Sinus arrhythmia is best described as which pattern?

Irregular R-R intervals that vary with respiration, with all other sinus rhythm criteria intact (Correct answer)
Irregular rhythm due to multiple P wave morphologies
Irregular rhythm with absent P waves
Regular rhythm with variable PR intervals

Correct answer: Irregular R-R intervals that vary with respiration, with all other sinus rhythm criteria intact

Sinus arrhythmia is a normal variant where heart rate increases with inspiration and decreases with expiration, producing slight R-R interval variation while maintaining normal sinus P waves and PR intervals.

Respiratory sinus arrhythmia results from vagal modulation: inspiration decreases vagal tone (increases HR), expiration increases vagal tone (decreases HR). All P waves are upright in II with constant morphology, the PR interval is constant, and QRS is narrow. Rate variation exceeds 10% or 0.12 seconds across the respiratory cycle. It is a normal finding, most prominent in young patients and during slow, deep breathing.

Question 4: A sinus pause is defined as an absence of the expected sinus beat lasting at least how long?

Two or more seconds (or twice the preceding R-R interval) (Correct answer)
0.5 seconds
500 ms
One cardiac cycle only

Correct answer: Two or more seconds (or twice the preceding R-R interval)

A sinus pause is defined as an unexpected absence of a P wave lasting at least 2 seconds (or more than twice the preceding R-R interval), indicating a failure of the SA node to generate an impulse.

Sinus pause (sinus arrest) occurs when the SA node fails to initiate an impulse, creating a gap in the rhythm. The pause duration is not a multiple of the basic P-P interval (distinguishing it from sinoatrial exit block where the pause IS a multiple). Pauses > 3 seconds are clinically significant. If the SA node fails to restart, an escape rhythm should emerge from the AV junction or ventricles.

Question 5: A patient's rhythm strip shows a regular rhythm at 55 bpm with upright P waves in lead II and a PR interval of 160 ms. How should this rhythm be classified?

Sinus bradycardia (Correct answer)
Junctional rhythm
Normal sinus rhythm
First-degree AV block with sinus bradycardia

Correct answer: Sinus bradycardia

Upright P waves in II, constant normal PR interval (160 ms is within 120-200 ms), and rate below 60 bpm with regular rhythm meets criteria for sinus bradycardia specifically.

This rhythm has: upright P in II (sinus origin confirmed), PR interval 160 ms (within normal 120-200 ms — not first-degree AV block), regular rhythm, and rate of 55 bpm (below the 60 bpm sinus threshold). Therefore the correct classification is sinus bradycardia. The PR interval of 160 ms is normal — first-degree AV block requires PR > 200 ms.

Question 6: Sinus tachycardia is most appropriately managed by which approach?

Identifying and treating the underlying cause (fever, pain, hypovolemia, anxiety, hyperthyroidism) (Correct answer)
Immediate cardioversion
IV beta-blockers as first-line treatment always
Adenosine administration

Correct answer: Identifying and treating the underlying cause (fever, pain, hypovolemia, anxiety, hyperthyroidism)

Sinus tachycardia is almost always a secondary, compensatory response — the appropriate treatment is to identify and address the underlying physiological cause, not to suppress the tachycardia directly.

Sinus tachycardia is driven by sympathetic stimulation or reduced parasympathetic tone, usually in response to: hypovolemia, pain, fever, anemia, hypoxia, anxiety, hyperthyroidism, medication effects (beta-agonists, stimulants), or heart failure. Treating the underlying cause resolves the tachycardia. Rate-slowing medications are rarely appropriate for sinus tachycardia and may be harmful by removing a compensatory mechanism.

CRAT Sinus Rhythm Interpretation 2

Normal sinus rhythm requires which set of criteria?