Overview
- Normal physiological phenomenon most commnonly seen in young, healthy individuals
- Normal variation in sinus rhythm that occurs during the inspiratory and expiratory phases of respiration
- Beat-to-beat variation in the P-P interval (time between successive P waves), producing an irregular ventricular rate
- P-P interval gradually lengthens and shortens in a cyclical fashion, usually corresponding to the phases of the respiratory cycle
- Differential diagnosis for an irregular ventricular rate: second-degree AV block Mobitz I (Wenckebach phenomenon), frequent premature atrial contractions, sinoatrial exit block type I
Rhythm Characteristics
- Regularity: Regularly irregular (>120 ms [>3 small boxes] variation in P-P interval)
- Rate: 60-100 BPM (adults; age-dependent in children)
- P wave: present with constant morphology (normal sinus P waves; no evidence of premature atrial contractions)
- Axis (frontal plane): 0° to +75° (leftward and inferior)
- Configuration: upright in I, II, V4-V6; inverted in aVR; upright, inverted, or biphasic (positive-negative) in V1-V2; variable in III and aVL (related to respiratory cycles)
- P wave:QRS ratio = 1:1
- PR interval: constant, normal (120-200 ms; no evidence of 2nd-degree AV block Mobitz I), unless underlying AV conduction defect or aberrancy
- QRS interval: normal (70-110 ms), unless underlying intraventricular conduction defect or aberrancy
- Grouping: none, but may be present
- Dropped beats: none, but may be present
Mechanism
- Heart rate varies due to reflex changes in vagal tone during the different stages of the respiratory cycle
- Inspiration increases the heart rate by decreasing vagal tone
- Expiration causes a restoration of vagal tone, producing a subsequent decrease in heart rate
- Incidence: decreases with age, presumably due to age-related decreases in carotid distensibility and baroreceptor reflex sensitivity
- Note: “non-respiratory” sinus arrhythmia (ie, sinus arrhythmia not related to the respiratory cycle) is less common, and typically occurs in elderly patients; it is more likely to be pathological (eg, due to heart disease or digoxin toxicity)
ECG Example
Patient: 36-year-old asymptomatic healthy female
Interpretation: Sinus bradycardia with sinus arrhythmia, otherwise normal
– Ventricular rate: 59 BPM
– PR interval: 182 ms
– QRS duration: 84 ms
– QT/QTc interval: 406/401 ms
– P-R-T axes: 51 7 43
Key Points from Example ECG
- Regularly irregular rhythm at 59 BPM
- Variation in the P-P interval
- Constant P wave morphology and normal P wave axis (upright in I and II, inverted in aVR)
- Narrow QRS complexes (<110 ms wide)
- Each P wave is followed by a QRS complex (ie, P wave:QRS ratio = 1:1)
- Constant PR interval (no evidence of AV block)
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