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Sinus arrhythmia is essentially a variant of normal sinus rhythm producing an irregularly irregular rhythm. Besides the altered rhythm, the rest of the EKG will be consistent with a normal sinus rhythm. Most importantly, this includes evaluation of the monomorphic P waves in leads II and V1 that are consistent with a sinus origin of the atrial activity (upright in II and biphasic or inverted in V1). The variations in rhythm usually correspond with the respiratory cycle. This is most commonly seen in young, healthy people due to reflex changes in vagal tone. This tends to disappear with age as the carotid baroreceptors have decreased sensitivity.
There is a 'non-respiratory' variant that is typically seen in older patients and is indicative of pathology. This variant does not correlate with the respiratory cycle and is commonly seen with either degenerative changes of the sinus node or digoxin toxicity.
The main differentials are Wenckebach and frequent PAC's. Sinus arrhythmia should not have a variable PR interval. Patients with frequent PAC's usually have a wide complex QRS as the ventricles are usually still partially in their refractory state.
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Example
Sinus node reentrant tachycardia is a very rare rhythm. It is usually paroxysmal, with an abrupt increase in heart rate (120-180bpm) and abruptly returns to the baseline. However, it is noted as the P wave morphology is consistent as this is from the same pathway as a normal sinus rhythm. Occasionally, it requires vagal maneuvers to terminate.
Example
Example
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