Sinus Arrhythmia

ECG Recognition:

• The rhythm is irregular. 
• The P wave and PR interval are normal
• The P to P interval varies by 0.16 sec.  

There are 4 Types/Variants of Sinus Arrhythmia:

• Phasic
• Non-phasic
• Ventriculophasic
• Wandering Atrial Pacemaker

Phasic Sinus Arrhythmia

The rate is dependent on the respiratory cycle which increases with respiration and decreases with expiration. The pacemaker site in the sinoatrial node (SAN) shifts with respiration. As the pacemaker site shifts higher the SAN, the heart rate and P wave amplitude/size in II, III and aVF increase. As the pacemaker shifts lower the SAN, the P wave amplitude and heart rate decrease. The heart rate changes gradually and rhythmically, thus differentiating it from non-phasic sinus arrhythmia. 

Phasic Sinus Arrhythmia. The rhythm is irregular. The P wave is prominent at the beginning but there is a decrease in amplitude or size as the rate decreases. 

Non-phasic Sinus Arrhythmia

For non-phasic (Non-respiratory) sinus arrhythmia , the P wave and PR interval are normal but the PP intervals vary at random and independent of any physiological function.

Nonphasic Sinus Arrhythmia. The rhythm is irregular with upright P waves and normal PR interval. At a higher rate, the P wave is more prominent compared at slower rate.

Ventriculophasic Sinus Arrhythmia

The P to P interval encompassing a QRS complex are shorter than the intervals without an intervening QRS complex. Ventriculophasic sinus arrhythmia is noted in the presence of AV block.

Ventriculophasic sinus arrhythmia. The rhythm is still sinus because there are upright P waves seen in lead II. The atrial rate about 88 bpm with second degree AV block type II (Mobitz II) manifesting as 2:1 and 3:2 AV conduction (AV block is discussed later). The P to P interval encompassing a QRS is shorter compared to that without an intervening QRS complex (0.68 s vs 0.76 s). 

Wandering Atrial Pacemaker (WAP)

WAP is a variant of sinus arrhythmia. There is passive transfer of dominant pacemaker focus from the sinus node to latent pacemakers. The change (in P wave shape) occurs gradually. There is only one pacemaker that is in control. 

ECG Recognition:

The change in P wave contour is gradual and after several cycles the pacemaker shifts back to the sinus node. Do not confuse WAP with multifocal atrial rhythm (MAR).

Wandering Atrial Pacemaker. These 2 strips are 10 seconds long lead II strips that are seconds apart that were intentionally printed to illustrate gradual change of P wave contour/morphology.

Wandering Atrial Pacemaker . Long lead II strips of one patient taken at different time intervals to highlight the gradual disappearance of distinct P wave (upper strip) and gradual appearance of distinct P wave (lower strip).

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