Sinus Arrhythmia

Amara Estrada, DVM, DACVIM (Cardiology) University of Florida

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A normal S1 andS2 with a regularly irregular rhythm, consistent with a sinus arrhythmia, is appreciated in this heart sound.

This is still a sinus rhythm; however, it is irregular with a pattern that increases and decreases for a few beats (regularly irregular). The heart rate varies with respiration, increasing on inspiration and decreasing on expiration; however, this pattern is not consistently present since it mediated both centrally and by cardiovascular and respiratory reflexes.

A sinus arrhythmia can occur without the influence of respiration. A respiratory sinus arrhythmia occurs because of fluctuations in autonomic tone. On inspiration, the right atrium fills, and this stretch induces the Bainbridge reflex (increases heart rate through a reflex inhibition of vagal tone and an increase in sympathetic tone); pulmonary stretch receptors also play a role. As a result of this increase in heart rate (caused by the increase in sympathetic tone and decrease in vagal tone), blood pressure increases, thereby activating the baroreceptor arc which increases vagal tone. This slows the heart rate during expiration. Again, this is not always associated with respiration, as autonomic tone is also modulated centrally; in fact, most sinus arrhythmias occur without the influence of respiration.

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