Overview of Sinus Arrhythmia
A normal S1 and S2 with a regularly irregular rhythm is consistent with sinus arrhythmia.
This is still a sinus rhythm; however, it is irregular with a pattern that increases for a few beats and decreases for a few beats (regularly irregular). The heart rate sometimes varies with respiration, increasing on inspiration and decreasing on expiration; however, this pattern is not consistently present since it is 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.
Sound Library
Auscultation provides only part of the clinical picture when diagnosing heart disease. To review best practices, read The Basic Cardiology Examination.