What pattern is observed in sinus arrhythmia?

In sinus arrhythmia, the characteristic pattern observed is that the heart rate varies with the respiratory cycle. This means that the heart rate tends to increase during inspiration and decrease during expiration, resulting in a naturally occurring fluctuation in the heart rate. This phenomenon is often seen in young and athletic individuals and is considered a benign finding.

The underlying mechanism for this variation is related to the autonomic nervous system, specifically the influence of the vagus nerve and its relationship with respiratory movements. During inhalation, there is a reduced vagal tone that contributes to a higher heart rate, while exhalation allows for increased vagal tone, leading to a slower heart rate. This respiratory variation is a hallmark of sinus arrhythmia and distinguishes it from other types of arrhythmias that may present with more consistent heart rates or abnormal conduction patterns.

In contrast, the other patterns mentioned, such as constant P-R intervals, a regular heart rate, or widening of QRS complexes, do not apply to sinus arrhythmia. A constant P-R interval suggests a stable conduction through the atrioventricular (AV) node, which is not the case in sinus arrhythmia due to its rhythmic variability. A regular and constant heart rate characterizes other arrhythm