Under normal circumstances, normal sinus rhythm is a very regular rhythm. The sinus node functions as the body’s metronome, keeping an exact and regularly recurring cardiac beat. However, in the field of medicine, there are very few things that are exactly the same among all patients. Normal sinus rhythm is no exception to this rule.
In some patients, the normally occurring sinus complexes can have some slight irregularity in the timing of each of the complexes. That variability should never be by an interval greater than 0.16 seconds, or four small boxes, in the R-R intervals found in normal sinus rhythm throughout the entire strip. There are some patients, however, who have normally occurring complexes that originate in the sinoatrial (SA) node, but the irregularities vary by more than 0.16 seconds. We call this irregular rhythm sinus arrhythmia to differentiate it from normal sinus rhythm.
Generally, sinus arrhythmia is very easily identified because of the gradual widening and narrowing of the interval. Some cases, however, may not be so evident. For those cases, it is a good idea to know the specifics needed to make the diagnosis. The Additional Information box will show you how to be certain you are dealing with sinus arrhythmia.
Now that we know a little bit about sinus arrhythmia, let’s make matters more interesting. There are two kinds of sinus arrhythmia: the respiratory or phasic variant, which occurs during normal circumstances, and the nonrespiratory or nonphasic variant, which is found only in pathologic states.
Additional Information
Identifying Sinus Arrhythmia
The mathematical way to identify a sinus arrhythmia is to make sure all of the complexes are identical (within reason) and that the P waves are consistent with a sinus pacemaker. Calculate the P-P interval for the two fastest complexes (the ones that are closest to each other), then calculate the P-P interval for the two slowest complexes (the ones that are farthest from each other) (Figure 11-1). The difference between these distances should not exceed 0.16 seconds. If it does, the rhythm is sinus arrhythmia.
Figure 11-1 Identify sinus arrhythmia by measuring the longest and shortest P-P intervals. If the difference between these distances is greater than 0.16 seconds, it is sinus arrhythmia.
