7.7.3 Sinus arrhythmia
Sinus arrhythmia is the periodic waxing and waning of heart rate associated with changes in vagal tone. It occurs infrequently in resting horses with a high vagal tone and may be related to respiration. However, it is much less common at rest than in dogs or humans. More commonly, sinus arrhythmia occurs during the recovery period following exercise, particularly after light exercise. Usually, when the heart rate slows further, the rhythm becomes regular once more. In this circumstance it is known as post-exercise transient sinus arrhythmia.
Auscultation reveals a cyclical irregularity of rhythm which is frequently a source of confusion in assessment of animals during the post-exercise period. If sinus arrhythmia disappears after further rest or with more vigorous exercise it is a normal finding. The ECG will show cyclical changes in the R-R interval (Figure 7.5). The QRS complexes will be normal, but the P waves often vary inconfiguration due to changes in the site of impulse origin within the SA node or in its propagation through the atria (a 'wandering pacemaker'). The condition usually disappears with decreasing parasympathetic or increasing sympathetic tone. A radiotelemetric ECG is very useful to demonstrate that the arrhythmia is not present during exercise. Commonly, during a pre-purchase examination, this arrhythmia will be heard after the first stage of the exercise phase, causing concern on behalf of the examiner (see section 8.1.5). If it is not possible to record an ECG at the time of the examination, increasing the level of exercise will often result in abolition of the arrhythmia, because the horse loses parareserve. No treatment is required