The most common situation in which exercise is of use in clinical evaluations is to see whether arrhythmias detected at rest persist at higher heart rates and wheexercise induces arrhythmias. Vagally mediated arrhythmias such as 20AVB and sinus block are usually abolished by reducing parasympathetic tone and increasing sympathetic tone. This is easily detected on auscultation after trotting the horse in hand. However, some arrhythmias, which were infrequent or absent at rest, may become more frequent, paroxysmal or sustained during exercise. It is important that these are detected and characterised because they are often associated with poor athletic performance. Occasionally arrhythmias which may be associated with pathological conditions, such as ventricular premature beats, are no longer apparent at higher heart rates. While this makes them less likely to be a significant problem, it is important that they are recognised. Thus, the abolition of an arrhythmia by increasing heart rate is not in itself a diagnosis of a vagally mediated arrhythmia, all arrhythmias should also be recognised by their other characteristics and if necessary by electrocardiography.
Recording exercising ECGs is particularly useful if an arrhythmia is present only during exercise, or if the effects of high intensity exercise on its frequency need to be assessed (see section 4.1.6, section 7.8.3, section 7.8.6 and section 7.8.8). Portable ECGs, rushed to the side of an animal after exercise, may also give valuable information, but are less satisfactory than radiotelemetric recordings during exercise.