4.1.8 24-hour ECG monitoring
One of the principal problems with standard ECG recording is that the cardiac rhythm is only recorded for a very short period of time. Consequently, a sigarrhythmia may not be detected during the examination, particularly if it is intermittent. In addition, when the animal is truly at rest, a different rhythm may be observed than when the clinician is present.
A 'Holter' monitor can be used to record the ECG signal on magnetic tape over a 24-hour period. The electrodes are attached with adhesive and can be bandaged in place or held by a tight girth. Some horses will require a roller to stop them rolling and displacing the electrodes. The tape is then played through a computer which is programmed to detect specific arrhythmias. Although the recording monitors themselves are relatively cheap, the computer system is very expensive and the tapes are usually sent to a human hospital for analysis. The computer must be programmed to search for parameters suitable for equine rather than human arrhythmias.
There are a number of situations in which a 24-hour ECG recording is helpful. In animals which collapse intermittently, arrhythmias are often suspected. Since animals seldom collapse when the clinician is present, and an arrhythmia may not be present at the time of the clinical examination, a 24-hour ECG may be of benefit. However, even then it may only be helpful if the animal actually collapses during the period of the recording (see section 8.3.2). 24-hour ECGs are perhaps more useful in detecting paroxysmal bouts of arrhythmias such as atrial or ventricular tachycardia. Atrial premature beats may be found in animals with poor athletic performance related to myocarditis. Animals which have repeated bouts of AF, which either resolve spontaneously (paroxysmal AF) or have been successfully treated, may have APCs during the remission of AF. In this instance, the chances of recurrence of AF are increased and the animal should be rested (see section 7.8.5).
When atrial or ventricular premature beats are detected during a routine ECG, it may be difficult to decide how significant they are. They may be only very occasional, in which case they are unlikely to be significant. However, there is the possibility that they are occurring with increased frequency at times other than the period of examination and may be of clinical significance. Vagally mediated arrhythmias and bradycardia are much more frequently detected during a 24-hour recording than during a standard examination. Although it may be difficult to interpret the findings, these recordings give clinicians valuable information which would not otherwise be available.