4.1.3 Recording an ECG
Recording an ECG in a horse is simple. All that is required is a clear trace in which the P waves and QRS complexes are easily seen. Limb leads, particularly lead two, are often adequate. However, horses have a strong panniculus reflex when clips are attached to their upper limbs and movement will result in artefacts on the recording. A much simpler method is to record from the trunk of the body. Because the mean vector of ventricular depolarisation recorded on the body surface ECG is directed mainly cranially and dorsally, the largest signals are recorded with a bipolar lead placed in a ventro-dorsal or cranio-caudal plane. The two lead systems which are commonly used are a base-apex lead and a Y lead.
A base-apex lead has the positive electrode positioned over the apex beat area of the heart (i.e. on the chest wall just behind the left olecranon) and the negative electrode over the cardiac base. The left or right jugular furrow, or the loose area of skin just cranial to the scapula, are suitable as the position for the basal electrode.
A Y lead has the negative electrode attached over the manubriurn at the thoracic inlet and the positive electrode over the xiphoid.
In both these systems the QRS complex is largely negative because the cardiac vector is directed cranially and dorsally, away from the positive electrode. If the electrodes are attached the wrong way round the QRS complex will be largely positive, which does not prevent assessment of cardiac rhythm but can be confusing.
On most machines, in order to obtain a bipolar lead it is convenient to use the leads marked 'right arm' (usually coloured red in the UK) and 'left arm' (usually coloured yellow in the UK). If the lead selection dial is turned to record lead one the right arm lead will be negative and the left arm lead positive. A neutral electrode is required if a mains-powered machine is used (use the electrode labelled 'right foot', which is usually black). The thoracic inlet is a convenient site for a base-apex neutral lead, the neck for a Y lead.
When a trace is recorded, a paper speed of 25 mm/sec is usually appropriate, although a faster speed is required if a tachycardia is present. The amplitude of deflection should be marked on the trace at the beginning of the recording. The standard deflection used is 10 mm/mV; however, the amplitude of the QRS complex may be too large to fit on the paper, particularly with a base-apex recording, and 5 mm/mV may be preferable. With most machines, the thickness of the baseline is determined by the stylus heat. This can be turned up or down asappropriate. Paper is marked in either blue or black. Black traces duplicate better if they are to be photocopied or sent by facsimile. Electrode gel provides the best electrical contact; however, spirit can also be used.
The lead systems which are used in the horse are shown in Figure 4.1 and Table 4.1. The limb leads and chest leads are shown only because they are sometimes required by overseas veterinarians at pre-purchase examinations; however, in the view of the author, they very seldom aid diagnosis. If a rechargeable machine is used, it is worth remembering that it should be recharged after use and not left switched on.