How to use an ECG machine

An electrocardiogram (ECG) is a useful diagnostic tool in veterinary practice to assess heart rate and rhythm, but anecdotally, veterinary nurses seem reluctant to use the machine, citing two distinct challenges. First, knowing how to set up and use the machine correctly, and second, knowing what to look for when it is working. This practical article will explore both these issues, and suggest cases that might benefit from ECG monitoring.

Cardiac muscle requires an electrical stimulus to start a contraction. Figure 1 shows how specialised cells within the sino-atrial (SA) node start the conduction process, by firing an impulse that spreads across the atria depolarising (contracting) the muscle as it travels. The impulse passes through the atrioventricular (AV) node, to the ventricles using the His-Purkinje fibre network. As the impulse travels through the His-Purkinje network, the ventricles are depolarised. This depolarisation is the catalyst for the systolic and simultaneous action of oxygenated blood being pumped through the aorta, and deoxygenated blood out through the pulmonary artery. Finally, as the cardiac muscle relaxes, the ECG records the repolarisation (relaxation), as the muscle prepares for the next contraction. This process should result in a sinus complex, shown in Figure 2. Table 1 provides a step-by-step guide to each part of the P-QRS-T complex.