7.6.2 Sinus tachycardia
Sinus tachycardia occurs when sympathetic discharge increases and paratone decreases. A rate above 60 bpm would be considered a tachycardia. The SA node is the pacemaker. Sinus tachycardia is a normal response in order to increase cardiac output. This often results from pain, fear, excitement or exercise. At rest, it can be seen in animals with heart failure, haemorrhage, shock, pyrexia, or anaemia.
Auscultation reveals a regular, rapid rhythm. The intensity of the heart sounds may be greater than normal. An ECG demonstrates normal P and QRS comwith a regular R-R interval. At high heart rates, the P wave may be lost in the preceding T wave (Figure 7.2). The T wave is extremely labile and is profoundly affected by heart rate. At high heart rates it may be large and of a different configuration from normal. Before deciding that a tachycardia is due to heart failure, it is very important to be sure that the horse is truly at rest, preferably in its own environment, having become used to the presence of the examiner.