7.7.2 Second degree atrioventricular block
Second degree heart block (20AVB), also called partial atrioventricular block (PAVB), or a missed, dropped or blocked beat, is the most common arrhythmia found in horses and may be found in up to 30% of fit horses at rest. It is assowith a slow or normal heart rate and high vagal tone. The percentage of animals with the arrhythmia is probably higher than recorded because horses are naturally wary of auscultation or the use of an ECG. 20AVB is nearly always a normal homeostatic mechanism which is involved in the control of blood presdetected by baroreceptors and mediated by the cardiovascular centre in the medulla. Rather than reducing the sinus rate, which might predispose to atrial arrhythmias, the heart rate and therefore cardiac output can be reduced by blocking conduction of sinus beats. Blood pressure measurements will show a slight stepwise increase in the arterial blood pressure until a sinus beat is blocked. This allows the pressure to fall back to a basal level before the process is repeHowever, as soon as there is demand for increased cardiac output, the sinus beats are all conducted, immediately increasing the number of stroke volumes pumped per minute (see section 1.4.3).
Auscultation will reveal an A sound without subsequent S1 and S2 sounds. The A sound is sometimes heard best on the right side of the chest. An A wave may also be seen or palpated in the jugular vein. It is important to try and identify the A sound, but it cannot be heard in all cases. In addition, the arrhythmia is often noticeably regularly irregular', i.e. the blocked beats occur at regular intervals. Frequently, every third, fourth or fifth beat will be blocked. This may be a useful feature in distinguishing this physiological arrhythmia from AF (see section 7.8.5). It may be helpful to use one's foot or hand as a metronome, tapping in time with the cardiac rhythm. Often, the metronome will tap once in the middle of the long diastolic interval, and then again in time with the next conducted sinus beat.
The ECG will show a P wave which is not followed by a QRS complex. Most commonly, Mobitz type 1 or Wenckebach phenomenon is seen in the horse. If it is this form of block, there is variation in the P-R interval, often with gradual prolongation before an impulse is blocked (Figure 7.3). This is often recognisable on auscultation. In Mobitz type 2 20AVB, the P~R interval is fixed. In small animals this can indicate the initial stages of AV node disease leading to 30AVB, but there is no evidence that this is the case in the horse. Although usually only one sinus impulse is blocked, on occasions, double-blocked beats can occur. These are very seldom pathological.
If high vagal tone is abolished by excitement or exercise, normal sinus rhythm will return. This is the reason for the use of a trot-up as a way of assessing whether an arrhythmia is physiological or not. Many pathological arrhythmias will become more irregular with exercise, so the abolition of the arrhythmia at higher heart rates is a useful presumptive indicator of a vagal arrhythmia. Exciting an animal in its box has the same effect. However, 20AVB can also occur at higher heart rates under some circumstances. Sometimes it is noted while the heart rate is slowing during the phase of 'autonomic imbalance' (Figure 7.4).In these situations it is usually regarded as normal. It is has a similar mechanism to that of post-exercise transient sinus arrhythmia.
20AVB can also occur when atrial premature beats reach the AV node while it is still refractory. These APCs can be difficult to identify without careful examof the ECG. On rare occasions, 20AVB can be so profound that it can be considered an abnormal arrhythmia (see section 7.8.15). However, it must be emphasised that this is extremely uncommon.