A Combination of Atrioventricular Block and Sinoatrial Block in a Horse

In the process of carrying out a routine check up examination on a group of horses of a riding school in Shiraz, a horse demonstrated frequent long pauses in heart rate of 33 beats per minute on auscultation. A base apex electrocardiogram (ECG) was recorded to determine the type of conduction disturbance. At the time of ECG recording, the animal was normal with no apparent sign of any body disorders and respiratory movement and temperature were within normal range. CBC and serum biochemical analysis carried out were within normal limits of our school reference values. The horse was given a light exercise and was examined immediately after exercise and 15 minutes thereafter. The heart rate immediately after the end of exercise was 65 bpm and slowed down to 35 bpm 15 minutes after exercise with no sign of heart block.

ECG interpretation

The first two complexes of the ECG appeared to be conducted normally through the heart following initiation of the sinoatrial node (Fig. 1); however, the P-R interval was variable and the third P wave was blocked at the A-V node. The mean P-P interval was about 1.80 second indicating a sinus rate of 33 beats per minute. Following the first blocked P-wave, there was a long pause, which was slightly more than twice the previous P-P interval. There was no electrical activity during this pause indicating a concomitant sinoatrial block (SAB). The underlying rhythm was sinus arrhythmia as the P-P or R-R intervals were variable. There was also a single atrioventricular block (AVB) in the second trace. No single SAB without accompanying AVB was observed on the ECG.

Figure 1

Base-apex electrocardiogram recorded from a horse with concomitant second degree atrioventricular and sinoatrial block. The underlying rhythm is a slight sinus arrhythmia. The pause between the first blocked P wave and the next P wave is slightly longer than 2 P-P intervals indicating SAB. Paper speed is 25 mm per second and calibration 10 mm equal to 1 mV.

Discussion

The prevalence of cardiac arrhythmias among domestic animals is more common in the horse [6] than in other species and the majority of these dysrhythmias are not clinically significant. Sinus arrhythmia, sinoartrial block, wandering pacemaker, first and second degree atrioventricular blocks are considered to be vagally mediated conduction disturbances and waxing and waning of the vagal nerve is believed to be the cause of these cardiac irregularities [8]. Of these cardiac dysrhythmias, second degree AVB is more common [2, 1]. There are three types of AVB; that is, first degree AVB in which all impulses originating from the SA node conduct to the ventricle with longer than normal P-R interval (P-R > 0.50 sec.). Although this can be diagnosed on auscultation, ECG is needed for confirmation. In second degree AVB, some of the P waves are not followed by the QRS complexes, therefore, there are more P waves than QRS complexes and during the missed beat the fourth heart sound can be heard. There are two types of second degree AVB. In type I, the P-R interval lengthens gradually untill a P-wave is blocked but in type II the P-R intervals remain fixed. In third degree AVB, the electrical relationship of the atria and the ventricles are disrupted.

The prevalence of second degree AVB is around 15 to 40% in the equine population [8, 2]. It has been considered functional provided that it disappears after light exercise [2]. It has been stated that training has effect on the autonomic nervous function [7] and vagally mediated cardiac dysrythmias are more common in trained horses although AVB has also been reported in a 2.5 months thoroughbred foal [5]. There are also reports that cardiac lesions can be associated with AVB and SAB in the horse [3, 4]. The SAB occurs much less frequent than AVB in horses [9]. Although AVB and SAB may occur simultaneously on the ECG of a horse their association is very rare. A review of literature showed that only one case has been reported in a doctral thesis [9] therefore, this can be accepted as the first published case of a concomitant AVB and SAB in the horse. The horse was clinically normal and used for jumping so this type of conduction disturbance could be taken as a functional block. However, periodic checking of the cardiovascular system is recommended.