Bradydysrhythmias due to cardiac disease are very uncommon in horses. Synor weakness are the principal signs associated with these arrhythmias if they are sufficiently severe.
Profound second degree heart block
In rare situations, prolonged periods of 20AV block which are not abolished by increasing sympathetic tone may occur and these may cause clinical signs. It may be helpful to use radiotelemetry, or a 24-hour Holter monitor and timed observations, to see if syncopal attacks coincide with periods of severe AV block. Unfortunately, the intervals between these attacks are often very long and it may not be possible to monitor the horse for prolonged periods. Occasionally the arrhythmia may be related to an abnormally high vagal tone caused by extra-cardiac disease, in which case atropine may abolish the block. If AV node disease is present, atropine will have little effect on heart rate or the AV block.
If 20AVB is accompanied by syncope or weakness, an artificial pacemaker may be required. If it occurs in an anaesthetised animal and leads to a marked drop in blood pressure, anticholinergic drugs can be administered, and dopaor dobutamine can be used to raise cardiac output and increase vascular tone. However, sometimes the use of these catecholamines can in itself induce AV block because of the baroreceptor mechanism.
Third degree AV block
Third degree AV block (complete heart block) is a condition in which the AV node does not conduct any sinus impulses. The ventricles are 'rescued' by a regular slow rate resulting from a junctional or ventricular escape rhythm. Auscultation reveals regular ventricular contraction with A sounds heard during the diastolic intervals. Jugular venous pulses may be seen at the atrial rate, with cannon A waves occurring when atrial and ventricular contraction coincide. The heart rate may be as low as 12-20 bpm. The ECG will show normal P waves which do not bear any relationship to the escape complexes (Figure 7.15). There is usually a regular R-R interval between these escape complexes. The escape complexes are most likely to be junctional (and therefore appear normal or nearly normal) but may be ventricular (and therefore appear different from junctional beats, although they may not be particularly wide or bizarre). In a few cases ventricular tachydysrhythmias are thought to develop and further comcardiac output.
The condition is invariably pathological and carries a grave prognosis. Occasionally it is reversible, but this is probably the exception. It is exceptionally rare. Investigation should centre on identifying underlying disease. However, a specific cause is seldom found. Treatment consists of box rest and reversal of any underlying systemic or heart disease if possible. If no underlying disease is found or treatment is unsuccessful, an artificial pacemaker is required. Pacemakers have been successfully used in horses, but this requires specialist equipment and expertise. Reports are limited because severe bradyarrhythmias are very rare. With so little experience of their use in horses, it would be unwise to comment on their long-term value and possible complications.