Supraventricular premature depolarizations (SVPDs) are also known as atrial premature contractions (APCs). An occasional SVPD is common in healthy horses at rest and immediately after exercise. They are usually clinically insignificant but if they occur frequently and if they occur in runs (atrial tachycardia) they may require investigation, especially if accompanied by illness, reduced athletic performance, or any other overt signs of heart disease.
Diagnosis
The diagnosis is suspected on auscultation of the heart – there is a regular rhythm interrupted by a premature heart beat, followed by a normal regular rhythm. The diagnosis is confirmed by electrocardiogram (ECG), as seen below. The normal electrical complex is represented by a notched P wave, followed by a negatively deflected QRS complex (onset of ventricular contraction), followed by a T wave (repolarization). In the trace below the T wave is positively deflected. The evenly spaced P waves is interrupted by an early or premature P wave, which is followed by a normal QRS complex and T wave. In this example, the shape of the premature P wave is different to the ‘normal’ P waves (note that this is not always the case). These are referred to as P’ waves. Also, the next beat after the SVPD occurs at, or slightly after, a normal beat-to-beat interval – i.e., the SVPD resets the normal rhythm.
Some sick horses (eg, horses with colic, colitis, pleuritis) may have atrial tachycardia, with long runs of SVPDs. This can be hard/impossible to distinguish from a normal fast heart rate caused by pain, excitement, dehydration etc (sinus tachycardia). This requires frequent auscultation, and ideally a continuous ECG. Vets should always be suspicious of a heart arrhythmia if the heart rate is higher than expected in a horse that appears to be comfortable and well hydrated.
Horses with frequent SVPDs are predisposed to developing atrial fibrillation.
Tags: Cardiology