Page 1001 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

976 CHAPTER 8

VetBooks.ir ATRIAL PREMATURE CONTRACTION ATRIOVENTRICULAR DISSOCIATION

Overview/aetiology/pathophysiology
The aetiology is variable. Myocardial irritation or
Atrial premature contraction (APC) is usu- Aetiology/pathophysiology
ally a single event. Persistent or high frequency inflammation is most common. This may be asso-
may be caused by atrial myocardial disease or ciated with systemic toxaemia. An ectopic focus in
inflammation. ventricular tissue has a higher intrinsic rate than the
sinoatrial (SA) node, consequently the atria and ven-
Clinical presentation/diagnosis tricles function as independent entities. Technically,
There is a shortened PP interval and changes in con- this occurs in all cases of ventricular tachycardia, but
formation of the P wave. The QRS complex confor- AV dissociation is considered a separate entity when
mation is generally normal (Fig. 8.14). the ventricular rate is <100 bpm.

Management Clinical presentation/diagnosis
Usually no treatment is required. Dexamethasone There is no relationship between P waves and ven-
may be of benefit to control inflammation. Digoxin tricular complexes (Fig. 8.15). The ventricular rate
may be necessary to control rate if severe tachycardia is higher than the atrial rate (in contrast to third-
is present. degree heart block). Ventricular and atrial complexes
are often normal in conformation.

8.14

Fig. 8.14 Atrial premature contraction (APC). Base–apex lead recorded from a clinically normal 10-year-old
Standardbred gelding at rest. Two normal cycles are followed by an APC. The APC is characterised by early
occurrence of altered waveform P wave, followed by a normal ventricular complex.

8.15

Fig. 8.15 Atrioventricular dissociation. Base–apex lead recorded from a 16-year-old Quarter horse gelding
with colitis. A sudden increase in heart rate, which was not consistent with other clinical signs, precipitated
the ECG recording. All waveform morphology is normal; however, there is no relationship between P waves
and QRS complexes. The ventricular rate of 80 bpm is higher than the atrial rate of 60 bpm and some P waves
are obscured by the ventricular waveforms. A capture beat is present near the end of the recording, where a
relatively longer diastolic interval is followed by P, QRS and T complexes with a normal relationship.