An echocardiographic study of atrial fibrillation in horses: before and after conversion to sinus rhythm.

Two-dimensional and M-mode echocardiograms were recorded from 41 horses before they were successfully treated for atrial fibrillation. In addition, these examinations were performed in a subgroup of 20 horses after treatment, and the results were compared with pretreatment values. Atrial fibrillation in this group of horses was associated with a reduction of mean left ventricular fractional shortening (mean 31% +/- 5.24%), and 22 of the 41 horses were below the reference range. The remaining mean M-mode variables were within the normal reference range, although 12 horses had increased left ventricular lumen dimensions in systole, and 8 horses had decreased left ventricular ejection times. Abnormal motion of the mitral valve was present in all horses and was characterized by the absence of A peaks, which were replaced by small diastolic undulations in 55% of the horses. In horses 1 to 20, after conversion to sinus rhythm, the mean fractional shortening increased (35.34% +/- 5.4%, P = .004), but there were no significant differences in heart rate or left ventricular lumen diameters in systole or diastole. These results suggest that ventricular function may be compromised by the presence of atrial fibrillation. However, this improved after correction of the arrhythmia.