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The aim of this study was to investigate the potential haemodynamic effects of valvular insufficiency and recurrent airway obstruction (RAO) in horses with atrial fibrillation (AF). Therefore in ten healthy horses (group 1) and 40 horses with AF a clinical examination, a lung examination, echocardiography and right heart catheterization for measurement of intracardic and pulmonary pressures were performed. According to the clinical findings the horses with AF were subdivided into 4 groups (group 2: AF; group 3: AF/valvular insufficiency; group 4: AF/RAO; group 5: AF/valvular insufficiency/RAO). Most of the horses of group 3 and 5 suffered from two valvular insufficiencies (mitral and tricuspid valve insufficiency: n=11, mitral and aortic valve insufficiency: n=2). The remaining horses showed a single mitral (n=6), tricuspid (n=2) or aortic valve insufficiency (n=1) or more than two valvular insufficiencies (n=4). In group 2 right ventricular mean pressure (RVPm) was higher than in group 1 and 4 (P<0.025); diastolic right ventricular pressure was higher than in group 1; PWP was higher than in group 1 and group 4; PDP was lower compared to group1. Compared to group1 in group 3 left atrial diameter (LA) was greater; the PAPs was higher and the PDP lower (P<0.05). In group 4 RVPm and PWP was lower compared to group 2. In group 5 LA, fractional shortening and diastolic left ventricular diameter were greater, PWP and PAPs were higher and PDP lower compared to group1. Twenty six of the 40 horses with AF (65%) were treated. Successful cardioversion to sinus rhythm occurred in 15 horses (58%). Therapy was successful in 50% of the treated horses of group 2 and 3, in 67% of the treated horses of group 4 and in 63% of the treated horses in group 5. In conclusion the presence of valvular insufficiency or RAO influences the haemodynamics of horses with AF.

The heart rate and the pulmonary artery wedge pressure (PWP) was measured in 10 healthy warmblood horses and in six warmblood horses with atrial fibrillation (AF) at rest and during standardised treadmill exercise. During treadmill exercise, the increase in heart rate was significantly higher in the horses with AF than in the healthy horses. Horses with AF showed a significantly higher increase in PWP at treadmill velocities of 5m/s and faster, than did the healthy horses. The differences in PWP between both groups increased with treadmill strain. The present study demonstrates that there is an influence on the haemodynamics in horses with AF during treadmill exercise, which could explain exercise intolerance in some horses with lone AF.

In 12 healthy warmblood horses and 10 horses with mitral valve insufficiencies (MVI) of various degrees heart rate and pulmonary artery wedge pressure (PWP) was measured at rest and during standardised exercise on a high speed treadmill. There was a significant increase in PWP with each change in speed of the treadmill (p < 0.01). The PWP of horses with mild mitral valve regurgitation under working conditions was not significantly different compared to the healthy horses. The horses with moderate mitral valve regurgitation showed a significant higher pulmonary artery wedge pressure at rest and during exercise compared to the healthy horses (p < 0.01) at rest and during treadmill velocity. The tendencies were seen that mild mitral valve regurgitation results only in mild hemodynamic changes during exercise, while moderate MVI have an important influence on haemodynamics.