2.2.10 Diastolic failure
The term diastolic failure refers to heart failure caused by abnormal ventricular filling. In other species, this may result from stenotic AV valves; however, this abnormality is exceptionally rare in horses. Diastolic failure may also result from pericardial effusion leading to tamponade, constrictive pericarditis and myodiseases which prevent normal relaxation. These conditions prevent an increase in right ventricular filling at times of increased demand and therefore limit cardiac output. This may cause exercise intolerance and even collapse. It will also result in increased end-diastolic ventricular pressure which, because the right ventricle has a lower filling pressure than the left, may cause systemic venous congestion and signs of right-sided CHF. Fortunately, these conditions are uncommon in horses.
Diastolic function can also be reduced by diseases which primarily affect the left side of the heart, because increased wall thickness, ventricular dilation, or increased myocardial oxygen consumption can make the ventricles stiff. This may lead to a higher enddiastolic pressure. Arrhythmias can also affect venfilling. This is particularly true in the case of atrial fibrillation when the pump-priming' action of atrial contraction is absent. Signs are most apparent at high heart rates, when there is less time for ventricular filling.
The causes of diastolic and systolic failure are listed in Table 2.2.