2.2 The pathophysiology of heart failure
Despite the fact that NYHA class 3 and 4 heart disease is relatively uncommon in horses compared with other species, it is helpful to understand the changes which lead to signs of heart failure.
Heart failure may be acute or chronic, compensated or decompensated. The majority of animals with heart failure are in a compensated state in which there is a stable balance between abnormalities and the homeostatic responses to these changes. Treatment of animals with heart disease is aimed at reversing the aetiological factors if possible, in order to return the heart to normality. Howbecause heart valves cannot be replaced in horses, a complete cure is only possible with some arrhythmias. Treatment of animals with compensated heart disease is therefore based on alleviating clinical signs without reducing cardiac reserve. Treatment of animals with decompensated heart failure is aimed at returning the animal to a compensated state, if the long-term prognosis is sufficiently good for treatment to be worthwhile.
Heart failure has been classified in many different ways, potentially leading to some confusion. Many classifications overlap; animals seldom fall only into one specific category. All of the descriptions are over-simplifications of the pathoprocesses which occur. However, they are useful concepts because they make one think about the physiological changes which are causing clinical signs.
2.2.1 Congestive heart failure
2.2.2 Left-sided congestive heart failure
2.2.3 Right-sided congestive heart failure
2.2.4 Biventricular failure
2.2.5 Oedema formation
2.2.6 Low output failure
2.2.7 High output failure
2.2.8 Myocardial failure
2.2.9 Systolic failure
2.2.10 Diastolic failure
2.2.11 Forward failure
2.2.12 Backward failure
2.2.13 Volume overload
2.2.14 Pressure overload
2.2.15 Decompensated heart failure
2.2.16 The pathophysiology of reduced exercise tolerance