The cardiovascular system has enormous compensatory reserves and heart failure only occurs when the reserve capacity is overwhelmed.
Venous oxygen reserves. The normal end-capillary or venous oxygen tension is 30-50 mm Hg. Providing this figure remains greater than 30 mm Hg increased oxygen can be extracted by the tissues as the demand for oxygen increases. Anaerobic metabolism occurs below a critical level of 21-22 mm Hg and results in lactic acid production in skeletal muscle tissue and signs of exercise intolerance.
Maximum effective heart rate. In dogs this is approximately 130 beats per minute. Above this figure there is inadequate filling of the ventricles during diastole with a resultant decrease in cardiac output.
Stroke volume. Normally only 50-65% of the diastolic volume is ejected during each cardiac cycle. This figure increases as the demand for oxygen increases.
Coronary reserve. Coronary blood flow can be increased to meet the increased oxygen demand of the myocardium.
Cardiac enlargement. In response to an increased work load the heart either hypertrophies or dilates to maintain cardiac output and tissue perfusion.
Preload and atterload
Preload is defined as the stretching force that determines the precontract ion length of each cardiac muscle fibre. A moderate increase in preload results in a greater contractile force (Frank-Starling relationship). Hence preload is related to left ventricular end-diastolic pressure. Afterload represents the tension which develops in a muscle fibre before contraction occurs, that is, the tension in the myocardium which is necessary to overcome the forces opposing ventricular ejection. As the ventricles increase in size, more tension must be generated to produce a given pressure (Laplace’s law).
Functional classification of heart failure
The functional classification of heart failure has recently been reviewed by the International Small Animal Cardiac Health Council (1994). The new classification scheme consists of three stages, the criteria for each being based on the anatomic diagnosis and severity of clinical signs at rest.