Heart failure in cats

Heart failure is not a specific disease or diagnosis, but a syndrome in which severe systolic and/or diastolic dysfunction results in decompensation of the cardiovascular system. There are limited and specific mechanisms by which heart disease can result in decompensation of the cardiovascular system. As a result, there are limited and specific clinical signs that can develop as a result of heart failure. Heart failure can be divided into 4 functional classifications: systolic myocardial failure, impedance to cardiac inflow, pressure overload, and volume overload.

Systolic Myocardial Failure

Systolic myocardial failure is a general reduction in the ability of the myocardium to contract. This can be identified on echocardiography as an increase in end-systolic diameter and end-diastolic diameter, as well as a reduction in the percentage change in volume following ventricular contraction and reduced wall motion during ventricular contraction. If the reduction in contractility is significant, normal cardiac output cannot be maintained. Primary systolic myocardial failure is an idiopathic condition resulting in myocardial failure and is called dilated cardiomyopathy. Secondary myocardial failure is myocardial failure resulting from some insult such as neoplasia, heat stroke, electric shock, trauma, infectious disease (bacterial, viral, fungal, protozoal), nutritional deficiency (eg, taurine deficiency), drugs (eg, doxorubicin), or toxins.

Impedance to Cardiac Inflow

Heart failure resulting from impedance to cardiac inflow may result in a decrease to ventricular volume and, as a result, a decrease in cardiac output. This may result from external compression of the heart (eg, pericardial effusion, constrictive pericarditis), diastolic dysfunction resulting in a stiff ventricle and reduced ventricular filling (eg, hypertrophic cardiomyopathy, restrictive cardiomyopathy), or anatomic abnormalities resulting in impedance to ventricular filling (eg, atrial tumor, mitral and tricuspid valve stenosis).

Pressure Overload

Heart failure caused by pressure overload occurs as a result of chronic increases in systolic wall stress. This may result from impedance to cardiac outflow (eg, subaortic stenosis, pulmonic stenosis) or increased vascular resistance (eg, systemic or pulmonary hypertension). The response of the myocardium to these conditions is concentric hypertrophy (i.e., increased wall thickness of the affected chamber). Pressure overload to the adult right ventricle can result in dilation of the chamber as well as increased wall thickness, however.

Volume Overload

Volume overload heart failure occurs secondary to any disease that results in an increase in ventricular volume. Grossly, the ventricular chamber increases in diameter with a relatively normal wall thickness ( eccentric hypertrophy). This still represents ventricular hypertrophy because the overall mass of the ventricle is increased. Sarcomeres, however, increase in series (end-on-end) as compared to in parallel (as with concentric hypertrophy). Typically, the process begins with a volume overload to the ventricle (such as volume overload to the left ventricle caused by a left-to-right shunting patent ductus arteriosus), resulting in a compensatory increase in left ventricular chamber diameter/eccentric hypertrophy. Myocardial contractility typically remains normal, but may be reduced (such as when myocardial failure develops). The increase in ventricular volume means that the same degree of ventricular contraction produces an increased cardiac output. Though this can temporarily normalize cardiac output, eventually compensatory mechanisms are overwhelmed and signs of congestive heart failure (CHF) develop. Diseases that result in volume overload myocardial failure include valvular insufficiencies (eg, degenerative valve disease of the AV valves), left-to-right shunts (eg, patent ductus arteriosus, ventricular septal defect), or high-output states (such as those caused by hyperthyroidism or anaemia).