2.1REDUCED CARDIOVASCULAR FUNCTION 2.1 REDUCED CARDIOVASCULAR FUNCTION

The cardiovascular system is the most important organ system in the body because maintenance of blood flow to and from cells is essential for the normal function and survival of all body tissues. When cardiovascular function is impaired it has deleterious effects on many organ systems, including the cardiovascular system itself.



Reduced tissue perfusion


Reduced tissue perfusion may result in:
(1) hypoxia
(2) poor cell nutrition
(3) reduced supply of immune mediators
(4) failure to remove metabolic waste products and toxins
(5) failure to transport physiologically active substances.


Compensatory mechanisms

In the presence of reduced cardiac function compensatory mechanisms come into operation which attempt to maintain blood flow to vital organs such as the brain and heart, and in the process blood may be diverted away from other tissues such as the abdominal viscera. These haemodynamic changes are helpful in hypovolaemia or shock and may have no serious consequences in the short-term for healthy individuals, but they can be deleterious when maintained over a long period of time, particularly in elderly animals in which tissues may be more susceptible to the adverse effects of reduced or increased perfusion.

Many ageing dogs have cardiovascular lesions but they are in a stable, compensated state with perhaps an audible murmur but no signs of heart failure. Veterinary cardiologists traditionally do not recommend therapeutic intervention until the animal is decompensated and clinical signs of failure are present, but it is important to realise that the compensatory mechanisms themselves may have adverse effects on the body. Prolonged sympathetic stimulation, for example, may induce insulin resistance and reduce glucose tolerance, with important metabolic consequences for the individual.



Cardiac workload

One objective in the management of geriatric animals with clinical or subheart disease should be to reduce unnecessary workload on the heart. The sum of the forces acting upon the myocardium to stretch the muscle fibres at the end of diastole is called the preload, and the sum of the forces opposing myocardial contraction during systole is called the after-load.

Restriction of dietary salt intake is one simple mechanism by which preload can be decreased, and avoidance or reduction of obesity can also significantly reduce cardiac workload. In the presence of clinical disease therapeutic agents such as diuretics can be used to reduce preload and vasodilators to modify preload and afterload.



Secondary heart disease

Control of cardiac activity depends upon the normal function and co-ordination of several organ systems including the peripheral nervous system (sympathetic and parasympathetic), endocrine function (e.g. adrenal and thyroid),the cardiovascular system itself and the metabolic state of the animal (e.g. electrolyte balance, acid-base balance).

Various chemical agents, toxins, nutritional abnormalities (deficiencies, excesses or imbalances), systemic diseases or metabolic diseases partithose of the liver, kidneys, lungs or endocrine system may interfere with cardiovascular function causing secondary heart disease.



Concomitant disease

In geriatric patients presenting with cardiac disease it is important to consider the likelihood of concomitant disorders in other organ systems:


(1) as a primary cause of the heart disease, e.g. hyperthyroidism;
(2) secondary to the heart disease, e.g. reduced renal function;
(3) reducing the efficacy of therapeutic agents, e.g. reduced efficacy of diuretics in the presence of hypoproteinaemia associated with reduced liver function;
(4) enhancing the toxicity of therapeutic agents, e.g. digoxin toxicity in the presence of renal failure or hypokalaemia.