Obesity Obesity

In dogs the incidence of obesity increases with age. Obesity can impair cardiovascular, respiratory, hepatic and musculoskeletal function. Even in thin animals there is an increase in the body fat to lean body mass ratio with increasing age. When calculating the dose of an anaesthetic it is important to base it on the lean body weight - not on total body weight so some assessment of the degree of obesity is necessary. Large amounts of body fat alter drug pharmacokinetics.

Anaesthetic agents (being fat soluble) are taken up into body fat stores during prolonged administration such as intravenous infusion or during inhalation maintenance ana~sthesia. These fat deposits act as a reservoir for the agent and prolong recovery time. The initial induction dose of short acting agents such as thiopentone sodium and methohexitone is not taken up into fat, but subsequent doses saturate skeletal muscle, and then are redistributed to fat. Repeated doses should be avoided in obese individuals.

Prolonged administration of halothane delays recovery because it has a high blood/fat solubility coefficient. On the other hand isoflurane is relatively insoluble in blood and fat and is probably more appropriate for use in obese old patients.

If the obesity is associated with hepatic lipidosis, drugs such as pentobarbitone which requires liver detoxification are probably best avoided.

Obesity may confound the identification of anatomical landmarks for the administration of local anaesthetic agents and it also increases the surgical risk of wound dehiscence and postoperative wound infection. Large amounts of perithoracic and intrathoracic fat may restrict chest wall excursion, lung inflation and compromise cardiac function.

Obesity sometimes occurs secondary to other conditions and this can present a diagnostic challenge to the clinician. Whenever possible uncomplicated primary obesity should be corrected by dietary management before elective surgery and anaesthesia.