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
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