Serum T4 concentrations decrease by approximately 0.07
mg/100 ml per year in dogs with advancing age (Belshaw and Rijnbeck, 1979; Weller
et al. 1983). If this fall
has a significant effect thermoregulatory problems (hypothermia), and cardiovascular
disturbances such as bradycardia or impaired myocardial contractility might be expected
to result. Older animals might also be expected to have a reduced metabolic rate and a
predisposition to develop obesity, which they do.
Hypothyroidism is relatively common in older dogs and
may be associated with concurrent obesity. In some of these animals anaesthesia will be
complicated by both the hypothyroidism and hypoinsulinaemia or insulin resistance.
Hypothyroid animals are more susceptible to develop hypothermia and the vasodilatory
effects of agents such as acepromazine and halothane may induce profound hypotension.
Aldosterone responses decrease in humans with advancing
age and these changes are thought to be secondary to reduced renin secretion from
the juxtaglomerular apparatus in the kidneys. In view of the high incidence of renal
pathology in old animals it is reasonable to assume that such a decline might also
occur in animals.
It has been suggested that corticosteroids should be administered
to geriatric animals during prolonged periods of stress, surgery or anaesthesia to counter
Hyperadrenocorticism (Cushing's syndrome) is most common
in middle-aged to old dogs. It causes muscle weakness, reduced expiratory reserve volume,
reduced chest wall compliance, increased blood volume and increased systolic and
diastolic blood pressures (Feldman & Nelson 1987).
Adrenaline concentrations may increase, particularly in
the presence of major organ system failure such as congestive heart failure. Plasma
nor-adrenaline concentrations increase with age due to reduced clearance, but receptors
compensate by becoming less sensitive.
Glucose tolerance deteriorates with advancing age and
may be associated with hypoinsulinaemia (diabetes mellitus) or the development of
peripheral insulin resistance. The administration of fluids containing glucose needs to
be carefully considered in such patients, particularly if nutritional support is
going to be given by total parenteral nutrition (TPN) when 50% dextrose solutions
may be advocated. Chronic diabetics may have abnormal serum electrolyte concentrations
which should be corrected before surgery.
In the presence of hyperinsulinaemia, hypoglycaemia may
be precipitated during general anaesthesia and, as even a transient hypoglycaemia may cause brain
damage, blood (and in some cases urine) glucose concentrations should be monitored