Age-related changes that may affect anaesthesia or surgery include:
- Nervous System
- Cardiovascular system
- Respiratory system
- Nervous system
- Central nervous system
Reduced functional tissue in the CNS is probably one of the factors that reduces the anaesthetic dose needed in older patients.
Old patients often have sluggish, impaired or absent reflex responses (e.g. pupillary light reflex) which may complicate monitoring during anaesthesia.
Loss of function of the special senses such as sight and hearing may lead to apprehension in strange environments (especially in cats), and sometimes sedation is needed to reduce preoperative stress which otherwise can significantly increase sympathetic simulation.
Geriatric animals have reduced ability to generate body temperature and are susceptible to develop hypothermia, particularly during prolonged surgery or the postoperative recovery period. In this context it is important to remember that core body temperature may differ from peripheral measurements, and the use of oesophageal thermometers or infra-red thermometers (applied in the aural canal) may be preferable to rectal temperature recording.
Peripheral nervous system
Supersensitivity of postsynaptic receptors may prolong the action of muscle relaxants.
Interpretation of the significance of poor reflex responses during anaesthesia is more difficult in older patients than in the young.
Subclinical and clinical cardiac disease is common in older dogs, and impaired cardiovascular function should be expected in elderly patients.
Baroreceptor function may be impaired, particularly in patients with chronic congestive heart failure, and the cardiovascular system’s ability to compensate for surgical haemorrhage, or for the vasodilatory effects of anaesthetic agents may be inadequate resulting in severe hypotension.
Existing cardiovascular conditions such as congestive heart failure, cor pulmonale, sick sinus syndrome and the cardiac signs associated with hyperthyroidism in cats should be stabilised before general anaesthesia.
Anaesthetic agents depress cardiac function and cardiac arrest can be precipitated if cardiac arrhythmias are present, particularly ventricular arrhythmias that are refractory to therapy, right bundle branch block and bradycardia (heart rate below 60 in dogs; below 80 in cats).
Monitoring heart rate and systolic and diastolic blood pressures by direct or indirect methods is desirable throughout anaesthesia as is ECG recording, monitoring pulse character and regular examination of visible mucous membranes for capillary refill time.
Age-related degenerative changes progressively decrease pulmonary function and physical changes occur in the lungs and chest wall. There is reduced alveolar surface area and diffusion capacity, pulmonary fibrosis, reduced lung elasticity, and reduced mechanical ventilation reserve. Chronic obstructive lung disease is common in old dogs and cats.
All of these changes impair gaseous exchange during anaesthesia hence oxygen supplementation is beneficial, and in some cases the use of bronchodilators may be indicated.
Impaired laryngeal function (sometimes laryngeal paralysis in older dogs -particularly Labrador retrievers) and increased respiratory dead space necessitate correct endotracheal intubation during anaesthesia and careful preparation of the patient to avoid vomiting. The use of antiemetic drugs (e.g. metoclopramide) might be indicated in patients requiring emergency surgical treatment, those with oesophageal or gastric motility problems or
those with conditions likely to cause nausea (e.g. uraemia). A recent study suggests that older animals have a greater risk of developing gastro-oesophageal reflux during anaesthesia.
Pulmonary embolism is a common postoperative complication in old people, and may be more common than is currently appreciated in veterinary patients. In human general surgery patients the incidence of deep vein thrombosis is reported to be as high as 45% in those aged over 40, and 65% in patients over 71 ().
Impaired renal function prolongs the plasma half-life of drugs eliminated via the kidney and may alter fluid, electrolyte and acid-base balance, so screening for renal function is important before anaesthesia.
It is good practice to administer a balanced solution intravenously before and during anaesthesia to facilitate control of fluid, electrolyte and acid-base balance and to maintain renal perfusion. If renal hypotension occurs during anaesthesia tubular ischaemia may result leading to acute tubular necrosis and renal failure. Advancing age and general anaesthesia are both important risk factors for the development of acute renal failure. Other risk factors include the administration of non-steroidal anti-inflamdrugs (NSAIDs), aminoglycosides, tetracyclines and other nephrotoxic drugs ().
Most liver function tests usually remain normal in geriatric patients and this probably reflects the huge reserve capacity of this organ, however, in humans bromsulphalein (BSP) retention does increase with age.
Hepatic lipidosis is common in cats, and may or may not be associated with obesity. Up to 50% of liver biopsies taken from cats in the USA are reported to demonstrate lipidosis on histological examination (). This condition can be secondary to diamellitus, hyperadrenocorticism, hypothyroidism or protein-energy malnutrition and affected animals may show gross hepatomegaly, elevated liver enzyme concentrations (serum alanine aminotransferase (ALT) and alkaline phosphatase (AP) and disturbances of liver function.
Cirrhosis is a chronic progressive disease usually affecting older animals resulting in loss of parenchymal mass and therefore reduced function. Primary and secondary hepatic neoplasia may also occur, eventually causing reduced liver function.
In the presence of impaired hepatic function the plasma clearance rate for drugs may be decreased resulting in increased duration of action. At the same time drugs and nutrients that need to be converted to an active form by the liver may exhibit reduced activity.
Serum T4 concentrations decrease by approximately 0.07 mg/100 ml per year in dogs with advancing age (). 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 ‘adrenal exhaustion’.
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 ().
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 during anaesthesia.
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 anaesthesia. 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.