On a daily basis, it is very common to be called to administer general anaesthesia to elderly animals. In recent years, it has been noted that dogs and cats are living longer. This, combined with the alteration of owners’ attitudes towards their pets and the advancement of veterinary geriatric medicine, makes it even more important to know about geriatric anaesthesia.
Broadly, a veterinary patient is considered geriatric when they have completed 75 to 80 percent of their expected lifespan. Therefore, there is a variation in the life expectancy between different breeds, with small dog breeds having longer life expectancies and large breeds having shorter ones.
Old age is not a disease, and it is not a reason to deny anaesthesia, but some changes that occur over time can contraindicate anaesthesia. Hence, it is important to discuss and understand physiological and pathological age-related changes before administering anaesthesia to geriatric patients.
Cardiovascular system
Geriatric patients have a decreased cardiovascular reserve that decreases their ability to respond to circulatory changes or stress, making them more prone to hypotension. Older individuals are also more likely to have conduction system changes that predispose them to arrhythmias. In addition, cardiac pathologies such as valvular lesions (especially mitral valve insufficiency in dogs), cardiomyopathies (especially hypertrophic cardiomyopathy in cats) and endocardiosis can predispose to arrhythmias. Age lessens the autonomic nervous responses, increases the vagal tone, decreases the sensitivity of baroreceptors and chemoreceptors, and decreases responsiveness to catecholamines. Furthermore, old animals have slightly decreased blood volume.
The consequence of these age-related changes can be an increase in myocardial oxygen consumption, which can lead to hypoxia and an overall decreased ability for senior patients to deal with hypotension. To cope with hypotension, geriatric animals tend to increase cardiac output by increasing heart rate, stroke volume and left ventricular end diastolic volume. However, the range of this increase in heart rate is limited for geriatric animals, making it necessary to ensure adequate preload (known as the “Frank Starling law”).
| Auscultate carefully! Does the patient have a murmur? If yes, investigate and amend the anaesthetic protocol as needed. |
Respiratory system
Geriatric patients have decreased ventilatory reserves. Functional residual capacity decreases while the closing capacity increases, leading to an increased risk of alveolar/perfusion mismatch and atelectasis and a decrease in resting arterial oxygen tension. Elderly animals also have decreased lung elasticity due to fibrosis and calcifications, decreased chest wall compliance due to calcification of costochondral joints, and muscle atrophy. Furthermore, obesity, which is more common in geriatric patients, can increase thoracic fat. This may restrict chest wall expansion, lung function and cardiac function. Additionally, laryngeal and pharyngeal reflexes are reduced, which can contribute to aspiration. The diameters of the trachea and the larynx are bigger in this group of animals, leading to an increase in anatomical dead space. Finally, in older patients, it is more common for diseases such as chronic bronchitis, lung neoplasia and laryngeal paresis to occur, all of which can contribute to the reduction of ventilatory reserves.
The consequence of the above is that older animals have an increased risk of hypoxaemia and hypercapnia. Hypercapnia can lead to decreased myocardial oxygen supply, increased myocardial oxygen demand, tachycardia, increased vascular resistance and hypertension, as well as peripheral vasodilation, which may lead to intraoperative haemorrhage. Hypoxaemia initially activates carotid chemoreceptors and increases ventilatory drive; however, the respiratory oxygen exchange becomes less efficient due to the decreased concentration gradients and hypoxic alveolar damage.
| Preoxygenate your patients before anaesthetic induction. Be ready to ventilate your patient’s lungs if the end-tidal carbon dioxide gets too high. Due to poor chest wall compliance, ventilation may decrease the patient’s blood pressure. |
Central nervous system
Progressive neuronal atrophy, decreased neurotransmitter release and reduced receptor affinity to neurotransmitters are described with advanced age. There is decreased cerebral blood flow and increased blood–brain barrier permeability, which change the patient’s drug pharmacokinetics. Thermoregulatory ability is also decreased in older patients, which can increase the risk of hypothermia when in combination with changes in body composition.
The consequence of the above is a decrease in the minimum alveolar concentration (MAC) of the inhalation agent, making overdose more likely.
Additionally, elderly individuals usually have visual and auditory issues that contribute to a higher level of stress.
| Handle your elderly patient gently, as they can get stressed more easily. Lower doses of premedication can be used. |
Renal system
Geriatric patients have decreased renal reserve as kidneys cannot regenerate. A very common clinical presentation of elderly patients is polydipsia and polyuria. Polydipsia and polyuria can occur because of the reduction in the kidneys’ ability to conserve sodium, concentrate urine and excrete acid. Geriatric animals are less responsive to antidiuretic hormone, which also impacts the above. Uraemia can also impact drug distribution, as well as the binding capability of albumin and the blood–brain barrier permeability of certain drugs. It is not uncommon for older patients to present with chronic renal disease and/or have fluid and electrolyte imbalances.
As mentioned earlier, elderly patients are more likely to have cardiac disease. Changes in the cardiovascular system may affect and reduce the renal blood flow, which, when combined with the reduced number of glomeruli and nephrons, can cause a decrease in glomerular filtration rate.
| Perioperative fluid therapy should be administered. If a patient has cardiac disease, fluid therapy rates may need to be conservative. |
Musculoskeletal system
Geriatric patients have decreased muscle mass (sarcopenia) and increased adipose tissue, even in non-obese animals. Obesity is also more common in older individuals. These changes can lead to sarcopenic obesity. In humans, sarcopenic obesity has been associated with a plethora of clinical complications, such as frailty, fractures, cardiovascular diseases, neoplasia and an increased risk of hospitalisation and mortality. Increased adipose tissue can alter the compartments for drug distribution resulting in prolonged duration of action of certain drugs. Additionally, increased body fat can alter the anatomical landmarks for nerve blocks and epidural injections. When it comes to epidural injections, adipose tissue in the epidural space, in combination with the stenosed intervertebral foraminae, may result in a more cranial spread of injectate. Therefore, the volume of drugs administered into the epidural space should be decreased. Increased body fat in combination with the thermoregulatory impairment previously mentioned can increase the risk of either hypothermia or hyperthermia.
Many older patients have osteoarthritis in multiple joints in their bodies. Hence, this can be uncomfortable when they are being restrained or positioned while under general anaesthesia, especially if the hind legs are placed in abnormal positions.
| Don’t forget to account for the chronic pain your patient lives with because of osteoarthritis. |
Gastrointestinal system
Geriatric patients have decreased lower oesophageal sphincter tone and increased gastric acidity. These factors increase the risk of regurgitation and oesophagitis. Combined with the reduced laryngeal reflexes, these conditions increase the risk of aspiration. Elderly individuals can also have malabsorption problems and nutritional deficiencies. To be more specific, iron and vitamin B12 deficiency can be a factor for anaemia.
| Old dogs are more likely to regurgitate under anaesthesia. |
Hepatic system
Geriatric patients can have decreased hepatic blood flow due to their decreased cardiac output. Reductions in hepatic blood flow can affect the plasma clearance of drugs, potentially prolonging their action. Additionally, elderly animals may have a decreased hepatocyte number and function, which can lead to decreased albumin production, decreased clotting factor production and decreased glucose regulation.
The above can lead to increased active drug concentration, haemorrhage and hypoglycaemia.
| Most drugs are metabolised in the liver and may have a prolonged duration of action. Check preanaesthetic bloods for your geriatric patients! |
