Module 11 - Anesthesia

Animal Factors in Anaesthesia

There are a number of factors related to the animal that impact on the quality of anaesthesia. These factors should be considered when the type of anaesthetic agent is being chosen.

Species. Different species require different doses of anaesthetic agents. This applies particularly to the injectable anaesthetics. In general, the smaller animals require a higher dose in mg/kg of a given anaesthetic than larger animals. Familiarity with the effects of an anaesthetic agent in one species should not be assumed in another species. The volatile anaesthetics are more consistent in their application between species. The mean alveolar concentration of the anaesthetic agent required for anaesthesia is similar among species and this is controlled by the concentration of the agent in the inspired gases. Differences in the respiratory tract in birds (fixed lungs, air sacs) and other non-mammalian species must be considered when administering inhalation anaesthetics.

Strain. Strain differences have been noted even within the same species. Some strains of pigs are more susceptible to malignant hyperthermia during halothane anaesthesia than others (a genetic trait).

Age. Young animals and old animals may have an increased risk for anaesthetic complications. In older animals, pathological changes if present in the respiratory system, may result in complications. Young animals may not have developed all the processes required to metabolize the drugs and so may have longer than expected recovery from anaesthesia. Volatile anaesthetics allow more refined control of the anaesthesia in both groups.

Weight. Very fat animals may not breathe as effectively during anaesthesia as thinner animals, leading to the problems associated with hypoventilation. In addition, if an agent is given on a mg/kg basis, there may be a relative overdose because the fat does not participate to a great degree in the circulation and distribution of the drug. If part of the recovery from an anaesthetic depends on its removal from the blood into tissues including fat (e.g., the short acting barbiturates) then animals with very little fat (e.g., greyhounds, calves) may experience longer than usual recovery from anaesthesia.

Sex. There is some evidence for a difference between the sexes for some anaesthetics.

Health of an Animal. Pre-existing disease or pathology may complicate an otherwise smooth aneasthesia. Any disease in the lungs will further compromise respiration during anaesthesia. Liver disease may interfere with the metabolism of anaesthetic agents and kidney disease may limit their excretion. Surgically altered animals (e.g., hypophysectomy, adrenalectomy, thyroidectomy) may be at increased risk at subsequent anaesthesias

Demeanor. An exited animal with high levels of circulating adrenalin, elevated heart rate and blood pressure is at an increased risk when undergoing anaesthesia.

Previous Anaesthesia. Some of the injectable anaesthetics are not completely cleared from the body for several days (e.g., pentobarbital), even if the animal has recovered consciousness and is behaving normally. Care must be taken if a second anaesthetic quickly follows the first. For those anaesthetics that are extensively metabolized as part of the excretory process, a second anaesthetic may result in more rapid metabolism of the drug than the first, with a shorter period of anaesthesia.

Other Factors. Some non-anaesthetic drugs have effects on anaesthetic agents. Chloramphenicol may lengthen the duration of pentobarbital anaesthesia and some antibiotics potentiate the actions of muscle relaxants.