Introduction
All anaesthetics produce a reversible depression of central nervous system activity, and on occasion, the degree of depression may become excessive and the animal dies. This should be a very rare event when anaesthetising healthy laboratory animals. The death of an animal during anaesthesia should stimulate a review of the entire process of animal selection, choice of anaesthetic and pre- and intra-operative care. In people, anaesthetic mortality rates are approximately 34 per million. Mortality rates in veterinary clinical practice have been reported as 1:400 (in cats) and 1:600 (in dogs). When anaesthetising healthy, young adult laboratory animals, it does not seem unreasonable to expect a mortality rate of <1:1000. Good pre-anaesthetic preparations, as described above, careful assessment of the physiological state of the animal during the anaesthesia can result in a dramatic improvement in recovery rates.
Different anaesthetics have differing effects on an animal’s body systems, but the vast majority of agents depress the respiratory and cardiovascular systems and depress thermoregulatory mechanisms. This can result in hypoxia, hypercapnia, acidosis, reduced tissue and organ blood flow and hypothermia. Changes such as these can influence research outcomes, as well as increasing the risk of serious complications during anaesthesia. All of these changes are dose-dependent, so it is important to give only sufficient anaesthetic to produce the required depth of anaesthesia. Ensuring that an animal is neither too lightly nor too deeply anaesthetised requires regular and careful assessment of anaesthetic depth. Monitoring the vital signs of the animal throughout the anaesthetic period and during recovery will allow the detection and correction of any problems that may arise.
Simple observation of the animal and assessment of its responses are valuable, but the monitoring process can be greatly improved by the use of electronic devices. Selecting these, and using them effectively, requires an understanding of their functions and their limitations. It is also important to be able to interpret the information gained from both clinical assessments and electronic monitoring devices, in order to use this information to prevent or correct problems and emergencies. After completion of the period of anaesthesia, an appropriate level of monitoring and supportive care needs to be maintained during the recovery period. Adopting this approach will benefit both the quality of research data obtained and the welfare of the animals involved.
Electronic monitoring devices
Although simple clinical observation by the anaesthetist should never be neglected, when the roles of anaesthetist and surgeon or anaesthetist and theatre technician are combined, uninterrupted or even regular observation is often impossible. In addition, fatigue during prolonged procedures may lead to human error, so the use of electronic equipment to provide continuous monitoring of physiological variables can be invaluable. Certain variables can only be measured directly by using electronic equipment. When anaesthetising animals for prolonged periods or during complex or high-risk procedures, the additional information provided by such apparatus can greatly assist in anaesthetic management. A further advantage of electronic monitoring equipment is that it usually enables acceptable limits for each monitored variable, for example, respiratory rate, to be set at the start of the period of anaesthesia. An audible or visible alert is triggered when these preset limits are exceeded. As mentioned earlier, the degree of monitoring required will depend upon the nature and duration of the surgical procedure.
Whatever monitoring is to be undertaken, it is essential to make a record of the information obtained. Problems during anaesthesia almost always develop gradually, rather than occurring as sudden catastrophes. If the observations made are recorded, preferably as simple graphs, adverse trends are easily detected and appropriate corrective action can be taken.
As discussed earlier, it is only through practical experience that the ability to assess the significance of changes of physiological variables during the administration of a particular anaesthetic can be developed. Although the production of written records may seem unduly time-consuming, these records provide an invaluable source of reference both for the current anaesthetist and for less experienced staff who may be required to undertake the procedure in the future.
Following the administration of an anaesthetic, it is also essential to assess that the required depth of anaesthesia has been achieved.