Observing the movements of the chest can be used to monitor the pattern and depth of respiration, although this becomes difficult if surgical drapes have been placed. This problem can be overcome by using electronic monitors, for example, a respiratory monitor to detect each breath, or a pulse oximeter to measure the adequacy of oxygenation.
Although both the pattern and rate of respiration change during anaesthesia, this varies greatly depending upon the anaesthetic regimen used. Becoming familiar with one or two regularly used regimens allows changes to be interpreted more reliably.
In general, once anaesthesia has been induced, respiratory rate reduces markedly, especially in animals such as rabbits that show tachypnoea prior to induction of anaesthesia.
Protective blink reflex
Anaesthetised animals lose their protective blink reflexes, and their eyes should be protected both from physical damage and from drying.
An ophthalmic ointment can be placed in the eyes, or the lids can be taped closed with micropore tape during longer ( >15 mins ) periods of anaesthesia and whenever you are using injectable anaesthetics, as recovery will last for at least 30-60 minutes.
Monitoring heart rate
To minimise the risk of circulatory failure, the cardiovascular system should be monitored during anaesthesia.
Heart rate and rhythm can be monitored in most species, and in larger animals, this can be done either by palpating a peripheral pulse or by use of an oesophageal stethoscope.
The small size of rodents and rabbits, and their rapid heart rate can make it difficult to monitor heart rate and rhythm.
Anaesthetists should always record details of the entire procedure, including the drugs given, the dose rates used, and any monitoring or supportive procedures (e.g. use of a warming pad). They should also keep a careful record of any complications or abnormal responses for each individual animal.
This will allow research workers to identify individual animals where problems have occurred. This could help them interpret unexpected results later in the study as well as allowing us to identify animals that may need additional care during the post-operative period.
Records also need to include details of each of the people involved, what techniques they were responsible for and their competence.
If respiratory depression occurs, it must be monitored carefully, and if severe depression ( <40% of estimated resting rate ) or respiratory arrest occurs, it must be treated promptly. If severe hypoxia occurs and is uncorrected, this can lead to cardiac failure.
If respiratory depression or arrest occurs, or animals become hypoxic, then administer oxygen if this is not already being done – it is advisable to provide oxygen immediately following induction of anaesthesia with injectable anaesthetics since all of the agents used produce some degree of respiratory depression.
We can also assist ventilation by manually compressing the thorax, and providing oxygen by face-mask. Attempts to ventilate the lungs using a face-mask are often relatively ineffective.
In small rodents such as the rat, ventilation can be assisted temporarily by positioning the animal with its head and neck in extension and placing the barrel of a plastic syringe over the nose. Gently blowing down the tube will usually enable the lungs to be inflated.
We can also administer a respiratory stimulant such as Doxapram ( 10mg/kg, i/v, s/c, i/p or sublingual ).
Hypothermia is a fall in body temperature of more than 2 degrees C. It has major effects on different body systems ( for example very cold animals may have a cardiac arrest ).
Hypothermia also alters numerous physiological processes, such as cell membrane transport systems, enzyme activity etc and this could significantly influence your research data.
Depression of metabolic rate and enzyme activity can also greatly prolong recovery, especially when injectable anaesthetics have been used.
To minimise the risk of hypothermia
- Use a heating pad. Turn on before you start, and check it has reached the correct temperature either by using an electronic thermometer placed on the blanket or by checking the blanket controller’s display.
- Monitor body temperature using an electronic thermometer
- Avoid using excessive amounts of skin disinfectants, minimise the area of fur shaved at the site of surgery, and avoid unnecessary exposure of the abdominal viscera
- Warm all fluids to body temperature before administration.
- Use forced-air warming blankets in larger species.
- Heat loss can also be reduced by insulating the animal using bubble wrap or other materials.