Monitoring respiratory function during surgical or prolonged procedures

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. In larger animals, monitor movement of the anaesthetic breathing system’s reservoir bag can be monitored if one is present. In addition, an oesophageal stethoscope can be used to monitor breath as well as heart sounds, or a conventional stethoscope placed on the chest wall. 

This problem can be overcome by using electronic monitors, for example a respiratory monitor to detect each breath, or apulse oximeterto measure 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. Typical respiratory rates during anaesthesia are 50-100 breaths per minutes for small rodents, 30-60 breaths per minute for rabbits, and 10-30 breaths per minute for larger species. A reduction to less than 50% of the estimated normal respiratory rate should give cause for concern. It is more usual to see gradual changes in rate, rather than a sudden reduction, so keeping an anaesthetic record is helpful when assessing the state of the animal during anaesthesia.

Pulse oximeters can be used to monitor both the adequacy of oxygenation, and also the heart rate, but not all instruments function in small rodents. The high heart rates may exceed the upper limits of the monitor, and the low signal strength may not be detected. A monitor with an upper limit of at least 350 bpm is needed for rodents and rabbits and it is useful to have a range of different probe designs. A reliable signal can usually be obtained from across the hind foot in small rodents, or across a toe in larger rabbits, but the tail, tongue and ear are also useful in some individuals.

Capnographs, which measure the concentration of carbon dioxide in expired air, are extremely useful for monitoring respiration. When an animal is breathing normally, the end-tidal carbon dioxide is between 5 and 6%. In small mammals (<500g), mainstream capnographs (which are placed in the anaesthetic breathing circuit close to the animal) introduce too much equipment dead space even when the connectors are modified. Side-stream capnographs (which extract gas out of the anaesthetic circuit to be analysed) can be used, but the volume of gas sampled may be very large in relation to the animal’s tidal volume, so the measurements made may not be very accurate.

If respiratory depression occurs, it must be monitored carefully, and if severe depression (<40% of estimated resting rate) or arrest occurs, it must be treated promptly. If severe hypoxia occurs, and is uncorrected, this can lead to cardiac failure.

• 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.

• Assist ventilation  – if an endotracheal tube is in place, this is easy to achieve (see below), if not, assist ventilation by manually compressing the thorax, and providing oxygen by facemask.  Attempts to ventilate the lungs using a facemask 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. 

• Administer a respiratory stimulant such as doxapram (10mg/kg, i/v, s/c, i/p or sub-lingual).

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