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. Ausculation of the chest wall is possible in rabbits and guinea pigs. Although it is not usually possible to palpate the pulse in the periphery in small rodents, the heartbeat can be detected by palpating the chest wall in all species. However, since the heart rate often exceeds 250 bpm in many of these animals, it is not possible to count the heart rate accurately. Problems can also arise when an ECG is used, as many instruments have an upper heart rate limit of 250 or 300 beats per minute, and may also be unable to detect the low amplitude signals generated in small rodents.
The adequacy of the peripheral circulation can be assessed using the capillary refill time. The mucus membranes on the inside of the lip are blanched by pressing with a finger, and the refill time assessed when pressure is removed. The colour of the membranes should return to normal in less than a second. The small size of rodents usually prevents