What does it tell us?

The electrocardiogram only monitors the electrical activity of the heart and the heart rate is derived from this. Electrocardiograms are not the first choice for cardiovascular monitoring because they tell you nothing about the mechanical function of the heart or the state of the circulation. They are essential for the diagnosis and treatment of arrhythmias, and ECG monitoring is advised during cardiac procedures such as catheterisation. They are useful in critical situations where the blood pressure has fallen so low that peripheral pulses are not palpable.

Respiratory monitors Pulse oximeter How does it work?

The pulse oximeter shines red and infrared light through a thin piece of tissue such as the tongue and measures the relative absorption of the two wavelengths. From this it calculates the oxygen saturation, i.e. the ratio of oxyhaemoglobin to deoxyhaemoglobin, from absorption curves programmed into the device (Alexander, Teller and Gross 1989). What makes the pulse oximeter so useful is that it only looks at the pulsatile component of the light absorption, not the background level. The pulsatile change in tissue absorption is due to blood entering the arterioles and thus the pulse oximeter measures arterial oxygen saturation (Sa02%).

What does it tell us?

The pulse oximeter measures the adequacy of arterial oxygenation. Pulse oximeters are very useful monitors in anesthesia for two reasons. Firstly, arterial hypoxaemia is the final "common pathway" for many problems that occur in anesthesia such as hypoventilation, airway obstruction and equipment-related problems. A pulse oximeter can therefore pick up problems of different etiology. Secondly, hypoxia is a common and serious problem so noticing it early will prevent disasters. A pulse oximeter will detect hypoxaemia long before the animal becomes cyanosed. An added bonus is that they are relatively cheap (<US$1000) because the human market is so large.

There are many pulse oximeters on the market, including some specifically designed for veterinary use. These are good because the clips are designed for animals' tongues rather than human fingers. The best site for the pulse oximeter clip varies between species. The tongue is the best site and works for larger species (Huss et al. 1995). The nasal septum can be used in pigs as well as the tall. In rodents the clip can be placed across the hind foot, and in primates the cheek works well.

The absolute accuracy of pulse oximeters seems to be a little lower for animals than humans (Erhardt et al. 1990), but the trends are still important. In general a saturation of >95% is good. If it falls to <90% then the anesthetist should take note but corrective action may not be necessary, especially if the cause is known and self-limiting. When the saturation fails below <80% action should be taken to improve oxygenation.

Tidal volume and respiratory rate

There are no good, inexpensive respiratory monitors on the market. Simple "beepers" are available which measure gas temperature at the endotracheal tube connector. They detect the change in temperature between inspiration and expiration. Such monitors are fiddly to set up and give no indication of depth of respiration. At the other end of the spectrum there are human multi-function anaesthetic monitors that measure airflow and airway pressure and calculate compliance (Datex Engstrom, Tewkesbury, MA) but they are expensive.

Basic respiratory monitoring, therefore, tends to be based on clinical observations. If an anaesthetic circuit is used that has a reservoir bag then the rate can be counted and some idea of volume obtained by looking at the bag movements. In smaller animals this is not possible and one is limited to looking at chest movement. It is important to know the difference between normal and paradoxical respiration - inexperienced anesthetists often assume that because the chest is moving there must be airflow. A useful teaching aid is to occlude the endotracheal tube for a few breaths and note how the pattern of respiration changes. With normal respiration the abdomen and chest rise together. When the endotracheal tube is occluded there is paradoxical respiration: the abdomen moves in as the chest rises.

Rabbits and rodents are commonly ventilated during anesthesia because it seems to improve their general condition, especially during long procedures. During ventilation the tidal volume and respiratory rate are controlled by the anesthetist and do not need to be monitored, but they should be set correctly! People using ventilators should know that carbon dioxide is the main stimulus for respiration, and that if the arterial carbon dioxide level is reduce below 354OmmHg the animal will not attempt to breathe. They should appreciate how altering the ventilator settings changes arterial carbon dioxide levels. A common mistake is to under-ventilate so that the animal fights the ventilator. This is misinterpreted as the animal being too light, and more anaesthetic is given. The end result is a hypercapnic animal that is too deeply anesthetized. Inappropriate ventilator settings also lead PIs to use neuromuscular blocking agents when they are not necessary.