What do they tell us?

The circulation should be monitored continuously during anesthesia (Wagner and Brodbelt 1997) and blood pressure measurement is one way to achieve this. The mean arterial pressure (MAP) is the overall judge of the state of the circulation. At the tissue level the fluctuations in arterial pressure are damped out so it is the MAP that determines how well tissues are perfused. As the MAP falls vital organ autoregulation and perfusion is lost. The susceptibility of tissues to poor circulation varies tremendously depending upon their metabolic rate. Skin, for example, is very tolerant of loss of perfusion whereas the heart, brain, retina and kidneys will not tolerate any significant ischaemia.

The diastolic pressure (DAP) and the pulse pressure (PP, = SAP - DAP) give information about the degree of vasoconstriction and the adequacy of ventricular ejection. if the peripheral vessels dilate the DAP falls because arterial run-off increases. if cardiac ejection is good then SAP will be preserved and the low DAP leads to a large PP which can be felt as a bounding pulse. Conversely, if the animal is vasoconstricted then the DAP, SAP and MAP are all high. The PP is low so the pulse is paradoxically weaker. A low PP and low DAP indicates poor cardiac output.

Electrocardiogram How does it work?

The electrocardiogram (ECG or EKG) monitors the electrical activity of the heart. With each beat the atria and ventricles depolarize and repolarize. The depolarization and repolarization are synchronized in each chamber and thus the action potentials from each fiber summate, producing a signal that is large enough to be measured at the surface of the body. The electrical signal is picked up by electrodes, amplified and displayed on a screen.

The ECG is always measured as the difference in voltage between two electrodes. Depending upon the placement of the electrodes the ECG has different shapes. If the electrodes are placed on each arm a lead I waveform is obtained. Lead II is measured from the right arm to the left leg, and lead III is measured from the left arm to the left leg. For anesthetic monitoring the lead configuration is largely irrelevant because we are not going to make detailed measurements of the complex heights and we just need a waveform that contains all the main components. A lead II type trace, with positive P, R and T wave is usually chosen. In theory only two electrodes are needed to record an ECG, since it is the voltage difference between the two electrodes. In practice a third ground electrode is needed to reject interference and the typical ECG lead arrangement for anesthetic monitoring has leads placed on the left (black connector) and right (white connector) arms and one leg (red connector). It is often easier to place the arm electrodes on either side of the chest rather than on the arms. The leg electrode can go on the inside of the thigh where the hair is tin. In primates, good results are also obtained by sticking the electrodes to the palms of the hands and feet.

Getting a good electrical connection to the skin is not as simple as it appears. The ideal electrode should have a low resistance, a stable half-cell voltage and not harm the animal. The first two requirements need some explanation. The ECG machine will reject noise best if the electrodes have a low resistance, which in practical terms means they should have a reasonable surface area and the skin should be moistened with a conductive solution. The "half-cell" voltage or electrode potential is the voltage formed whenever a metallic conductor meets a conducting solution. The junction between the two forms one half of a battery and generates a characteristic voltage, the value of which depends upon the particular chemistry involved but which is always several hundred times greater than the ECG voltage. This voltage must be very stable if it is not to interfere with the ECG waveform. The ideal electrode uses silver coated with silver chloride, which has a stable half cell voltage and is compatible with the chloride ions in plasma. Commercial ECG electrodes have a piece of silver-plated foil in the center which is coated with saline gel and backed up by an adhesive pad. The problem with these electrodes is that they don't stick well to animals. We have had better success with electrodes designed for human "stress testing", which involves recording the ECG while the person exercises. These electrodes are much stickier than the normal ones.

Electrodes used for veterinary work are often crude modifications of human leads. The problem is that the self-adhesive electrodes have a button connection in the middle and commercial ECG cables are made up with the corresponding clip. These clips are often cut off and other connectors substituted. "Crocodile" clips are commonly used but they are painful, they don't give a reliable contact, they have a high resistance and they corrode easily. Subcutaneous stainless steel needles are also used but they tend to fall out. Another problem with modifying the leads is that it is impossible to provide proper strain relief for the cable and it will inevitably break at the point where it is connected to the clip. In larger animals it is therefore best to use the original cable with human stress testing electrodes, but they are too big for rodents. A solution is to use pediatric electrodes on the paws and secure them with adhesive tape.