Why do we need monitoring equipment?

There are three main reasons why we need monitoring equipment to augment our senses.

1. Our senses are limited. For example, by palpating the peripheral pulse we get a very good indication of heart rate but only a poor measurement of arterial blood pressure. This is because the quality of the pulse reflects the pulse pressure (the difference between systolic and diastolic pressure) rather than the absolute pressure. Palpation of the pulse may tell us that the heart rate is erratic but gives us no clues about the type of arrhythmia present and thus the treatment needed.

2. Monitors free up the anesthetist's hands to tend to the patient. Once the sensors are placed on the patient, the data are displayed on a screen where they can be read at a glance. A secondary benefit is that the data can be read by anyone in the room. During critical procedures, for example vascular surgery, it is important that both the surgeon and anesthetist are aware of the blood pressure. If the surgeon and anesthetist are the same person, as is common in laboratory animal work, the patient can be monitored without breaking sterility although the anaesthetic record cannot be filled in.

3. It is impossible to monitor many parameters simultaneously. During critical procedures the anesthetist needs constant updates on several parameters at once which cannot be done manually.

Cardiovascular monitors Arterial blood pressure

There are two basic methods for measuring arterial blood pressure, direct and indirect.

How do they work?

Direct blood pressure measurement involves placing a catheter in an artery and connecting it to a transducer. In cats, rabbits and larger species the arteries are large enough to be cannulated percutaneously. A standard 22g or 24g venous catheter is suitable in many cases. Specialized arterial catheters, designed for human use, are available to make the task easier. One useful technique is the "wire-guided" or Seldinger method which makes it easier to advance the catheter up the artery. Arrow International (www.arrowintl.com) makes wire-guided arterial catheters. For smaller animals the artery has to be exposed surgically, the usual site being the femoral or carotid.

Once the catheter has been placed in the vessel and secured, it is connected to a pressure transducer via a fluid-filled line. Drip tubing is often used but for the best results the tubing should be narrower and stiffer. It is important that there are no bubbles in the system. The transducer is connected to an amplifier and display unit, which shows the waveform and the systolic/diastolic/mean pressures calculated from the waveform. The shape of the waveform gives useful information about the state of the circulation, in particular the peripheral resistance.

Indirect blood pressure involves inflating a cuff around the limb and monitoring the blood flow in the limb distal to the artery. The classic technique for monitoring the blood flow in humans is to listen for the Korotkoff sounds with a stethoscope. This cannot be done in animals because the arteries are too small. Blood flow can be monitored optically (e.g. on a rat's tail), using ultrasound (the Doppler system) or by the pulsations it induces in the cuff itself. The pressure in the cuff is measured with a mechanical gauge or a transducer.

Automated systems such as the Dinamap are easy to use. A cuff is simply wrapped around the limb and the unit turned on. They work well in larger species such as dogs, primates and sheep but not in smaller species such as rabbits and rodents, and are marginal in cats. The Doppler system is more sensitive and works well in cats (Binns et al. 1995), but requires manual intervention to obtain a reading. Indirect systems for rodents do exist but are not used for routine anesthesia because they are fiddly to set up. They are used for research purposes.