Direct or indirect recording of systemic arterial blood pressure can be carried out in most animal species. Direct measurements are invasive, requiring arterial cannulation, but this is relatively straightforward in most larger species. Cannulation can be carried out either following surgical exposure of a suitable artery or by puncture through the skin using a specially designed catheter system. The femoral artery can be cannulated in this way in dogs, pigs, sheep and larger primates. Percutaneous cannulation of the femoral artery in the cat and rabbit requires considerable technical skill. In rabbits and sheep, the central ear artery can be catheterised easily using an “over-the-needle” catheter.
Direct blood pressure monitoring has the advantage of providing a rapid indication of changes in pressure and of recording accurately over a wide range of blood pressures. Blood pressure can also be monitored indirectly and non-invasively using a sphygmomanometer. Instruments designed specifically for use in animals are now available, and these are preferable to those used in people since an appropriate-sized cuff for occlusion of the artery must be used for accurate measurement. The use of a Doppler probe to detect arterial blood flow, coupled with an inflatable cuff and a pressure sensor, can be used to measure arterial blood pressure in a range of animal species. These instruments are available commercially and can be used to measure arterial pressure in the caudal artery of rats.
The main disadvantage of indirect monitoring is the intermittent nature of the information obtained. The most widely used automated instruments, which use an oscillometric technique to detect the arterial pressure changes, take readings at a minimum interval of a minute. During periods of cardiovascular instability, this interval may be too long to allow effective emergency treatment. A second problem is that when blood pressure falls, the instrument may fail to detect the reduced amplitude signals.
Pulse oximetry, which is described above, provides a measure of heart rate and gives a crude but effective indication of the pulsatile flow in the tissues.