In human medicine, blood pressure has traditionally been measured using a stethoscope and a sphygmomanometer, with a cuff placed around the arm. These simple instruments allow maximum and minimum pressures, systolic and diastolic, to be measured easily and accurately. Inflating the cuff overcomes blood pressure and briefly stops blood flow. As cuff pressure is reduced, blood begins to flow through the artery again; the pressure at that moment is the maximum pressure. A stethoscope detects the turbulent sound, which follows a rhythm similar to the heartbeat. These characteristic Korotkoff sounds disappear when cuff pressure falls sufficiently for the artery to regain a diameter that allows blood to flow normally, without turbulence, known as laminar flow.
In exotic animals, most of which are small, such as birds and rabbits, this flow is very difficult to hear. Even a paediatric stethoscope can be more than three times the size of a rabbit’s or bird’s forearm, and these sounds are inaudible. We therefore use a Doppler device, which produces sound when a pulse is present. It lets us locate the artery and detect the return of blood flow as cuff pressure is reduced. This provides the maximum pressure. Although it does not provide the minimum pressure, it allows us to investigate hypertension in birds and small mammals. We cannot detect minimum pressure with this method because the Doppler detects the pulse itself rather than turbulent flow, and the pulse remains after the turbulence has stopped.
