4. What alternative measurement devices are there?
The mercury sphygmomanometer is the device which has seen the longest use for indirect measurement of blood pressure. There are a number of alternatives, most of more recent origin.
A variety of auscultatory devices use other means of measuring the pressure inside the cuff. Aneroid manometers, which contain no fluid, work by a system of bellows and/ or levers which move a pointer on a dial. Electronic devices measure the pressure in the cuff with an electrical transducer. They display the pressure in a readout, which can simulate a mercury column, an aneroid gauge, or simply display a number.
A similar range of devices automate detection of the Korotkoff sounds , so that the operator does not need to listen. These automated auscultatory devices can, in principle, use any of the pressure sensing methods, but all have a microphone, electronic circuitry, and a display to indicate when the sounds are registering. Most commonly, the operator has to record the indicated pressure when a light begins flashing (for systolic pressure) and then again when it stops flashing (for diastolic). These devices are mainly used at present for patients with an irregular heartbeat, for whom other modern methods are unsuitable, and for measuring blood pressure during exercise.
The main alternative to the auscultatory method is the oscillometric technique, so-called because it detects changes or oscillations in the pulsation of the artery as the cuff is pressurised and depressurised. This method also appears in many automated devices, including ones which feature automated inflation and deflation of the cuff, and are especially suitable for home use and for 24-hour ambulatory monitoring. Some devices operate from the wrist or even the finger instead of the arm.
The oscillometric technique does not yield systolic and diastolic pressures directly. The oscillations or vibrations of the artery which happen as the cuff pressure is increased or decreased reach a maximum at the mean arterial pressure. The incorporated electronics convert this recording of pressure waves into a readout of systolic and diastolic pressures, according to a computation which is specific to each model of the device.
A few devices exist which combine methods. For example, some non-mercury devices for professional use incorporate automated electronic measurement using the oscillometric technique and auscultatory measurement by the operator.