PRESSURE TRANSDUCER

Typical Number in Hospital: 30

Cost Bands:

References: 3,9

Physiological pressures range from about 18 kPa (140 mmHg) for arterial blood pressure to 0.26 kPa (2mmHg) for airways pressure. These and other pressures may be measured and recorded for diagnostic or monitoring purposes. Because the mercury manometer is used so much for blood pressure recording it is very common to quote physiological pressure in millimetres of mercury (mmHg) rather than in SI units (Pa - pascals). Also some pressures such as bladder pressure and central venous pressure are often quoted in centimetres of water head. This is particularly convenient with water-filled measurement tubing since changes in the height of the transducer or patient make the same changes in the measured pressure. Useful conversions are as follows:

1 mmHg = 133 Pa = 0.133 kPa

1 mmHg = 1.36 cmH2O

1 cmH2O = 0.735 mmHg

Although static pressures can be measured using a column of mercury (e.g. the mercury sphygmomanometer) or water, changing pressures require a pressure transducer which can respond to rapid changes. These usually work by producing an electrical signal from the bending of a membrane or diaphragm. The strain gauge which measures the bending may be part of the pressure diaphragm itself as in semiconductor gauges or it may be attached to the diaphragm (usually a thin steel plate) by a rod. The strain gauges change their electrical resistance as the diaphragm is bent by the pressure. The gauges form part of a bridge circuit which provides a voltage output corresponding to the applied pressure. A differential amplifier is required to amplify this signal before delivery to a CRT monitor or pen recorder. Inductive, capacitance, and piezoelectric strain gauges also exist.

Physiological pressure transducers are usually 2-3 cm in diameter and are mounted in a clamp close to the patient and connected to the site of measurement (artery, heart, bladder, lungs etc.) by a narrow tube which is usually filled with water or saline. Special miniature versions also exist which are mounted on the tip of a narrow tube which can be passed directly into the body.

Favourable characteristics for a pressure transducer are high-frequency response, low drift with temperature and time, low compliance, and electrical isolation.