Pressure Measurement

Pressure is defined as force per unit area and may be measured in various units in the clinical setting. Some examples are shown in Figure CM.14.

UNITS IN THE CLINICAL MEASUREMENTOF PRESSURE

Quantity measured

Unit

Blood pressure

mmHg

Airway pressure

cmH2O

Partial pressure of blood gas

kPa

Gas cylinder pressure

Figure CM.14

In anaesthesia, pressure measurements are usually applied to gases (cylinders, anaesthetic machines, breathing circuits) or liquids (intra-arterial pressure monitoring).

Pressures are not usually absolute measurements but are generally measured relative to atmospheric pressure. When interpreting pressure measurements various factors should be considered, such as:

• Transmission path—pressure transducers are often remote from the site at which pressure is sampled. The pressure is transmitted to the transducer by a length of tubing. The dynamic characteristics of this transmission path can significantly affect the final pressure measurements and signal displayed

• Sampling site—pressure may be sampled at a site remote from where the measurement is actually required, due to lack of access. Although static pressures may be equal throughout a closed system, pressures may differ significantly in a dynamic situation. A common example is the measurement of proximal airway pressures, which may not necessarily reflect distal airway pressures

• Static or fluctuating conditions—if the pressure is not varying rapidly it can be considered as static in which case the dynamic characteristics of the transmission path and measuring system may not affect the measurement significantly. This may not be the case when measuring a rapidly fluctuating pressure

Common types of device used to measure pressure in gases or liquids include the aneroid gauge, manometer and piezoresistive strain gauge.

Aneroid Gauge

This type of gauge is a mechanical device that uses the pressure being measured to operate a mechanism coupled to a pointer. It can be used to measure high or low pressures, and is usually employed when measuring pressures greater than one bar.

In the Bourdon gauge the measured pressure is applied to a spiral tube which uncoils as the pressure increases. This uncoiling movement is coupled to a pointer that indicates the pressure. Another form of aneroid mechanism relies on the expansion of a capsule produced by connection to the sampled pressure. This expansion drives a pointer over a scale (Figure CM.15).

• Advantages—simple technology, mechanically robust and convenient to use. Operate in any position and do not require power supply. Suitable for high or low pressures

• Disadvantages—not suitable for very low pressures (< 5 cmH2O). Not easily re-calibrated

Figure CM.15 Aneroid gauge

Manometer

The manometer is the most basic device for measuring pressure and, because of its simplicity, represents a standard method of calibrating other devices. The unknown pressure is measured by balancing it against the pressure due to a column of a liquid (Figure CM.16). The liquids used most commonly are water for lower pressures and mercury for higher pressures. Pressure units are, thus, commonly cmH2O and mmHg. Accuracy and sensitivity can be increased by angling the manometer tubing and using a liquid with a lower density than water (e.g. alcohol). Surface tension between the liquid and the manometer tubing can cause an error, which causes the water manometer to read too high and the mercury manometer to under read.

• Advantages—simplicity of mechanism and no need for calibration. A standard method used to calibrate other techniques of pressure measurement

• Disadvantages—bulkiness of the device and lack of a direct reading

Level Gaugge Pressure
Figure CM. 16 Manometer

Piezoresistive Strain Gauge

This device is based on a semiconductor material with piezoresistive properties that cause it to vary in electrical resistance when subjected to a mechanical strain. The semiconductor is deposited on to the surface of a thin diaphragm that flexes when a pressure difference is applied across it. The distortion of the diaphragm produces a strain in the piezoresistive material that forms one arm of a bridge circuit etched onto the diaphragm. This results in a small signal current from the transducer that can then be amplified and processed.

• Advantages—versatility, since it can be used for measurement of high or low pressures. The electronic signal and display making it suitable for on line display, automated data logging and linking to a computer. It is also easily adaptable for measuring differential pressures since the diaphragm can be mounted with each face of the diaphragm enclosed in its own chamber and isolated from the other. Differential pressures can be used to measure gas flows, with the use of a suitable pneumotachograph head

• Disadvantages—requires power supply and signal processing unit. Susceptible to electrical interference but has to be used in electronically hostile environments (operating theatres and intensive care units)

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