Sources of Error in Indirect Methods

In comparison with direct methods of blood pressure measurement (the 'Gold Standard'), indirect methods tend slightly to under read, with the diastolic pressure showing the greatest degree of variability. The sources of error include:

• Detection of Korotkov sounds, which are complex with a large proportion of the sound energy being below the audible range. This reduces sound transmission to the observer. In addition, observer detection of the sounds will be dependent on aural acuity. The generated sounds are flow dependent and, thus, factors affecting flow can introduce inaccuracy (e.g. in high output states, and post exercise, Phase V may not occur)

• Cuff size—width of the cuff effects the measured value of blood pressure; too narrow, then there is a tendency to over estimate, too wide, then there is a tendency to under estimate. As a consequence, there have been efforts to standardize the widths of blood pressure cuffs. The World Health Organisation (WHO) recommends that adult cuffs should be 14 cm wide, and should cover two-thirds of the length of the upper arm, or its width should be 20% greater than the diameter of the arm. Suggested widths are shown in Figure CM.18

• Zero and calibration errors, particularly in aneroid devices

• Pneumatic leaks

RECOMMENDED CUFF WIDTHS FORDIFFERENT AGES

Age (years)

Cuff width (cm)

Adult

12-14

4-8

9

1-4

6

Neonate

Figure CM.18

• Speed of deflation—when too fast, then there is insufficient time to detect audible change Direct Method (Intra-Arterial Pressure Monitoring)

Provides an invasive, continuous measure of blood pressure by beat-to-beat reproduction of the arterial pressure waveform. It is particularly useful in the following situations:

• Cardiovascular instability

• Where blood pressure manipulation is required (inotropes or vasodilators)

• Where non invasive blood pressure measurement is likely to be difficult and/or inaccurate (obesity)

The method requires the insertion of a short parallelsided cannula into an artery. A continuous flow of either saline or heparinized saline at rates between 1 and 4 ml/H is used to reduce clot formation in the cannula. The cannula is connected by a short length of narrow bore, non compliant plastic tubing containing saline to a pressure transducer, which is usually of the piezoresistive strain gauge type. More recently, catheter tip pressure transducers have been developed but remain comparatively expensive. The piezoresistive strain gauge produces a low amplitude signal requiring signal processing before analysis and display.

The design of an intra-arterial pressure monitoring system must take into account the following considerations:

• The frequency and phase shift responses of the system has to be adequate to allow good reproduction of the arterial signal. An approximate guide is that acceptable accuracy requires a frequency response extending to 810 times the maximum heart rate expected. In man, the most important information is contained within the frequency range 0-20 Hz. The system can, thus, be designed to have an upper cutoff frequency > 20 Hz

• The transducer and connecting tubing should be chosen to avoid natural frequencies or resonances occurring within the desired frequency response. Mechanical resonances due to the properties (compliance and inertial elements) of the transducer and column of saline in the connecting tubing, can be shifted above the desired cutoff frequency by reducing the diameter of the connecting tubing

• Components must also be chosen to provide the optimum degree of damping. Usually 'critical damping' is aimed for but since frequency response, phase shift response and damping requirements may conflict a compromise may have to be arrived at. It is important to be able to recognize abnormal levels of damping to interpret the arterial waveforms appropriately. Figure CM.19 illustrates the effect of damping on arterial pressure waveforms

Aortic Pressure Waveforms Dampening
Figure CM.19 Measurement of system damping

• Advantages—provides a continuous display of the pressure wave form, providing an immediate assessment of blood pressure which is regarded as the 'Gold Standard'

• Disadvantages—cannulation can be difficult, particularly in low-output states, and may require consideration of multiple sites before success. Disconnection: if unrecognized this may result in serious blood loss and, in the extreme, exsanguination. Infection is particularly relevant in cases of prolonged use. Distal vascular insufficiency may result directly from cannulation, or arise from subsequent thrombosis of the cannulated artery. This risk is increased by insufficient collateral circulation, which should always be checked before cannulation. Emboli (air or thrombus) can cause distal vascular occlusion

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