Signal-Processed Pressure Measurement (Noninvasive
Blood Pressure Monitor)
Systolic pressure can be estimated by noting return of the flow
pulse after occlusion of the artery by a cuff. The return of flow can be detected
by (1) simple palpation of the radial artery, (2) recording with a Doppler device
over the radial artery, or (3) the use of a pulse oximeter. Most anesthesiologists
are familiar with the loss of pulse oximeter signal when the noninvasive blood pressure
monitor is cycling.
The automated noninvasive blood pressure monitoring devices used
in most operating rooms make use of a more sophisticated application of this principle.
These devices monitor the oscillating signal generated in the cuff by the arterial
pressure changes. The cuff first inflates to above systolic pressure, at which point
the signal and oscillations are abolished. Then the cuff slowly deflates in a stepwise
fashion. The pressure at which the signal first appears is interpreted as the systolic
pressure. The signal increases in amplitude as the cuff pressure decreases. The
point at which the signal is at maximal amplitude is interpreted as mean arterial
pressure. As cuff pressure decreases further, the oscillations drop off rapidly.
Diastolic pressure is mathematically inferred from the systolic and mean values
( Fig. 30-15
).[8]
Errors can be introduced in the same manner as for manual auscultation of Korotkoff
sounds: too small or too large a cuff requires a higher or a lower pressure to occlude
arterial flow, and stiff atherosclerotic arteries are resistant to compression.
External compression caused by patient motion or the surgeon leaning on the noninvasive
blood pressure monitoring cuff can cause oscillations in cuff pressure that are not
related to arterial pressure and may result in an erroneous reading, most commonly
a high diastolic pressure.
Mercury sphygmomanometers are being phased out of use in some
countries and hospitals, which leads to questions regarding the accuracy and precision
of alternative devices such as the aforementioned noninvasive automated blood pressure
monitors and aneroid sphygmomanometers.[9]
[10]
Figure 30-15
Noninvasive blood pressure measurement. Using the signal
from the arterial pulse, oscillometric blood pressure measurements are obtained by
determining the point at which the signal is first detected, its maximal amplitude,
and the signal decay rate.[8]
(Adapted
from Ehrenwerth J, Eisenkraft J: Anesthesia Equipment: Principles and Applications.
St Louis, Mosby-Year Book, 1993.)
