Despite recognized shortcomings, mercury sphygmomanometry remains the gold standard for blood pressure measurement since being introduced by Riva Rocci. Various inherent errors can be attributed to the observer, the instrument itself or the stethoscope. The error is further confounded by the variability of blood pressure and the number of measurements taken. In pregnancy alone, the associated hemodynamic changes relating to peripheral vascular resistance, cardiac output and the use of antihypertensive/ fluid therapy has the potential to influence the characteristics of arterial oscillations and blood flow at a given mean arterial pressure. Subsequently, it can also influence the accuracy of indirect measurement using automated oscillo-metric devices.
Recommendations for measurement in pregnancy are shown in Table 18.3.
Common errors experienced with the mercury sphygmomanometer include oxidized mercury in the glass tubing that obscures clear visualization, incorrect calibration, i.e. the meniscus does not settle at zero before inflation and cracked tubing.
Inappropriate cuff dimensions can also lead to errors in measurement. It is recommended that the length of the cuff should be at least 80% and the width 40% of the circumference measured around the middle of the upper arm (Petrie et al., 1986). Using a cuff that is smaller than this recommendation will result in overestimation of blood pressure by between 5 and 10mmHg for diastolic and 7—13mmHg for systolic pressures (Maxwell et al., 1982). If the opposite occurs, i.e. using a cuff that is too large for the patient, diastolic pressure will be underestimated by 3—5mmHg. It is clear from these examples that ''under-cuffing'' is more of a problem than ''over-cuffing.'' The upper arm should be measured at the approximate mid-point to establish the correct size to use and the arm should be supported at heart level.
The cuff should be inflated rapidly over 3—5 s. The device must be able to reach 200mmHg or 40mmHg above the estimated systolic blood pressure. Sometimes a period of silence is observed between the systolic and diastolic points. This is known as an auscultatory gap. If the cuff is not sufficiently inflated the returning sounds following this gap might be mistaken for the systolic point resulting in underestimation of systolic blood pressure. It is therefore important to initially inflate the cuff while palpating the distal (radial) pulse to determine the approximate systolic pressure prior to auscultation.
A smooth deflation rate of 2—3mmHg per second or pulse (using the control release valve) should be maintained. Too slow deflation may cause sufficient discomfort to the patient to increase the blood pressure (Petrie et al., 1986) and too rapid or jerky deflation will cause under-and over-estimation of the systolic and diastolic blood pressure, respectively.
The stethoscope can also contribute to error in measurement by influencing the quality of sound available to the observer. As the length of the tubing increases, the sound transmission will diminish.
During measurement the observer can be subject to ''digit preference," i.e. rounding the reading to the nearest 5 or 10 mmHg. It is recommended that blood pressure be measured to the nearest 2 mmHg (Petrie et al., 1986). The observer may also choose to ignore blood pressure measurements that would require any action. This is known as threshold avoidance. Other factors that can influence his/her ability to obtain an accurate measurement include concentration, reaction time, hearing and visual acuity.
In pregnancy, posture can play a particularly important role in accurate blood pressure measurement. Measurements are frequently taken with the woman lying in the left lateral position. This is due to the fact that during advanced gestation, lying supine will cause compression of the inferior vena cava by the gravid uterus. Blood pressure seems to be lowest when measured in the left lateral position, using the uppermost arm, and it increases on sitting or standing (Wichman et al., 1984).
When the inherent inaccuracies of indirect blood pressure measurement compared with true intra-arterial pressure are considered and are added to the worst observer practices (measuring blood pressure to the nearest 10mmHg), a diastolic measurement of 90mmHg may be as high as 128mmHg or as low as 74mmHg (de Swiet, 1991). Management based on the extremes of this range would be entirely different.
The debate surrounding the abandonment of mercury in the clinical setting is ongoing. It is a bio-accumulable substance and is toxic to the environment. However, the quantity of mercury contained in sphygmomanometers around the world is not nearly equivalent to that used in the industry and crematoriums, for example. Nevertheless, there is a notion to eliminate its use from the clinical setting.
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