Vital Signs of Blood Pressure Measurement
Studies addressing the measurement of blood pressure with a sphygmomanometer have focused on issues such as the accuracy of indirect blood pressure, palpation versus auscultation cuff size, position of arm during measurements and health care workers technique.
Direct versus Indirect
Several studies have compared direct (intra-arterial) and indirect (auscultation) measurements of blood pressure. There appears to be little significant difference in systolic pressures measured by either method, with differences ranging from 3 mmHg in two studies to 12mmHg in a third.
Differences in diastolic blood pressure are greater, and are influenced by the refer- ence point that is used. When the phase V Korotkoff’s sound is used (disappearance of the sound), both methods provide similar pressures. However when the phase IV Korotkoff’s sound (muffling), is used, auscultated measurements are significantly greater than intra-arterial pressures (see table one). A study in children reported the use of either auscultation or palpation overestimated systolic pressure. See table two for current recommended practice for the measurement of blood pressure.
Palpation versus Auscultation
A comparison between systolic blood pressure measurements taken by auscultation and palpation found both were within 8 mmHg. While palpation has been commonly limited to the measurement of systolic blood pressure, one study reported that diastolic pressures could be accurately palpated using the brachial artery to identify the sharp phase IV Korotkoff’s sound, However, the value of this technique in clinical practice, and its accuracy when used by health care workers, has yet to be demonstrated.
The length and width of the inflatable cuff (bladder) that is used during the measurement of blood pressure may be a source of error. Much of the research has focused on cuff width, (the dimension across along the bladder) as the potential source of this error. The standard width of currently available cuffs is approximately 12cm, with both larger and smaller sizes also available. Studies have shown that the use of a cuff that is too narrow results in an overestimation of blood pressure, and a cuff that is too wide underestimates blood pressure. Length of cuff appears to have little influence on accuracy.
For obese people it has been suggested that large cuffs (15cm width) will be required when the person’s arm circumferences is between 33 -35cm, and a thigh cuff (18cm width) may be needed if the arm circumferences is greater than 41cm. However, difficulties in applying thigh cuffs to large arms have been reported. Cuff width may also be important when measuring blood pressure in neonates and a cuff width equal to approximately 50% of the arm circumference has been recommended.
Arm and Body Position
Comparisons of blood pressures measured in the sitting person with their arm supported horizontally or with the arm resting at their side, have found an average difference in systolic pressure of 11mmHg and diastolic pressure of 12mmHg. When the arm was placed above or below the level of the heart, blood pressure measurements changed by as much as 20mmHg. As a result of this, it has been recommended that blood pressures be taken in the sitting position with arm supported horizontally at approximately heart level.
Bell versus Diaphragm
The accuracy of blood pressures measured with the bell or the diaphragm of the stethoscope have been investigated. One study found the bell of the stethoscope resulted in higher readings than those taken using the diaphragm. These results were supported by another study, with researchers recommending the use of the bell for all blood pressure measurements.
Health Care Workers Technique
The technique used by health care workers to measure blood pressure has been shown to differ from recommended practice. Using the American Heart Association Guidelines as the standard, one study found that 57% of nursing students failed to comply with these guidelines in areas such as cuff placement, estimation of systolic pressure by palpation, calculation of proper inflation pressure, and proper stethoscope placement. Another study of 172 health care workers concluded that nurses and physicians evaluated blood pressure in an inadequate, incorrect and inaccurate way, and that only 3% of general practitioners and 2% of nurses obtained reliable results. Two studies evaluating the impact of education programs on blood pressure measurement, found they increased agreement between the different blood pressure readings and also significantly reduced differences in operator technique.
A descriptive study of blood pressures in critically ill patients who had suffered a cardiac arrest highlighted some limitations to these measurements. Of the 15 patients investigated, 5 patients had adequate intra-arterial blood pressures, but unreadable cuff pressures. Four patients had cuff pressures approaching normal, but had an inadequate cardiac output. This study suggests that indirect blood pressure measurements do not always accurately reflect haemodynamic status of critically ill people.