Many factors affect blood pressure monitoring and measurement, including patient factors such as stress or activity level; equipment factors, such as calibration of monitoring equipment; and operator factors, or technique in blood pressure measurement. Accurate assessment of blood pressure requires consideration of all of these factors to provide the most accurate measurement of blood pressure at any point in time and the most useful information in monitoring over time.
Significance
Physicians need accurate blood pressure measurements to diagnose and prescribe antihypertensive medication in the right dose and monitor its effects. When blood pressure is monitored over time, accurate, consistent measurement provides assurance that changes in blood pressure are the result of changes in therapy or changes in the patient’s condition, rather than inaccurate or inconsistent measurement.
Technique
To capture the first sound and systolic blood pressure measurement, the nurse or physician inflates the cuff on the sphygmomanometer to 30mm Hg above the point where the brachial or radial pulse is no longer palpable. Slow deflation at a rate not exceeding 2mm Hg per second allows accurate auscultation of the second sound, the diastolic blood pressure. Faster deflation rates can result in erroneously low systolic readings and erroneously high diastolic readings.
Equipment
The mercury sphygmomanometer has been the mainstay of blood pressure measurement since it was first introduced to clinical medicine by Scipione Riva-Rocci in 1896. However, increased concern about the potential for mercury poisoning and the widespread availability of automated blood pressure monitoring devices since the late 1990s has all but made the mercury sphygmomanometer obsolete.
Aneroid sphygmomanometers provide a cost-effective alternative to mercury sphygmomanometers and still widely used in health care settings. Aneroid sphygmomanometers consist of a cuff, inflatable bladder like a mercury sphygmomanometer and a round pressure gauge with a needle indicator. Aneroid, as well as mercury manometers offer the advantage of direct auscultation of the Kortkoff sounds which indicate the systolic and diastolic blood pressure. Aneroid as well as mercury sphygmomanometers require regular calibration for accuracy and inspection of the cuffs and tubing for defects.
According to The Sustainable Hospitals Project, electronic blood pressure monitoring devices may be slightly less accurate than aneroid or mercury sphygmomanometers, but easier to use and more reliable. Mercury manometers are still necessary for verifying the accuracy of all non-mercury blood pressure measuring devices, according to the American Heart Association.
Considerations
For a resting blood pressure, measurement is taken after a patient has been sitting comfortably in a chair for at least five minutes, his feet resting flat on the floor. Crossing the legs can increase blood pressure as much as 2 to 8mm Hg, notes the AHA. Alternatively, blood pressure may be measured with the patient in a supine position, the arm elevated on a pillow to cause it to be at the level of the right atrium.
Arm placement, relative to the heart affects blood pressure readings. Measurements will be erroneously high if the arm is held below the level of the right atrium of the heart, and low if the arm is held above this level. Erroneously high readings may also result if the patient holds his arm up during measurement.
Cuff size, cuff placement and rate of deflation also affect blood pressure readings. Too narrow a cuff results in an erroneously high reading and too wide of a cuff results in an inaccurately low reading. Error in measurement is minimized when cuff width is 46 percent of the arm circumference. For greatest accuracy, pressure on the cuff should be released at a rate of no more than 2 to 3mm Hg per second, advises the AHA.
Time Frame
The AHA recommends that at least two blood pressure measurements be taken a minute apart and the two readings averaged for the most accurate assessment of a person’s blood pressure. If there is more than a 5mm Hg difference in readings, the AHA recommends one or two additional readings. The average of all measurements should be recorded as the patient’s blood pressure reading. The frequency with which blood pressure measurement occurs varies according to an individual’s health status. In critical care, for example, continuous blood pressure via an arterial catheter and electronic monitoring system provides health care professionals with up-to-the minute information about blood pressure.
References
- National Guideline Clearinghouse: Recommendations for Blood Pressure Measurement in Humans and Experimental Animals
- NHLBI: National High Blood Pressure Education Program
- "The Annals of Pharmacotherapy": Timing of Blood Pressure Measurement Related to Caffeine Consumption
- American Heart Association: Miscuffing A Problem with New Guidelines
- Sustainable Hospitals: Comparing Mercury and Aneroid Sphygmomanometers
