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The Best BP

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Vol. 7 •Issue 26 • Page 35
The Best BP

Eight tips to a more accurate blood pressure

Taking blood pressures is a daily nursing task, yet not all nurses follow the recommended procedure for taking blood pressures.

In one study, nurses and nursing assistants performed less than 50 percent of the steps recommended in the blood procedure.1 Only 40 percent of the nurses and nursing assistants participating in the study followed the recommended procedure for taking blood pressures. The percentage of participating nursing students and instructors using correct procedures was 70 percent.1 This finding was confirmed by another study which showed that only 61 percent of the nurses conformed to the current standards for blood pressure monitoring and only 51 percent could correctly identify faulty equipment.2

Factors Influencing BP

Besides the failure to follow the recommended procedure for taking blood pressure, specific user errors are described in the research. The most commonly occurring error was the selection of an inappropriately sized cuff. Another common error was the preference for a terminal zero.1 More than three-quarters of the nursing personnel participating in a study recorded the blood pressures they took as ending with zero for both the systolic and diastolic blood pressures.1

In addition to user error, incomplete patient preparation can lead to inaccurate blood pressure readings. Patient anxiety, talking during blood pressure measurement, recent cigarette smoking and recent coffee intake may alter the blood pressure. Anxiety can elevate the blood pressure as much as 30 mm Hg due to the "fight or flight" reaction.3

Other patient factors can influence blood pressure readings as well. Some of these factors are modifiable and some are not. Blood pressure varies during the day according to the circadian rhythm with the lowest blood pressures being recorded while the patient is sleeping. Blood pressure also is influenced by age and race. Other patient factors that can affect blood pressure readings include pain, alcohol use, meals, temperature and bladder distension.3

User error and improper patient preparation may result in an increase in systolic blood pressure of as much as 20 mm Hg, which may result in the inappropriate diagnosis and treatment of hypertension.3 Therefore, the decision to treat hypertension should not be based on a single elevated reading but rather on the mean of three readings.4

8 Tips for Increasing Accuracy

The following 8 tips can help you increase the accuracy of the blood pressures you take.

Position the patient correctly. Different patient positions affect blood pressure readings. For example, diastolic blood pressures are approximately 5 mm Hg higher when the patient is supine. So to standardize blood pressures, standardize the position of your patient.5

Patients should be seated in a chair with their back and legs supported for 5 minutes before taking a sitting position with their feet flat on the floor. An elevation in both systolic and diastolic blood pressures occurs when the patient crosses his legs at the knee.6,7

Positioning is important to blood pressure readings with the patient in the supine position. Allow the patient to rest at least 1 minute in the supine position before taking his blood pressure. Even the supine patient must have his arm elevated to the level of the right atrium. An easy way to elevate the supine patient's arm to the appropriate height for an accurate blood pressure is to prop the patient's arm on a pillow.

Frequently, patients need to be monitored for orthostatic hypotension. Detecting orthostatic blood pressure requires changing the patient's position. Take the first blood pressure with the patient supine and then repeat the blood pressure with the patient standing. Allow the patient to stand no more than 3 minutes prior to repeating the blood pressure. Orthostatic blood pressure occurs when there is a drop of 20 mm Hg in the systolic blood pressure or 10 mm Hg in the diastolic blood pressure.5

Use only well-maintained blood pressure equipment. Only use cuffs that are in good condition. Discard the cuff if the Velcro no longer grips the cuff or if the tubing is cracked and has air leaks. You cannot measure accurate blood pressures with a cuff that has air leaks.5

Use aneroid and electronic blood pressure units that have been tested for accuracy. Electronic and aneroid blood pressure units used in place of the mercury sphygmomanometer require preventive maintenance to ensure ongoing accuracy. Problems can occur with calibration and validation. Calibration ensures that the unit starts at zero. If the machine is not accurately calibrated, the blood results may be higher or lower than the patient's actual blood pressure. Validation makes sure that the blood pressure results are accurate over a wide range of blood pressures, clinical conditions and ages.8

Use the appropriately sized cuff. Errors can occur when the cuff is not the appropriate size. The ideal cuff size has a bladder that is 80 percent of the width of the arm and 40 percent of the circumference of the arm. A cuff that is too long in relation to the arm circumference results in an underestimation of the blood pressure. A cuff that is too short for the arm circumference results in a false high blood pressure. Using a cuff that is too small may result in the inappropriate diagnosis of hypertension.5

Obese patients and patients with prominent biceps may require a longer and wider cuff to obtain an accurate blood pressure.5 Often the nurse or nursing assistant will take the blood pressure on the lower arm instead of getting a larger cuff. This technique results in an overestimation of blood pressure and may falsely increase the prevalence of hypertension in the obese.9

Use the arm with the higher blood pressure. Every new patient should have his blood pressure taken on both arms. Even women who have had a mastectomy can have their blood pressure taken on both arms unless they have lymphedema. Some patients have differences of up to 10 mm Hg between the two arms. For ongoing blood pressure monitoring, take the blood pressure on the arm with the higher blood pressure.5

Apply the cuff correctly. Roll up the patient's sleeve and apply the cuff with the midline of the cuff over the palpated brachial artery. When you roll the sleeve, be careful not to create a tourniquet effect. Place the cuff snuggly on the arm. If the patient does not have a brachial artery due to peripheral obstructive disease, then a reliable blood pressure is not possible.5

Support the patient's arm at heart level. Arm position has a major effect on blood pressure readings. The best position for the arm is supported at heart level or to the middle of the sternum. If the patient's arm is left hanging at his side when the blood pressure is taken, then the blood pressure will be increased.10 Allowing the patient to rest the arm on the arm rest of his chair also will raise the blood pressure.11 Raising the patient's arm above the level of the heart will decrease the blood pressure. If the patient holds his own arm at heart level then the blood pressure reading will be higher due to isometric activity. This artificial rise in blood pressure is increased when the patient has hypertension or is on a beta blocker.

Deflate the cuff slowly. When taking a manual blood pressure it is essential to deflate the cuff at a speed of 2 mm/sec. Faster deflation will lead to a significant underestimation of systolic blood pressure and an overestimation of diastolic blood pressure.5

Repeat the blood pressure. Multiple blood pressure readings are better than one, so retake the blood pressure after at least 1 minute has passed and then average the readings. The first blood pressure reading is usually the highest. If the difference between the first two readings is greater than 5 mm Hg then repeat the blood pressure several times. At least 1 minute should elapse between these serial blood pressure readings. This repetition of blood pressure readings is especially important in the elderly, who have an increased risk for isolated systolic hypertension (systolic BP >140 with diastolic BP <90) or pseudohypertension (the muscular layer of peripheral arteries become rigid because of advanced arteriosclerosis). In both these cases, the cuff must be pumped higher to compress the brachial artery.5

Obtaining accurate blood pressures is an essential nursing skill. Use these techniques recommended by the American Heart Association to improve the accuracy of the blood pressures you take.

References

1. Veiga, E., et al. (2003). Assessment of the techniques of blood pressure measurement by health procedures. Arquivos Brasileiros de Cardiologia, 80(1), 89-93.

2. Armstrong, R.S. (2002). Nurses' knowledge of error in blood pressure measurement technique. International Journal of Nursing Practice, 89(3), 118-126.

3. O'Brien, E., Beevers, G., & Lip, G. (2001). Blood pressure measurement: Part I: Sphygomomanometry: Factors common to all techniques. British Medical Journal, 322, 981-985.

4. Marshall, T. (2004). When measurements are misleading: modeling the effects of blood pressure misclassification in the English population, British Medical Journal, 328, 933.

5. Pickering, T., et al. (2005). Recommendations for blood pressure measurement in humans and experimental animals. Circulation, 111, 697-716.

6. Foster-Fitzpatrick, L., et al. (1999). The effects of crossed leg on blood pressure measurement. Nursing Research, 48(2), 105-108.

7. Keele-Smith, R., & Price-Daniel, C . (2001). Effects of crossing legs on blood pressure measurement. Clinical Nursing Research, 10(2), 202-213.

8. Lewis, C. (2002, September-October). Checking up on blood pressure monitors. FDA Consumer Magazine. Retrieved Aug. 7, 2005 from the World Wide Web: http://www.fda.gov/fdac/features/2002/502_hbp.html

9. Pierin, A., et al. (2004). Blood pressure measurement in obese patients: Comparison between upper arm and forearm measurements. Blood Pressure Monitoring, 9(3), 101-105.

10. Hemmingway, T., Guss, D., & Abdelnur, D. (2004). Arm position and blood pressure measurement. Annals of Internal Medicine, 140(1), 74-75.

11. Netea, R., et al. (1999). Arm position is important for blood pressure measurement. Journal of Human Hypertension, 13(2), 105-109.

Fran Kestel is a clinical educator at Albert Einstein Healthcare Network, Philadelphia.




     

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