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Self-measurement of blood pressure: a paper for health professionals Prepared for the National Blood Pressure Advisory Committee by Dr Michael Stowasser PhD, FRACP, MBBS, NHF Postdoctoral Fellow, Robin Armstrong RN, BHSc, Hypertension Nurse, Hypertension Unit, Greenslopes Hospital, Brisbane, Queensland. The advantages of self-measurement Patient self-measured blood pressure levels can complement those measured by the doctor to give a more complete, accurate picture of the patients blood pressure, and help doctors to make more informed therapeutic decisions. Blood pressure levels measured by the patient at home or at work are usually lower than those measured by the doctor in the clinic setting (the white coat effect ), and more closely approximate levels measured by 24-hour ambulatory blood pressure monitoring (ABPM)1,2. Evidence from cross-sectional studies suggests that self-measured blood pressure levels correlate more closely than clinic readings with target-organ damage1,3. One longitudinal study has also demonstrated tighter correlation of self-measured blood pressure with regression of left ventricular hypertrophy in response to treatment4. Data from prospective outcome trials which are becoming available are confirming that outcome is better predicted by ambulatory blood pressure than by clinic measurements. Among patients diagnosed with mild hypertension on the basis of clinic readings, approximately one in five exhibit normal blood pressure levels when measured by the patient at home 5. Such individuals with white coat hypertension appear to be at much lower risk of cardiovascular morbidity and mortality than those with sustained hypertension 6,7. Most investigators believe that lifestyle modification is sufficient intervention and antihypertensive drug treatment can be safely avoided in these patients so long as home blood pressures remain normal and evidence of target-organ damage (eg. left ventricular hypertrophy) does not develop during close long-term follow-up8. Because of the white coat effect , and because blood pressure varies markedly from moment to moment, a single clinic measurement is unlikely to be closely representative of the level throughout the day or on other days. When patients measure their own blood pressure at several specific times of the day for a week, many readings can be averaged to give a more complete picture of the blood pressure under conditions of everyday living. Problems such as postural hypotension can be better defined, the effect of changes in antihypertensive medications more fully assessed, and changing trends in blood pressure levels more quickly detected. In addition, self-measurement provides the opportunity to confirm or exclude any suspected relationship between episodic symptoms and periods of high or low blood pressure (eg. light-headedness thought to be caused by medication-induced hypotension).

Self-measurement of blood pressure promotes better patient understanding of the disease and the effects of treatment, and allows the patient to become more involved in the management of their condition, resulting in improved compliance9,10. Most patients are able to take their own blood pressures confidently after adequate instruction11 without fear or anxiety12. In fact, the demonstration of well-controlled self-measured blood pressure in the face of elevated clinic readings is a source of reassurance to them. Self-measurement of blood pressure has been estimated to be the most costeffective means of assessing hypertension13, and, unlike 24-hour ABPM, is particularly useful in the long-term follow-up of patients with both sustained and white coat hypertension. Levels measured by patients living in remote regions, precluding frequent clinic attendances, can be sent to the doctor together with any comments and questions for assessment. By reducing over-treatment consequent on the white coat effect, reducing visits to the doctor and improving prognosis, the potential for cost savings are considerable. Guidelines for valid blood pressure self-measurement A good technique and an accurate device are critical to the value of BP selfmeasurement. Prior to commencing self-measurement, patients should undergo a practical instruction session by trained personnel, followed by an assessment of their accuracy and competence. They should be provided with written instructions for reference at home. Follow-up sessions are important to ensure that the patient has grasped the procedure and adapted it to his/her home environment, and, in the long term, to prevent bad habits in technique from developing. (i) Equipment Devices used for home BP measurement are usually aneroid or electronic in type. In general, the aneroid machines are very accurate14,15 provided they are regularly calibrated (every 6 to 12 months) against a mercury sphygmomanometer16. Calibration can be readily performed using a yconnection and two pieces of blood pressure tubing16. One piece connects to the gauge of the aneroid machine to one arm of the y-connection, and should be the same length as the tubing coming from the mercury column, which is connected to the other arm of the y-piece. The stem of the y-piece is then connected to the inflation bulb of the mercury machine, using the second piece of tubing. The mercury column is then pressurised to 300 millimetres of mercury and slowly reduced, stopping at intervals of no more than 30 mm Hg17 to compare the pressures indicated on each sphygmomanometer. Gauges registering greater than 3 millimetres of mercury difference from the mercury sphygmomanometer should be repaired or replaced18. Inability to hold the rate of deflation at 1mm Hg per second or less when using an aneroid machine may herald the need to repair or replace a faulty exhaust valve screw or leaks in the rubber bladder. Aneroid machines are relatively inexpensive and easier to maintain and repair than

electronic devices. While their use requires more training, most people master the technique with little difficulty11 and can retain their skills over time. Occasionally, patients are unable to use an aneroid machine because of problems with memory, hearing or vision deficits, or lack of the fine motor control required to regulate the rate of deflation. In such situations, alternative valves can be fitted, a friend or relative can be taught to take the patient s blood pressure, or a reliable electronic device can be used. Electronic devices vary widely in cost and reliability19. It is essential that all electronic devices be validated against a mercury sphygmomanometer before use as well as periodically (6 to 12 monthly) during use as the machine ages. This is best done using the simultaneous same arm approach 8, 19, in which a mercury column is connected by a y-piece to the cuff tubing and a stethoscope applied over the brachial artery, allowing an observer to take a mercury reading at the same time and on the same arm that the electronic device takes its measurement. The average difference from the mercury reading after several measurements should be less than 4 mm Hg8. It is important that validation be performed on the patient who uses the device. Also, the deflation rate of the device should be controlled at 2-3mm Hg per second all the way down to zero16. An overly rapid deflation rate can result in serious measurement errors and can impede validation of the device, especially in patients with relatively slow pulse rates. Whichever type of machine is used, care should be taken to ensure that the cuff size is appropriate for the patient. The bladder width should be at least 40%, and the bladder length at least 80%17,20 of the arm circumference at midpoint between the acromion process and olecranon. (ii) The technique Patients should be instructed to take measurements in a warm, comfortable, quiet place without the distractions of television, loud music or people talking, and to wait for at least 5 minutes after sitting or lying and at least two minutes after standing before taking their blood pressure. Measurements should not be taken within two hours of drinking tea or coffee, or smoking cigarettes if the patient has not yet given up, as these substances can acutely raise blood pressure. Whenever possible, the blood pressure should be measured under the same quiet conditions and in the same manner every time. For all machines, patients should be shown how to apply the cuff correctly and to position it so that the pressure-sensing device is placed directly over the brachial artery with the lower border of the cuff approximately 3-4 cm above the elbow crease. For aneroid machines, patients should also be taught: how to use the stethoscope and identify Korotkoff sounds

how to use the valve screw and to manipulate it finely during deflation in order to regulate the deflation rate at 2-3 mm Hg per second the importance of the systolic gap 6 and how to detect this by listening to Korotkoff sounds during the inflation phase and inflating the cuff to 40 mm Hg above the point where the sounds disappear before commencing the deflation phase to wait for several seconds before deflating the cuff to allow for any temporary rise in blood pressure which may occur in association with the action of the cuff inflation. Because blood pressure varies from moment to moment, many readings are required to get an overall picture21,22. For example, patients may be instructed to take lying and standing measurements in the morning before rising, then 2-4 hours after taking their morning medication, and lastly before retiring at night. The average reading for each time and position over the entire week can then be calculated, which is useful for detecting a diurnal pattern or postural effect. Once it has been established that the levels are stable and under control, the frequency of readings can be reduced. However, it is preferable that patients continue to take a full week s readings at a time and progressively increase the interval between measurement weeks rather than to increase the interval between each measurement time or day. During the weeks when they are not taking a full week s readings, patients should be advised to take their blood pressure two or three times a week to keep up the skills necessary for accurate measurement. Blood pressure measurements should ideally be recorded immediately on a chart which indicates the date, time of day and body position when the recorded blood pressure was taken. The chart should provide space to allow the patient to record their medications and dosages, any comments such as symptoms or stressful events that may be relevant when interpreting the recorded readings, and the average systolic and diastolic blood pressures calculated for each week of readings. The blood pressure chart can be taken to the doctor at each clinic visit, or sent if necessary in between visits, and will help the doctor to assess if blood pressure control is adequate, or whether changes to treatment are necessary. The effects of such changes can then be assessed by examining subsequent readings recorded on the chart. References 1.

Kleinert HD, Harshfield GA, Pickering TG, et al. What is the value of home blood pressure measurement in patients with mild hypertension? Hypertension 1984; 6:574-8.

2.

Sega R, Cesana G, Milesi C, et al. Ambulatory and home based blood pressure normality in the elderly. Data from the PAMELA study. Hypertension 1997; 30:1-6.

3.

Verdecchia P, Bentivoglia M, Providenzia M, et al. Reliability of self recorded and arterial pressure in essential hypertension in relation to the stage of the disease. In Germano G ed. Blood Pressure Recording in the Clinical Management of Hypertension. Rome. Ediziono Pozzi, 1985, pp 402.

4.

Mancia G, Zanchetti A, Agebiti-Rosei, et al. Ambulatory blood pressure is superior to clinic blood pressure in predicting treatment-induced regression of left ventricular hypertrophy. Circulation 1997; 95:1464-70.

5.

Pickering TG, James GD, Boddie C, et al. How common is the white coat hypertension? J Am Med Assoc 1998; 259:225-8.

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Palatini P, Mormino P, Santonastaso M, et al. Target-organ damage in stage 1 hypertensive subjects with white coat and sustained hypertension. Results from the HARVEST study. Hypertension 1998; 31:57-63.

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Verdecchia P, Portellati C, Schillaci G, et al. Ambulatory blood pressure: an independent predictor of prognosis is essential hypertension. Hypertension 1994; 24:793-801.

8.

Pickering TG. Recommendations for the use of home (self) and ambulatory blood pressure monitoring. Am J Hypertens 1995; 9:1-11.

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Haynes RB, Sackett DL, Gibson ES, et al. Improvement of medication compliance in uncontrolled hypertension. Lancet 1976; 1:1265-8.

10. Edmonds D, Foerster E, Groth H, et al. Does self measurement of blood pressure improve patient compliance in hypertension? J Hypertens 1985; 3:31-4. 11. Armstrong R, Barrack D, Gordon R. Patients achieve accurate home blood pressure measurement following instruction. Aust J Adv Nursing 1995; 12: 15-21. 12. Burns-Cox CJ, Rees JR, Wilson RSE. Pilot study of home measurement of blood pressure by hypertensive patients. Brit Med J 1975; 2-80. 13. Wilson MD, Johnson KA. Hypertension management in managed care: the role of home blood pressure monitoring. Blood Pressure Monitoring 1997; 2:201-6. 14. Zachariah PK, Sheps SG, Smith RL. Clinical use of home and ambulatory blood pressure monitoring. Mayo Clin Proc 1989; 64:1436-46.

15. van Egmond J, Lenders JWM, Weernik E, Thien T. Accuracy and reproducability of 30 devices for self-measurement of arterial blood pressure. Am J Hypertens 1993; 6:873-9. 16. Perloff D, Grim C, Flack J, et al. Human blood pressure determination by sphygmomanometry. Circulation 1993; 88:2460-8. 17. American National Standard: Non-automated sphygmomanometers. Association for the Advancement of Medical Instrumentation. 1994. 18. Australian Standard: Sphygmomanometers. Standards Australia, 1989. 19. Mann S. Inaccuracy of electronic sphygmomanometers. Clin Exper Pharmacol Physiol 1992; 19:304-6. 20. O Brien ET, Petrie JC, Littler WA, et al. Blood pressure measurement: recommendations of the British Hypertension Society. 3rd edition. British Medical Journal Publishing Group, London, 1997, pp 6, 17-18. 21. Gordon RD, Ziesak M. Home blood pressure measurement. Curr Concepts Hypertens Cardiovasc Disorders 1981; 2:6-13. 22. Stergiou GS, Skeva II, Zourbaki AS, et al. Self-monitoring of blood pressure at home: how many measurements are needed? J Hypertens 1998; 16:72531. September 1999/PP322