Blood pressure measurement in primary care

1. Blood pressure measurement in primary care Frank Lefevre MD Associate Professor of Medicine Division of General Internal Medicine, Northwestern Feinberg School of Medicine

2. Rationale • Achieving optimal outcomes in the treatment of HTN requires accurate BP assessment • Current practice patterns for measuring BP are suboptimal

3. Overview • How accurate are various methods of BP measurement? • Office BP measurement • Out of office BP measurement • Patient self-measurement • Ambulatory BP monitoring • Can the use of out of office BP measurements improve outcomes? • Diagnosing HTN • Monitoring treatment • How can BP measurement be improved?

4. Sources of error in BP measurement • Measurement error • Random variability • White coat effect

5. Patient should be: seated in relaxed environment for 5min Legs resting on floor Back supported No conversation Bare arm supported on table, midpoint of upper arm at level of heart Examiner technique: Place cuff 1-2cm above antecubital fossa, Inflate cuff, palpate to estimate SBP Place bell of stethoscope over brachial artery, do not wedge under cuff Inflate cuff 20-30mm above estimated SBP Deflate at 2mm/sec, listen for Karatkov sounds Allow subject to rest for at least 30sec Repeat measurement and take average of both measurements Standardized BP measurement(AHA guidelines, Circulation, 1993;88:2460)

6. Do MD’s follow standardized approach?(McKay et al, J Hum Hyper, 1990;4:639) • Observation of 114 primary care physicians • Assessed potential for measurement error • Accuracy of sphygmomanometers • 40% off by  4mm; 30% off by 10mm • Physician technique

7. Impact of errors in technique(McAlister et al, BMJ, 2001;322:908)

8. Effect of random variability on the diagnosis of HTN(Mar et al, J Med Dec Mak, 1998) • Modeling study • Simulated predictive value of diagnosing mild HTN with 3 measurements (office BP measure), as compared to 24 measurements (ambulatory BP measure), accounting for random variability • Did not consider white coat effect or measurement error PPV • 3 BP measurements: 0.64 • 24 BP measurements: 0.84

9. The “white coat effect” (WCE) • Generally defined as: (office BP - out of office BP) • Alerting response causing acutely elevated BP • May be large; up to 40% of pts have WCE > 20/10mm Hg • Magnitude dependent on number of office readings • Larger magnitude: • Taken by physician • Older patients • Higher baseline pressure

10. Difference in SBP readings between clinic BP and ABPM

11. Comparative accuracy of different methods of BP measurement • Lack of true gold standard • Accuracy estimated by: • Predictive ability for future CV events (prospective studies) • degree of correlation with hypertensive end-organ damage (cross-sectional studies)

12. Prospective cohort studies(Perloff et al 1989: • 1,079 patients with essential HTN followed for 5.5 years. • Classified patients as ABP higher than predicted by office BP, same, or lower than predicted: • Patients with ABP lower than predicted had more favorable prognosis • Major limitations: • Did not specifically evaluate patients with WC HTN • Confounding by treatment of patients with “WC HTN” • Failed to consider covariates contributing to cardiac events

13. Prospective cohort studies (Verdecchia et al 1989) • 1,187 patients with essential HTN followed for mean of 3.2 years. • WC HTN defined as office BP >140/90 and ABP<136/87 (men) or 131/86 (women); n=228. • Compared with 205 healthy normotensive patients ACE/100 pt-yrs • True HTN: 1.79 • WC HTN: 0.49 • Normotensive: 0.47 • Major limitation - confounding by treatment in WC HTN group.

14. Prospective cohort studies (Khattar 1998) • : Longest cohort study: • 479 patients followed for over 9 years from one center in UK; 126 patients with WC HTN • rate of adverse cardiovascular events for WC HTN compared with sustained HTN ACE’s/100 pt-yrs • WC HTN 1.32 • Sustained HTN 2.56 • Major limitations: • no comparison with normotensive group • confounding by treatment (82% WC pts treated)

15. Cross-sectional studies • Numerous studies comparing accuracy of ABP and office BP by comparing correlation with end-organ damage (LVM) • Meta-analysis of 21 studies (Fagard et al 1995): Correlation with LVM • Ambulatory BP: r = 0.50 • Office BP: r = 0.35

16. Ambulatory BP monitoring – de facto gold standard?

17. Limitations of ABP monitoring • No good epidemiologic benchmarks for determining treatment threshold • Virtually all studies of treatment and prognosis have used office BP readings • One epidemiologic study of ABP/prognosis (Okhubo et al 1998): • Population based study of ABP and prognosis • 1542 patients from one city in Japan followed for 6.2 years • Ambulatory BP associated with best prognosis: • 120-133mmHg systolic • 65-78mmHg diastolic

18. Interpretation of ABPM results(Adapted from Okhubo et al, Hyperten, 1998;32:255)

19. Patient self-monitoring • Accuracy approaches that of ABPM in groups of patients in research studies • Accuracy/validity of measurements in individual patient less certain • Validity may vary by whether used for diagnosis of HTN vs management of known HTN

20. Comparison of office, home and ambulatory BP’s

21. Difference between clinic and self- measured blood pressure(Adapted from

22. Difference between ABPM and self- measurement of BP(Adapted from

23. Methods: 91 volunteer patients self-measured BP in the presence of trained technician Accuracy defined as systolic and diastolic BP within 10mm of values recorded by technician Results: 66% accurate 34% inacurrate Clinical and demographic factors not predictive of accuracy How often do individual patients get inaccurate self-readings?(Merrick et al, South Med J, 1997;90:1110

24. Performance characteristics for SBP measurement in diagnosing HTN (Little et al, BMJ, 2002;325:254)

25. RCT’s comparing ABPM with office BP for monitoring HTN

26. Outcomes of monitoring BP with ABPM vs office measurementStaessen et al • 419 patients with office DBP >95 randomized to follow-up with either ABPM or office BP • Medication adjusted in a stepwise fashion according to BP measurements

27. Improving BP measurement • “HTN clinic” approach • “Out of office” approach • “Individualized” approach

28. Improving BP measurement • “HTN clinic” approach • Dedicated personnel • Specific training in HTN • Standardized BP measurement • Patient education

29. Improving BP measurement • “Out of office” approach • Use out of office measurements to guide decision-making • Use self-measurement in patients with demonstrated accuracy • Use ABPM in others, or when validity is uncertain

30. Improving BP measurement • Individualized approach in managing BP • Assess absolute risk for ACE’s • Treat based on expected benefit • Absolute risk may vary markedly at any level of BP • Expected benefit closely related to absolute risk • RRR of treatment will be same regardless of whether BP measure is precise

31. Conclusions • Measurement of BP in the primary care office is highly prone to error • Out of office BP measurements can be more accurate than office BP’s • Lack of strong evidence demonstrating an improvement in outcomes associated with OOO-BP readings • Multiple potential areas for improvement in BP assessment