Blood Pressure Measurement: A New Frontier?

In this issue of The Journal of Clinical Hypertension, a respected group in Taipei, Taiwan, linked with the US National Institute on Aging at Johns Hopkins in the United States, described indices of arterial stiffness in a group of 57 hypertensive patients treated to current guidelines with three or more antihypertensive agents and compared with a group of 57 age‐ and sex‐matched normotensive patients from a community survey.1 Findings were of higher residual aortic stiffness (as carotid‐femoral pulse wave velocity [PWV]) and of early‐wave reflection (augmentation index [AIx]) in the treated hypertensive than in the normotensive group. The question that arises is how we can better manage such patients who are already treated to guidelines, in a center that has available methods, which are discussed in the recent American Heart Association Scientific Statement2 to measure aortic stiffness, wave reflection, and central aortic pressure (CAP). The Taiwan group has developed such instruments, with a commercial sponsor, and evaluated them over a period of 25 years.3, 4 They have shown that indices of aortic stiffness, including PWV, AIx, and reflection magnitude, are associated with higher risk of morbidity and mortality. The group of 114 patients is too small to measure conventional outcomes, and there is no direct evidence that further reduction of PWV or AIx will improve outcome, but there is increasing acceptance that treatment of aortic stiffness from middle age can reduce cerebrovascular lesions and delay development of intellectual deterioration and onset of dementia.5 There is evidence that such indices can predict reduction of left ventricular (LV) hypertrophy with relatively small patient numbers.2, 6, 7

This question is likely to become more relevant in clinical practice, with better knowledge and acceptance of aortic stiffness and pressure AIx as targets of therapy2 and pending Medicare reimbursement for measurement of CAP. A more aggressive approach to the management of systolic blood pressure (BP) will likely arise from the Systolic Blood Pressure Intervention Trial (SPRINT), which has been terminated by the supervising ethics committee on the basis of better results from reduction of systolic pressure to a target of <120 mm Hg than to <140 mm Hg.8

During the past 25 years, our group in Sydney has been working on the same issues as our colleagues in Taiwan and Baltimore and, having developed the almost identical transfer function9 as Chen and colleagues,10, 11 has used this in our clinical practice. Our approach is set out below.

Clinical Practice

Patients with hypertension in Australia are typically managed by general (primary) medical practitioners. Referral to a specialist usually occurs only when there is concern about hypertension secondary to other diseases; severe and/or refractory hypertension, complications of hypertension such as coronary disease, heart failure, or stroke; or uncertainty as to whether a patient meets criteria for drug treatment. General practitioners use conventional cuff sphygmomanometers and patients often have cuff sphygmomanometers for use at home.

There are very few general practitioners who use devices such as the SphygmoCor (AtCor Medical, West Ryde, Australia), which we use, or the Microlife device (Microlife Corporation, Taipei, Taiwan), which was used by our colleagues from Taiwan. About 20 years ago, reimbursement was approved for noninvasive measurement of radial artery pressure waveforms under Australian Medicare, but this was withdrawn when there was perceived inappropriate and excessive use by general practitioners and, at the time, no evidence of benefit. We now use the device in all referred patients, whether Medicare recipients or not, during the initial visit and then one or more times per year when referred back by general practitioners. We use the same reference values as those used in Taiwan for central pressure12 (130 mm Hg rather than 140 mm Hg) for systolic pressure and the same value for diastolic pressure (90 mm Hg).

The greatest value appears to be in the identification of a condition that we originally described as spurious systolic hypertension of youth.13 It now appears that this is caused by distortion and amplification of higher‐frequency (3–6 Hz) components of the pressure wave generated in the aorta by vigorous LV ejection.13, 14 While typically seen in tall adolescent males, it has been found in normal persons of all ages, with tachycardia or during exercise, and to be a normal phenomenon in males and females aged 10 to 18 years during childhood.15 This phenomenon can account for the innocuous nature of isolated systolic hypertension in young patients, particularly men younger than 50 years,16 and the endorsement for measuring CAP when the condition is suspected.17

In clinical practice, measurement of CAP waveforms can also be useful in making a decision on when to begin therapy for hypertension in a person whose systolic pressure is borderline high and on what therapy to use, ie, a β‐blocker in a person with tachycardia or evidence of vigorous LV contraction or a drug that reduces wave reflection when the aortic systolic peak is exaggerated by wave reflection.2

Use of pulse waveform analysis in induction and monitoring therapy is now warranted, especially since the SPRINT trial,8 to be discussed and presented at the American Heart Association Annual Meeting in November 2015, has been terminated prematurely, with outcome benefit shown for patients whose brachial BP was reduced to below 120 mm Hg as opposed to 140 mm Hg. Another incentive for taking such an approach is the reduction in progression of dementia and cerebral microvascular disease in patients treated with antihypertensive drugs, which reduce wave reflection and thereby reduce central aortic systolic and pulse pressure.2, 5, 18

24‐Hour BP Measurement vs Home BP Measurement

On the basis of the Japanese experience, we have come to use home BP measurement rather than 24‐hour BP. We recognize the deficiencies in clinic BP measurements but believe that these are not overcome by 24‐hour BP, on the basis of the following: set times for frequency of measurement by day and night; the arbitrary exclusion of data, which does not confirm with expectation; the alarm created by inflation of the cuff in different situations; the measurement in different positions and during different activities; the discomfort of the device; and the lack of reproducibility data. We believe that the benefit of multiple measurements can be overcome by multiple measurements in the clinic or at home. We are also impressed by the ability of different indices available from pulse wave analysis to predict regression of LV hypertrophy in small patient groups.6

Calibration of CAP to the Upper Limb (Radial) Pressure Waveform

In approving the use of the SphygmoCor device, the US Food and Drug Administration (FDA) insisted that the peak and bottom of the radial waveform be calibrated to the brachial systolic and diastolic cuff pressure. The logic of this was that any benefit in the use of the device must be incremental to the information provided by the cuff sphygmomanometer. The FDA was aware of the differences between noninvasive (cuff BP) and invasive values of systolic and diastolic pressures, and took this view as the appropriate compromise. When calibrating the radial pressure waveform, one can use whatever measure of cuff BP considered appropriate––the clinic value, 24‐hour BP value, or home BP value. The manufacturer complies with FDA requirements and refers to the “best estimate” of brachial cuff pressure as the appropriate calibration. The best estimate of brachial cuff BP is still in contention. The incremental benefit of measuring aortic stiffness and wave reflection is becoming less contentious.

What is BP? John Laragh asked us to address this question 25 years ago.19 Our views have not changed.

Disclosures

Michael O'Rourke is a founding director of AtCor Medical Pty Limited, a manufacturer of systems for analyzing arterial pulse, and Aortic Wrap Pty Limited, a developer of devices to improve aortic distensibility. Audrey Adji has no disclosures.