Blood Pressure Measurement – a Call to Action to Improve Reliability and Enhance Awareness of Underlying Mechanisms

In addressing the global burden of raised blood pressure, the recent Lancet Commission on Hypertension¹ aimed to raise awareness of the intertwined array of factors that contribute to determining the lifecourse strategy for reduction of cardiovascular risk. The Commission proposed four key actions: (i) prevention by lifestyle and environmental changes; (ii) blood pressure diagnosis and evaluation; (iii) pharmacological prevention and monitoring; (iv) blood pressure and health care systems, with emphasis on obtaining accurate data on blood pressure.

Reliable measurement of blood pressure is the sine qua non of the meaningful quantification of the health burden of high blood pressure, and the quantification is conventionally expressed as the presence or absence of the clinical condition of hypertension, as determined by guidelines established by professional societies². Thus, the quantitative information is critical in the identification of those at risk and in the diagnosis, treatment and management of hypertension.

Although it is recognized that the arterial pressure pulse varies in morphology and magnitude throughout the arterial tree³, the conventional evaluation of the blood pressure status in humans is based on the measurement of pressure in an inflatable cuff wrapped around the upper arm. The cuff pressure is associated with specific surrogate measures that are related to basic physiological phenomena of pulsatile blood flow in arteries, enabling a noninvasive estimate of systolic (SBP) and diastolic pressure (DBP), the key quantitative metrics used for diagnosing hypertension. Based on the underlying principles of sphygmomanometry, the main surrogate factors are the identification the Korotkoff sounds (auscultatory method), the change of the small pulsatile signal in the cuff during inflation or deflation (oscillometric method) and the presence or absence of an arterial pulse distal to the cuff (palpatory method, only for SBP). Although, on theoretical grounds, it should be possible to obtain highly reliable measures of SBP and DBP, the practical implementation of the brachial cuff measurement of blood pressure is subject to a large range of variables that can undermine the quality of measurement, and so affect the fiducial diagnosis of hypertension and reliability of treatment and management⁴.

The recent AHA scientific statement on Blood Pressure Measurement in Humans⁵ addresses the multiple and complex range of factors that play a significant role in obtaining reliable measures of SBP and DBP. It is the first update of the previous statement published in 2005⁶, with the current recommendation containing comprehensive evidence-based information of risk profiles associated with office, ambulatory and home measurements of blood pressure⁵. The statement makes a series of recommendations aimed at the whole “infrastructure” of blood pressure measurement involving attributes related to equipment, operator and environmental factors. Population data accumulated over the past three decades has shown the value of ambulatory blood pressure monitoring (ABPM) in comparison office and home measurements^{7, 8}. Hence, ABMP is the recommended modality for confirmation of hypertension once it is indicated by office measurements. However, although ABPM has assumed a prominent role in the latest guidelines, as it gives an estimate of the circadian changes in blood pressure profile, it is a modality that may not be generally available or tolerated by all patients. The AHA statement⁵ surveys emerging technologies that may complement the intermittent measurement of ABPM and provide more continuous blood pressure data. While of interest, the new technology has not yet undergone robust regulatory approval, lacking standards and reliable calibration procedures. However, it might be expected that future updates to recommendations on blood pressure measurement in the not too distant future would include use of data obtained by wearable devices⁹.

Notwithstanding the advocacy of ABPM, the cornerstone of blood pressure measurement is the brachial cuff sphygmanometer in the doctor’s office. Conventional office measurement of blood pressure is essential for initial screening of hypertension but has been shown to be different to measurements out of the office and is affected by ‘white coat’ or ‘masked’ hypertension¹⁰. A significant change in recommendations of office measurement of blood pressure has been potentiated by the progress made in Canada in promoting the use of automated sphygmomanometry. The automated office blood pressure (AOBP) measurement is an operator independent technique and has been shown to reduce the ‘white coat’ effect^{11, 12}. The AHA statement proposes AOBP as the recommended modality for office measurement of blood pressure, with the suggestion that the total time for unattended AOBP measurement is of order of 4-6 minutes (including 1-minute rest period between measurements), which his shorter than that time for conventional operator-dependent methods of 7-8 minutes (requiring a 5 minute rest period).

For many years, blood pressure has been measured with the auscultatory method using a stethoscope and a brachial cuff connected to a mercury column. The mercury column has now given way to pressure transducers and the stethoscope is gradually being replaced by cuff microphones or completely eliminated and the ausculatory method largely being superseded by the oscillometric technique. It is the gradual surge of the ubiquitous use of oscillometry in blood pressure devices that is facilitating the automated methodology. However, these changes have important implications regarding the accurate estimation of SBP and DBP. In the oscillometric measurement, the fundamental pressure measurement is the cuff pressure corresponding to the maximum oscillation of the oscillogram, that is, the mean arterial pressure. SBP and DBP are then estimated from proprietary algorithms using this characteristic and the shape of the oscillogram. From a regulatory standpoint, oscillometric devices are validated against the auscultatory method, which has inherent limitations associated with fiducial detection of sound energy, including dependence on operator hearing acuity. To account for these potential limitations, methodologies have been proposed to ‘visualise’ the Korotkoff sounds so as to improve calibration of oscillometric devices¹³. The AHA statement emphasizes the importance of using validated devices, as differences in blood pressure estimated with different algorithms from different manufactures can exceed 10 mmHg¹⁴. There are also concerted moves to arrive at universal standards for validation of blood pressure devices¹⁵.

The AHA statement⁵ addresses the issue of blood pressure variability mainly in the context of ‘masked’ and ‘white coat’ hypertension, where office and home measurements show a marked difference. The short-term variability, that is, the difference between the initial and subsequent measurements is considered with suggestions of averaging the measurements or exclusion of initial readings. However, it is conceivable that there is a range of time constants amongst individuals to arrive at a stable level of resting blood pressure after the initial cuff inflation. For example, for the home measurements, the recommendations are that 2 readings be taken at least 1 minute apart in the morning before taking antihypertensive medications, and 2 readings at least 1 minute apart in the evening before going to bed. This assumes that the representative stable and resting pressure of the individual is achieved after 1 minute following the initial cuff inflation. Since the home measurements are taken in a relaxed environment, it could be possible to suggest a longer measurement period, say 5 minutes, with at least 5 measurements taken 1 minute apart. The would guarantee a series of measurements to ascertain a physiological stable condition. A similar approach has been used to assess extended measurement periods for optimizing ABPM protocols¹⁶.

The update in the recent AHS statement⁵ includes a comprehensives description of methodologies and devices associated with the measurement of brachial pressure, including the noninvasive estimation of central aortic pressure from the peripheral pulse which is calibrated to the measured brachial pressure. In this context, an important consideration is the effect of heart rate. The relationship central aortic and brachial pulse pressure is highly dependent on heart rate due to the frequency-dependent propagation characteristics of the brachial artery¹⁷. This may provide information beyond brachial blood pressure when considering anti-hypertensive treatments and target organ damage. Anti-hypertensive treatment that causes significant alteration in heart rate, such as beta-blockers, will have a reduced effect on lowering central aortic systolic pressure compared with other agents that do not alter heart rate but produce similar reductions of brachial pressure¹⁸. This will have significant implications in the correct assessment of end-organ effects such regression of left ventricular hypertrophy, information which is not obtained simply by measuring brachial pressure alone¹⁹.

The authors of the updated AHA statement have undertaken a timely and important task in providing a set of clear and comprehensive recommendations on the most effective methodologies and strategies to measure arterial blood pressure in humans by noninvasive means using the brachial cuff sphygmomanometer. It is not a perfect device, but it is the device which has delivered most of the information on the risk factors associated with blood pressure and cardiovascular disease – a health burden that also has a significant potential for modification¹.