Correspondence Central aortic pressure: alive and well at 25 years Michael F. O’Rourke

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octors Narayan et al. [1] have conducted an exhaustive meta-analysis on methods used to estimate central aortic blood pressure by combining information from a conventional brachial cuff sphygmomanometer with features of the arterial pressure wave. As Founding Director of AtCor Medical, maker of the SphygmoCor device, used in 110 of their 164 (67%) eligible studies, I have a different perspective on measurement of central pressure noninvasively, using applanation tonometry and a validated generalized transfer function (GTF), and set this out as follows. Potential value of a GTF was first suggested by Raymond Kelly’s observation at cardiac catheterization that nitroglycerine consistently reduced central aortic systolic pressure to a greater degree than brachial systolic pressure in studies that used high fidelity micromanometry [2,3]. This was confirmed by others, and the transfer function was first published by us in 1993 [4] with virtually identical information published by Chen et al. [5] at Johns Hopkins in 1997. A series of validation studies were then undertaken and published between 1991 and 2001. Patent protection in the USA was gained in 1993 and the device gained clearance by the US Food and Drugs Administration (FDA) in 2001 for invasive use as K002742. The FDA required evidence that the method was stable under different conditions, at different ages in adults, with changes in heart rate, different diseases (significant aortic valve stenosis being the only exclusion) and vasomotor challenge, when radial and aortic pressures were measured invasively with catheter manometry of adequate frequency response. For noninvasive use, the FDA also required evidence of tonometer accuracy and frequency response. Calibration was an issue, and the FDA insisted that the noninvasive device be calibrated to the existing ‘gold standard’, the cuff sphygmomanometer’s systolic and diastolic pressure. Approval from the FDA was given in 2002 as K012487. SphygmoCor was sold from 1994 onwards and has been evaluated in a meta-analysis most recently by Roman and Devereux [6] with results that are quite different to those presented by Narayan et al. [1]. The Melbourne group [1] draw attention to disparities between results of studies comparing GTF-calculated central pressure and pressure measured directly in the ascending aorta at cardiac catheterization (predominantly with long fluid-filled catheters and unknown frequency response). Authors appear to take such direct invasive measurements of aortic pressure as the ‘gold standard’ for central pressure, whereas we (under instructions from the FDA) calibrated peak and nadir of the radial wave to brachial cuff systolic pressure and diastolic pressure We Journal of Hypertension

and the FDA were well aware of the difference between direct and indirect measurements of arterial pressure. The FDA and our approach was that to be of value, central pressure would need to have incremental information over and above brachial cuff pressure, and that we would have to establish normal values for central aortic pressure. We estimated that 130/90 (aorta) would correspond to the brachial pressure of 140/90 as the upper limit of normal, and this has been established by Cheng et al. [7] using a generalized transfer function first published as that of Chen et al. [5] with findings almost the same as our own. All of our validation work was conducted prior to 2000, which was the starting time for the authors’ meta-analysis, so that just one of our articles (authors’ ref 9) was included. Reference 9 shows very close identity of calculated central pressure calculated from the simultaneously recorded radial artery pressure and directly measured pressure, using paired manometers with adequate frequency response. The Melbourne authors’ own GTF had a mathematical error in phase so that central pressure could not be accurately calculated, as pointed out by Segers et al. in 2007 [8]. It is unfortunate that the Melbourne Group [1] discredit the most frequently used and respected noninvasive GTF method for measuring central aortic pressure. Certainly, their own GTF approach has been found wanting and has been successfully challenged [8] without subsequent recognition or response. Readers will find a more evenhanded evaluation of central pressure by Roman and Devereux [6] and outcome data from other independent studies conducted over the past 20 years.

ACKNOWLEDGEMENTS Conflicts of interest Please note that M.O’R. is a founding director of AtCor Medical Pty Limited, manufacturer of systems for analysing the arterial pulse and Aortic Wrap Pty Limited, developer of devices to improve aortic distensibility, and a consultant to Novartis and to Merck.

REFERENCES 1. Narayan O, Casan J, Szarski M, Dart AM, Meredith IT, Cameron JD. Estimation of central aortic blood pressure: a systematic meta-analysis of available techniques. J Hypertens 2014; 32:1727–1740. 2. Kelly RP, Gibbs HH, O’Rourke MF, Daley JE, Mang K, Morgan JJ, Avolio AP. Nitroglycerine has more favourable effects on left ventricular afterload than apparent from measurement of pressure in a peripheral artery. Eur Heart J 1990; 11:138–144. 3. O’Rourke MF, Safar ME, Dzau V, editors. Arterial vasodilation: mechanisms and therapy. London: Edward Arnold/Philadelphia: Lea & Febiger; 1993. 4. Karamanoglu M, O’Rourke MF, Avolio AP, Kelly RP. An analysis of the relationship between central aortic and peripheral upper limb pressure waves in man. Eur Heart J 1993; 14:160–167. 5. Chen CH, Nevo E, Fetics B, Pak PH, Yin FC, Maughan WL, Kass DA. Estimation of central aortic pressure waveform by mathematical transformation of radial tonometry pressure: validation of generalised transfer function. Circulation 1997; 95:1827–1836. 6. Roman MJ, Devereux RB. Association of central and peripheral blood pressures with intermediate cardiovascular phenotypes. Hypertension 2014; 63:1148–1153.

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Correspondence 7. Cheng HM, Chuang SY, Sung SH, Yu WC, Pearson A, Lakatta EG, et al. Derivation and validation of diagnostic thresholds for central blood pressure measurements based on long-term cardiovascular risks. J Am Coll Cardiol 2013; 62:1780–1787. 8. Segers P, Mahieu D, Kips J, van Bortel LM. The use of a generalised transfer function: different processing, different results. J Hypertens 2007; 25:1783–1787.

Journal of Hypertension 2015, 33:187–190 St. Vincent’s Clinic/University of New South Wales, Darlinghurst, Sydney, New South Wales, Australia Correspondence to Dr Michael F. O’Rourke, St Vincent’s Clinic, Suite 810, 438 Victoria Street, Darlinghurst, Sydney, NSW 2010, Australia. Tel: +61 02 8382 6874; fax: +61 02 8382 6875; e-mail: [email protected] J Hypertens 33:187–190 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins. DOI:10.1097/HJH.0000000000000450

Reply Om Narayan a, Anthony Dart b, Ian T. Meredith a, and James D. Cameron a

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e refer to Dr O’Rourke’s comments [1] apparently made in response to our recent meta-analysis [2]. Dr O’Rourke’s longstanding involvement with the SphygmoCor approach is well known as is his interest in, and defence of, the specific so-called validated arterial transfer function. In this instance, we are at a loss to understand the relevance of his comments, as our metaanalysis relates solely to variation in central–brachial BP estimations associated with differing technologies and analytical approaches that are employed in currently available devices [2]. We did not report on, raise or discuss any issues of precision and/or accuracy. The conclusion of our metaanalysis remains, namely that comparison of specific results of assessment of central BP may be problematic given differing, and often proprietary, technological approaches used. This is an issue we feel users of such devices should be aware of. Although not the subject of our meta-analysis, Professor O’Rourke is rightly concerned with the issue of validation. Unfortunately, and notwithstanding Professor O’Rourke’s protestations to the contrary, the SphygmoCor (or in fact any other) device has not in fact received rigorous validation either in terms of its accurate prediction of central SBP or the incremental value, over sphygmomanometric cuff measured brachial pressures, or of any SphygmoCor derived measure in relation to prediction of clinical outcomes. The former requires high fidelity, simultaneous, direct, measurement of central (aortic) pressure and noninvasive estimation by the SphygmoCor device. Studies in which the SphygmoCor device is calibrated from invasively acquired pressures are not adequate surrogates. To our knowledge, there have been no studies in which any

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SphygmoCor-derived parameter, including the now apparently defunct ‘augmentation index’, has been shown to be more useful than cuff-derived brachial pressures in the prognostication of clinical events, nor a superior method on which to base therapy. Dr O’Rourke does raise the interesting issue of regulatory approval and marketing. This has never been systematically discussed for this range of equipment, but it appears that the majority of the currently available devices have been presented as ‘substantially equivalent’ to some form of predicate device (often an earlier SphygmoCor version). To the best of our knowledge, the device quoted by Dr O’Rourke [to which 510(K) Number K002742, 2002 refers] utilizes invasively obtained, directly calibrated radial pressure waveforms and was accepted by the US Food and Drugs Administration (FDA) as substantially equivalent to an invasive pressure catheter. Interestingly, this 510(K) summary specifies that ‘the system is designed for use with a conventional invasive radial artery blood pressure monitor in the hospital setting’, this would appear to be a very different use and application to the general hypertension clinic or in epidemiological studies. Further equivalence was subsequently accepted on the basis of similarity of noninvasive tonometric (presumably uncalibrated) and invasive radial pressure waveforms (510K Number K012487, also 2002). As is usual for 510(K) acceptance, the only testing appears to have been to Electro-Medical safety standards and we are unable to identify any instance wherein actual comparative data of invasive and derived central BP waveforms calibrated with brachial pressures have been required or assessed by a regulatory body in relation to any of the available commercial devices. We would suggest to readers that the points raised in Dr O’Rourke’s letter are a good illustration of potential risks associated with a phenomenon of ‘creep’ that will inevitably occur in a sequence of studies based on repeated comparison to earlier devices, which were themselves considered ‘substantially’ equivalent within undefined limits when assessed against even earlier devices. We do not consider this rolling process of comparison of numerical values to constitute validation. Practical validation of devices for measurement of central BP, using any system, would require the demonstration of significant additional clinical benefit over existing BP measurement. We wait with interest for published results of any large study reporting validation based on hard clinical endpoints. We are not clear as to the relevance of Dr O’Rourke’s references. The review by Roman referenced by Dr. O’Rourke does not address the issue of device or methodological accuracy and fundamentally differs from the approach we employed in our systematic, quantitative meta-analysis [2,3]. Generalized arterial transfer functions have been a topic of interest in the literature for many years and there are a number of relevant publications for readers who are interested in a balanced discussion of the topic [4–10]. We agree with Segers et al. [11], Van Soender et al. [9] and Rezai et al. [4] (amongst others) that the manner of signal processing and pressure calibration makes a difference to the resultant estimated central BP. Indeed that measurement approach matters when comparing derived central BP is one of the key points of our report Volume 33  Number 1  January 2015

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Correspondence

and we have also commented on this issue by previous editorial commentary [12,13]. The authors of the manuscript referred to by Dr O’Rourke [2] and of this letter have no involvement with any company involved in measurement of blood pressure or arterial stiffness.

The 20  20 Latin American Society of Hypertension target

ACKNOWLEDGEMENTS

igh blood pressure (BP) is the leading cause of cardiovascular disease (CVD) and deaths globally, and is associated with 7.5 million deaths per year (representing one-eighth of all deaths) worldwide [1]. The importance of BP as a modifiable risk factor for CVD is well recognized and many effective and inexpensive BP-lowering treatments are commonly available. Therefore, BP control and prevention of related morbidity and mortality could be clearly achievable. However, the Prospective Urban Rural Epidemiology (PURE), a community-based study including 153 996 adults (35–70 years) from three high-income countries (HICs), ten upper middle and low middle income countries (UMICs and LMICs), and four lowincome countries (LICs), found that 40.7% of participants were hypertensive. A total of 13.3% of participants had a BP < 140/90 mmHg and only 32.5% of treated hypertensive participants had a BP < 140/90 mmHg [2]. Overall, only 12.5% of treated hypertensive patients received two or more BP-lowering medications, with a decreasing trend from wealthier to poorer countries (HICs 18.1%, UMICs 14.5%, LMICs 14.1%, and LICs 1.6%; P < 0.0001). Hypertension prevalence was highest in participants with diabetes (63%), and even though awareness was 74.4%, and the percentage of those who received treatment 69.3%, the control rate was only 23.3%. So, it is crucial to improve the control of BP in a high-risk group, such as the diabetic population. In Latin America, of those with hypertension, 57.1% were aware of this condition and 52.8% were receiving pharmacological treatment, but only 18.8% had controlled BP (

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