CORRESPONDENCE: LETTER TO THE EDITOR
Reply
Thananya Boonyasirinant, MD,
Prabhakar Rajiah, MD,
Randolph M. Setser, DSc,
Michael L. Lieber, MS,
Harry M. Lever, MD,
Milind Y. Desai, MD and
Scott D. Flamm, MD*
* Cleveland Clinic, Cardiovascular Imaging, J1-4, 9500 Euclid Avenue, Cleveland, Ohio 44195 (Email: flamms{at}ccf.org).
We thank Dr. O'Rourke and colleagues and Dr. Nemes and colleagues for the interest in our study (1). A common thread is that each promotes alternative methods for computing aortic stiffness to the method we chose to employ, namely pulse wave velocity (PWV) computed by magnetic resonance imaging (MRI).
We are in agreement with Dr. Nemes and colleagues that more work is needed between and among these various approaches. We also recognize that many techniques exist to quantify aortic stiffness, and each is subject to its own characteristic strengths and weaknesses. Reliable quantification of PWV is dependent on accurate measurement of both the aortic flow (or pressure) wave at 2 measurement sites as well as the distance between these 2 sites. Previously, in numerous studies, MRI has been shown as a reliable technique for quantifying the aortic flow wave; its longitudinal dimensional accuracy is unparalleled, although it does suffer from a relatively low temporal resolution when compared with alternate techniques.
One of the concerns raised by Dr. O'Rourke and colleagues was that our PWV values for normal controls were lower than previously reported. Despite this assertion, our values are, indeed, consistent with several previous reports (2–5). Nonetheless, it is important to keep in mind that 2 different techniques measuring the same physiological parameter are unlikely to provide precisely the same results, and, in fact, it has been shown recently that PWV results may vary not only based on the technique used to acquire the data, but also based on the analysis method used to determine their value (6).
There are several possible explanations for the differences in PWV between measurement techniques, and each could contribute to the observed discrepancies. For instance, PWV increases along the length of the aorta as the artery becomes less elastic (7), and has been demonstrated in a study using MRI (3). The technique of arterial tonometry, which is restricted to measurement sites at the carotid and femoral arteries, provides an average PWV between these points, but it is incapable of determining regional values such as those we reported between the ascending and proximal descending aorta. At least in normal subjects, PWV values by MRI, restricted to the thoracic aorta, should be somewhat lower than values obtained by arterial tonometry. In addition, nonimaging techniques are subject to errors in determining the aortic length between measurement sites, particularly in the case of tortuous aortas, which occur, for example, as part of the normal aging process and in the setting of significant atherosclerotic disease. We are unaware of corrections available to compensate for this uncertainty.
Dr. O'Rourke and colleagues also raised concerns about the relatively large SDs seen in the hypertrophic cardiomyopathy population. This would be a legitimate concern if large SDs were present in the normal control subjects; however, they were not. Instead, this more reasonably reflects the heterogeneity within this large group of hypertrophic cardiomyopathy patients including, but not limited to: variations in myocardial mass, the presence or absence of interstitial fibrosis, the degree and amount of interstitial fibrosis, and the presence or absence of left ventricular outflow tract obstruction.
Of course, there exist other alternative measures of aortic stiffness, some of which are mentioned by Dr. Nemes and colleagues. These measures are attractive because they offer a local estimate of stiffness, though they rely on accurate measurement of aortic distension and, in some cases, aortic pressure. For ease of measurement, arterial distension is sometimes approximated by its 1-dimensional analog (i.e., change in diameter), and cuff (brachial) pressure is often used as an approximation of central pressure, despite the fact there is an assumption of absence of central stenoses, and that amplification along the arterial tree can result in large differences between these pressures (7).
In this and previous studies, PWV has been shown to be a robust and completely noninvasive index of arterial stiffness, regardless of the method used in its quantification. However, when MRI is the method of choice, PWV can be included as part of a comprehensive and clinically validated imaging evaluation of cardiovascular disease, and with no additional time, cost, or equipment needed. As with any quantification technique, care must be taken to consider the methodology being used when comparing results between patient groups and between and among different studies.
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References
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1. Boonyasirinant T, Rajiah P, Setser RM, et al. Aortic stiffness is increased in hypertrophic cardiomyopathy with myocardial fibrosis: novel insights in vascular function from magnetic resonance imaging J Am Coll Cardiol 2009;54:255-262.[Abstract/Free Full Text]2. Groenink M, de Roos A, Mulder BJ, Spaan JA, van der Wall EE. Changes in aortic distensibility and pulse wave velocity assessed with magnetic resonance imaging following beta-blocker therapy in the Marfan syndrome Am J Cardiol 1998;82:203-208.[CrossRef][Web of Science][Medline] 3. Rogers WJ, Hu YL, Coast D, et al. Age-associated changes in regional aortic pulse wave velocity J Am Coll Cardiol 2001;38:1123-1129.[Abstract/Free Full Text] 4. Grotenhuis HB, Ottenkamp J, Westenberg JJ, Bax JJ, Kroft LJ, de Roos A. Reduced aortic elasticity and dilatation are associated with aortic regurgitation and left ventricular hypertrophy in nonstenotic bicuspid aortic valve patients J Am Coll Cardiol 2007;49:1660-1665.[Abstract/Free Full Text] 5. Grotenhuis HB, Ottenkamp J, Fontein D, et al. Aortic elasticity and left ventricular function after arterial switch operation: MR imaging—initial experience Radiology 2008;249:801-809.[Abstract/Free Full Text] 6. Boonyasirinant T, Setser RM, Rajiah P, Desai MY, Flamm SD. Pulse wave velocity in patients with bicuspid aortic valve and normal controls: discriminatory ability among multiple analysis techniques. Paper presented at: the 17th Scientific Meeting of the International Society for Magnetic Resonance in Medicine; April 18 to 24, 2009; Honolulu, HI. 7. Laurent S, Cockcroft J, Van Bortel L, et al. Expert consensus document on arterial stiffness: methodological issues and clinical applications Eur Heart J 2006;27:2588-2605.[Abstract/Free Full Text]
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