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Letters to the Editor |
Department of Imaging (MR Unit) National Heart and Lung Institute Imperial College of Science, Technology and Medicine London SW3 6NP, United Kingdom
To the editor:
Elsheikh et al. (1) describe how they attempted to assess central arterial hemodynamics in women with Turner’s syndrome.
The whole thrust of their paper suggests that the noninvasive central arterial assessments of "aortic" blood pressure that they adopted have been validated—but no data to support this assertion are actually offered or cited.
Indeed, in the measurements section of their paper, the authors write: "The aortic pulse waveform was derived from the radial pulse waveform by using a validated transfer function (4) implemented in the Sphygmocor software. Measurements using this method have been shown to correspond closely to intraarterially recorded waves (2, 3, 4)" (1). However, this is not correct.
Rather, examination of the three references cited from 1989 (2, 3) and 1996 (4) make no mention of the Sphygmocor/BPAS device (PWV Medical, Sydney, Australia) or its radial artery generalized transfer function (GTF). Indeed, none of them show that radial artery pulse waveforms can be used to derive central aortic pulse waveforms noninvasively. One article overviews the basic technique of applanation tonometry (2), and the other two references show how carotid artery tonometry [a technique not used by Elsheikh et al. (1)] can provide noninvasive assessments of ascending aortic blood pressure (3, 4).
The literature sometimes cited as having "validated" the radial artery GTF used within the Sphygmocor/BPAS system has recently been overviewed elsewhere (5). None of the above three articles even feature, because they have nothing to do with the radial artery GTF.
This issue is of importance because in the 7+ yr since the report describing the original radial artery GTF was published (6), and the U.S. Patent for the technique was granted (7), there has been an absolute paucity of validation work with the approach reported in the scientific literature. This lack of validation studies is particularly noticeable when the measurements have to be calibrated noninvasively, as in the study by Elsheikh et al. (1). Indeed, what little "validation" data for noninvasive use of the Sphygmocor are available in the literature, based on 20 patients (8), suggests the errors associated with the method for the estimation of central aortic blood pressure to be substantial. In fact, these errors are much greater than those permitted by the American Association for Medical Instrumentation under guidelines endorsed by the U.S. Food and Drugs Administration for noninvasive blood pressure measuring devices (5, 9). This has led to the conclusion that, at this time, there are no accurate or well validated methods for measuring blood pressure noninvasively in vivo in the aorta (10).
Notwithstanding this, driven it would seem by commercial claims (7) rather than evidence-based medicine, the Sphygmocor GTF approach now is being tried in all manner of disease states completely unrelated to the 14 subjects who provided data at cardiac catheterization for the original generation of the radial artery GTF (6). In this respect, even if one believes the claims that a single radial artery GTF can be used accurately and robustly to measure central aortic blood pressure noninvasively in all subjects of all ages, heights, weights, and blood pressures, both on and off medication (claims that are disputed because no evidence has, in fact, been published in the peer-reviewed literature), it seems completely implausible that this will work for all disease states as well.
Connected with this, a recent query about where is the evidence that the Sphygmocor system can be used to accurately and noninvasively predict central aortic blood pressure in patients with another endocrine disorder (diabetes; Ref. 11) was not able to offer any evidence or data to support the claims that were being made (12). Furthermore, a search on Medline up to September 2000 using "Sphygmocor" and "sphygmocardiography" as search terms confirms the absolute paucity of validation data that are available in the literature for the technique.
Other researchers have noticed this as well and have independently raised separate fundamental concerns about the use of the method, especially for the determination of the augmentation index (13, 14, 15) and central aortic blood pressure (16).
Unfortunately, citing papers from elsewhere in the field, which have not involved the Sphygmocor’s radial artery GTF, as "validation" references, may make it appear on a cursory inspection that there have, in fact, been more validation studies using the device than are really the case.
Given all the above, Elsheikh et al. (1) may wish to exercise caution in making claims about the "validity" of the noninvasive approach they have adopted, which at present remains completely unproven, especially in endocrine disease.
References
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