Published online before print September 22, 2003, doi: 10.1161/01.HYP.0000095611.09419.F2
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
(Hypertension. 2003;42:e15.)
© 2003 American Heart Association, Inc.
Letters to the Editor |
Clinical Assessment of Wave Reflection
UNSW/St Vincent’s Clinic, Sydney, Australia
To the Editor:
In their paper on wave reflection, Millasseau et al1 questioned whether quantification of wave reflection phenomena required synthesis of the ascending aortic waveform from the radial pulse through use of a generalized transfer function, or whether similar information could be extracted directly from the radial pulse. We had introduced the SphygmoCorTM system, not only to study central hemodynamics, but also because we could not reliably or consistently identify evidence of wave reflection on the falling systolic limb of the radial waveform. Problems were most commonly encountered when augmentation was low, or wave reflection came late (young persons), or was reduced by vasodilator therapy, or when heart rate was fast and augmentation obscured by the cardiac incisura. By generating aortic pressure, we sought to eliminate the effects of wave reflection within the upper limb so that we could determine effects of reflection from all parts of the body on aortic pressure throughout systole and on left ventricular load. The validity of this system has been confirmed,2 as has its reproducibility.3 Absolute synthesized values are within the Association for the Advancement of Medical Instrumentation (AAMI) SP10 criteria when compared with direct aortic pressure recordings.4
In the limited comparisons undertaken by Millasseau, greater variability of aortic augmentation than absolute pressure is predictable, because the former is measured from 2, and the latter from just 1 point on the wave. We were pleasantly surprised by the close fit between radial augmentation index (AIx) and calculated aortic AIx (authors’ Figure 4); this was the only real concern raised by Chen et al2 (authors’ Reference 9) in accurate calculation of aortic AIx. We cannot explain Millasseau’s difference between aortic AIx derived from carotid and radial waveforms in the 2 older groups, but note that carotid-derived values were rather higher than one would expect, and that the authors had experienced difficulty in accurately recording the carotid waveform. We have not seen such a dissociation in paired recordings from 548 subjects, except (as seen by Millasseau) when values of augmentation were low and, so, of little clinical importance (Figures 22.18, 22.19 in Nichols and O’Rourke5).
Millasseau et al tested their hypothesis by comparing aortic waveforms when generated by transfer functions at the 95% extremes of confidence limits as published by Chen et al2 and found greater difference in AIx than aortic systolic pressure. This is a tough test in itself, but was generated with just one (atypical) youthful waveform with 2 late systolic shoulders, rather than one.
We have repeated this test with more realistic radial waveforms taken in an older person with a single late systolic shoulder. Both under control conditions and after sublingual nitroglycerin, calculated systolic and augmentation differences were within 3 mm Hg when generated from the 95% confidence extremes of the transfer functions.
While radial waveforms may contain effects of wave reflection as seen in the synthesized aortic pulse, the automated process that generates the aortic pressure waveform is more practical and more consistent for automated feature extraction and more clinically useful than analysis of the radial pulse alone to determine ventricular load.
References
1. Millasseau SC, Patel SJ, Redwood SR, Ritter JM, Chowienczyk PJ. Pressure wave reflection assessed from the peripheral pulse: is a transfer function necessary? Hypertension. 2003; 41: 1016–1020.
2. 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. Circulation. 1997; 95: 1827–1836.
3. Siebehenhofer A, Kemp C, Sutton A, Williams B. The reproducibility of central aortic blood pressure measurements in healthy subjects using applanation tonometry and sphygmocardiography. J Hum Hypertens. 1999; 13: 625–629.[CrossRef][Medline] [Order article via Infotrieve]
4. Pauca A, O’Rourke M, Kon N. Prospective evaluation of a method for estimating ascending aortic pressure from the radial artery pressure waveform. Hypertension. 2001; 38: 932–937.
5. Nichols WW, O’Rourke MF. McDonald’s Blood Flow in Arteries. 4th ed. London, UK: Arnold; 1998.
Response: Augmentation Index and the Radial-to-Aortic Transfer Function
Clinical Pharmacology Department, St Thomas’ Hospital, King’s College, London, United Kingdom
We thank Professor O’Rourke and colleagues for their comments. We agree that the use of a radial-to-aortic transfer function (TF) for estimation of systolic blood pressure (SBP) and pulse pressure has been validated,1–3 although the accuracy of central blood pressure obtained in this way depends critically on the calibration of the radial pressure pulse.4–6 Agreement between aortic augmentation index (AIx) estimated using a TF and measured values is less convincing.1 Indeed, in a recent study, Hope et al7 report no correlation between measured and TF-estimated AIx. We suggested that a greater error in TF-estimated AIx than in TF-estimated central SBP could result from the greater variability of the TF at high frequencies that contribute more to AIx than to central SBP. We illustrated this with an example from a 46-year-old man. We have now run a similar analysis on all the subjects in our study and find greater variability in AIx than in central SBP, but we agree that other explanations, including additive errors, may account for the greater error in AIx than in central SBP. Because AIx typically ranges from -20% to +20%, a small absolute error may represent a large proportion of the pathophysiological range.
Carotid tonometry does require a degree of expertise to obtain optimal traces.8 However, the operator in our study (S.M.) was experienced in performing carotid measurements, and only measurements satisfying the default quality controls of the SphygmoCorTM were used. It is unlikely, therefore, that our findings were due to the poor quality of the carotid pulse recordings. The discrepancy between values of AIx obtained from the carotid and radial arteries was most marked in young subjects, and this is important for the many studies using AIx in young healthy volunteers.9–13 We do not dispute the use of a TF to estimate central blood pressure, provided the radial pulse wave is correctly calibrated. Although we remain to be convinced of an added benefit (beyond measurements taken directly from the radial pulse) of using a TF to assess wave reflection, we are enthusiastic in using pulse contour–derived measurements to study arterial structure and function. This is one of the many important contributions made by Professor O’Rourke and colleagues to the understanding of arterial hemodynamics.
References
1. Chen C-H, Nevo E, Fetics BJ, Pak PH, Yin FCP, Maughan LW, Kass DA. Estimation of central aortic pressure waveform by mathematical transformation of radial tonometry pressure. Circulation. 1997; 95: 1827–1836.
2. Pauca AL, O’Rourke MF, Kon ND. Prospective evaluation of a method for estimating ascending aortic pressure from the radial artery pressure waveform. Hypertension. 2001; 38: 932–937.
3. Siebenhofer A, Kemp C, Sutton A, Williams B. The reproducibility of central aortic blood pressure measurements in healthy subjects using applanation tonometry and sphygmocardiography. J Hum Hypertens. 1999; 13: 625–629.[CrossRef][Medline] [Order article via Infotrieve]
4. Cloud GC, Rajkumar C, Kooner J, Cooke J, Bulpitt CJ. Estimation of central aortic pressure by SphygmoCorTM requires intra-arterial peripheral pressures. Clin Sci (Lond). 2003; 105: 219–225.[Medline] [Order article via Infotrieve]
5. Smulyan H, Siddiqui DS, Carlson RJ, London GM, Safar ME. Clinical utility of aortic pulses and pressures calculated from applanated radial-artery pulses. Hypertension. 2003; 42: 150–155.
6. Davies JI, Band MM, Pringle S, Ogston S, Struthers AD. Peripheral blood pressure measurement is as good as applanation tonometry at predicting ascending aortic blood pressure. J Hypertens. 2003; 21: 571–576.[CrossRef][Medline] [Order article via Infotrieve]
7. Hope SA, Tay DB, Meredith IT, Cameron JD. Use of arterial transfer functions for the derivation of aortic waveform characteristics. J Hypertens. 2003; 21: 1299–1305.[CrossRef][Medline] [Order article via Infotrieve]
8. O’Rourke MF, Pauca A, Jiang XJ. Pulse wave analysis. Br J Clin Pharmacol. 2001; 51: 507–522.[CrossRef][Medline] [Order article via Infotrieve]
9. Mahmud A, Feely J. Effect of smoking on arterial stiffness and pulse pressure amplification. Hypertension. 2003; 41: 183–187.
10. Mahmud A, Feely J. Acute effect of Caffeine on Arterial stiffness and aortic pressure waveform. Hypertension. 2001; 38: 227–231.
11. Wilkinson IB, MacCallum H, Hupperetz PC, van Thoor CJ, Cockcroft JR, Webb DJ. Changes in the derived central pressure waveform and pulse pressure in response to angiotensin II and noradrenaline in man. Journal of Physiology. 2001; 530: 541–550.
12. Yasmin Brown MJ. Similarities and differences between augmentation index and pulse wave velocity in the assessment of arterial stiffness. QJM. 1999; 92: 595–600.
13. Westerbacka J, Wilkinson IB, Cockcroft JR, Utriainen T, Vehkavaara S, Yki-Järvinen H. Diminished wave reflection in the aorta: a novel physiological action of insulin on large blood vessels. Hypertension. 1999; 33: 1118–1122.
This article has been cited by other articles:
 |
|
 |
|
  P. Segers, J. De Backer, D. Devos, S. I. Rabben, T. C. Gillebert, L. M. Van Bortel, J De Sutter, A. De Paepe, and P. R. Verdonck Aortic reflection coefficients and their association with global indexes of wave reflection in healthy controls and patients with Marfan's syndrome Am J Physiol Heart Circ Physiol, June 1, 2006; 290(6): H2385 - H2392. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||