Letters to the Editor
Clinical Assessment of Wave Reflection
applanation tonometry and sphygmocardiography. J Hum Hypertens.
1999;13:625– 629.
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.
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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 SphygmoCor™ 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.
Response: Augmentation Index and the Radial-to-Aortic
Transfer Function
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 SphygmoCor™
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.
Sandrine C. Millasseau
James M. Ritter
Philip J. Chowienczyk
Clinical Pharmacology Department
St Thomas’ Hospital
King’s College
London, United Kingdom
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.
4. Cloud GC, Rajkumar C, Kooner J, Cooke J, Bulpitt CJ. Estimation of
central aortic pressure by SphygmoCor™ requires intra-arterial peripheral pressures. Clin Sci (Lond). 2003;105:219 –225.
5. Smulyan H, Siddiqui DS, Carlson RJ, London GM, Safar ME. Clinical
utility of aortic pulses and pressures calculated from applanated radialartery pulses. Hypertension. 2003;42:150 –155.
Michael F. O’Rourke
Albert Avolio
Ahmad Qasem
UNSW/St Vincent’s Clinic
Sydney, Australia
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
1
2
Letters to the Editor
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.
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.
8. O’Rourke MF, Pauca A, Jiang XJ. Pulse wave analysis. Br J Clin
Pharmacol. 2001;51:507–522.
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.
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Clinical Assessment of Wave Reflection
Michael F. O'Rourke, Albert Avolio and Ahmad Qasem
Downloaded from http://hyper.ahajournals.org/ by guest on June 17, 2017
Hypertension. 2003;42:e15-e16; originally published online September 22, 2003;
doi: 10.1161/01.HYP.0000095611.09419.F2
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 2003 American Heart Association, Inc. All rights reserved.
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