Response to Timing and Amplitude of Wave Reflection
I appreciate Drs O’Rourke and Nichols’1 kind words and thoughtful analysis of my recent commentary on techniques for assessing wave reflection.2 They have raised 2 major differences between the observations summarized in my commentary and their own convictions regarding wave reflection. However, these 2 differences boil down to a single question: what happens to reflected wave transit time (RWTT) as the aorta stiffens and aortic pulse wave velocity (PWV) increases? If the distance to the reflecting site is fixed, an increase in PWV should be associated with a proportional decrease in RWTT. In contrast, the concept of “impedance matching” predicts that reflecting sites will shift distally as PWV increases because impedance mismatch in the muscular arteries is diminished. Thus, the decrease in RWTT will be attenuated relative to the increase in PWV. Which model is supported by data? Studies by Kelly, O’Rourke et al,3 Mitchell et al,4 and McEniery et al,5 have concordantly shown that in large samples, timing of wave reflection does not change much with advancing age and certainly does not change in inverse proportion to the dramatic increase in PWV that occurs. For example, in the study by McEniery, timing of wave reflection decreased by ≈11% (men) to 14% (women) across 7 decades (age 20 to 90 years). At the same time, carotid-femoral PWV increased by ≈100%, suggesting that the location of the reflecting site shifted distally by ≈75% across this same age span. These findings were similar to our earlier results from Framingham.4
I agree with Drs O’Rourke and Nichols that there are individuals in whom RWTT is as high as 200 ms or as low as 80 ms. However, conclusions regarding general relations between arterial properties and wave reflection are based on average data, not anecdote. I disagree that it is very difficult to identify the RWTT from a central pressure waveform. The onset of the reflected wave is readily identifiable in the vast majority of high quality carotid artery pressure tracings, making failure to find the reflected wave an unlikely explanation for the attenuated reduction in RWTT with age.
In contrast to the suggestion by Drs O’Rourke and Nichols, the Westerhof triangulation technique will be of limited value as a tool for assessing timing of wave reflection, and Westerhof et al appropriately never suggested this application for their technique.6 By empirically setting peak flow at the observed RWTT or at 30% of the systolic ejection period, timing of wave reflection is assumed and therefore cannot be assessed using this technique.
I too welcome new approaches to analysis of central hemodynamics, but I would like to reiterate that the Westerhof triangulation paper was a concept paper only and still requires validation in a large, unbiased sample where both pressure and flow have been measured.
G.F.M. is owner of Cardiovascular Engineering, Inc, a company that designs and manufactures devices that measure vascular stiffness. The company uses these devices in clinical trials that evaluate the effects of diseases and interventions on vascular stiffness.
Mitchell GF. Triangulating the peaks of arterial pressure. Hypertension. 2006; 48: 543–545.
Kelly R, Hayward C, Avolio A, O’Rourke M. Noninvasive determination of age-related changes in the human arterial pulse. Circulation. 1989; 80: 1652–1659.
Mitchell GF, Parise H, Benjamin EJ, Larson MG, Keyes MJ, Vita JA, Vasan RS, Levy D. Changes in arterial stiffness and wave reflection with advancing age in healthy men and women: the Framingham Heart Study. Hypertension. 2004; 43: 1239–1245.
Westerhof BE, Guelen I, Westerhof N, Karemaker JM, Avolio A. Quantification of wave reflection in the human aorta from pressure alone: a proof of principle. Hypertension. 2006; 48: 595–601.