Response to Noninvasive Input Impedance of the Human Systemic Circulation
We thank O’Rourke1 and colleagues for their supportive comments on and concise interpretation and summary of our data,2 and we agree with most of the raised points. The points that we wish to comment on involve the 2nd and 3rd remarks.
Measurements of pulse wave velocity (PWV) and augmentation index (AIx) as measures of arterial stiffness and wave reflection, respectively, are certainly appealing in a clinical context. Wave separation analysis, however, yields wave reflection indices depending on the magnitude of wave reflection alone, not confounded by the timing of the reflected wave, nor sensitive to the identification of a characteristic point on the pressure waveform. Furthermore, in addition to stiffness (PWV) and reflection, the arterial cross-sectional area codetermines the characteristic impedance (Zc) and the total compliance of the arterial tree (the windkessel effect), and hence the input impedance of the arterial tree. With age and in hypertension, arterial stiffness, wave reflection, and aortic diameter increase,3 and all of these aspects are ideally to be taken into account when assessing the arterial system. As such, we will continue measuring impedance—in addition to PWV and AIx—in our future studies, whenever possible.
Responding to the 3rd point raised, it is indeed correct that blood flow velocities and vessel dimensions were acquired in the left ventricular outflow tract (LVOT) and not in the aorta. These measurements are optimally defined in the LVOT, and time integration of the flow waveforms yielded consistent and physiological values of cardiac output in men and women.2 We therefore feel confident to use the volume flow (not the flow velocity!) waveform measured in the LVOT as surrogate for the volume flow waveform at the entrance of the arterial tree. In our opinion, the assumption of using carotid pressure waveforms as surrogate for central aortic pressure may be more critical, as discussed.2 It is for this latter reason that we agree with O’Rourke et al that the derived Zc remains an approximate value. Direct measurement of aortic dimensions would indeed provide more direct evidence of age-related alterations in aortic size than the indirectly derived effective cross-sectional area.
In spite of these methodological limitations (inherent to any noninvasive protocol), our data are, however, remarkably consistent and indeed in overall agreement with existing literature. We therefore believe that the different evolution in men and women in total arterial compliance and characteristic impedance over the age range 35 to 55 is at least intriguing. At present, we are analyzing the local geometric and mechanical properties of the common carotid and femoral artery, which were also measured in the Asklepios population.4 These data should provide some insight in the age-related evolution of arterial vessel size and stiffness, and this for a large elastic and muscular vessel. On a longer time scale, we eagerly await our longitudinal data, and we look forward to verifying whether our data will adhere to the time-course of Zc, PWV and AIx predicted by Gary Mitchell in his editorial comment.5
Source of Funding
This research was funded by Fonds voor Wetenschappelijk Onderzoek Vlaanderen research grant G. 0427.03 (Asklepios Study).
O’Rourke MF, Nichols WW, Murgo JP. Noninvasive input impedance of the human systemic circulation. Hypertension. 2007; 50: e16.
Segers P, Rietzschel ER, De Buyzere ML, Vermeersch SJ, De Bacquer D, Van Borte LM, De Backer G, Gillebert TC, Verdonck PR. Noninvasive (input) impedance, pulse wave velocity, and wave reflection in healthy middle-aged men and women. Hypertension. 2007; 49: 1248–1255.
O’Rourke MF, Nichols WW. Aortic diameter, aortic stiffness, and wave reflection increase with age and isolated systolic hypertension. Hypertension. 2005; 45: 652–658.
Rietzschel ER, De Buyzere ML, Bekaert S, Segers P, De Bacquer D, Cooman L, Van Damme P, Cassiman P, Langlois M, van Oostveldt P, Verdonck P, De Backer G, Gillebert TC. Rationale, design, methods and baseline characteristics of the Asklepios Study. Eur J Cardiovasc Prev Rehabil. 2007; 14: 179–191.
Mitchell GF. Impedance progress. Aortic diameter rears its head again? Hypertension. 2007; 49: 1207–1209.