In a recent editorial, Mather and Lewanczuk (1) describe the potential value of a generalized transfer function to synthesize the ascending aortic pressure wave from the radial pressure waveform, as recorded indirectly by applanation tonometry. The Food and Drug Administration (FDA) had previously accepted validity under different conditions of a generalized transfer function used in a commercial device: “The SphygmoCor SCOR-Px can calculate the calibrated ascending aortic pressure waveform using the radial artery pressure waveform recorded noninvasively from a radial artery tonometer and a brachial cuff measurement” (2). On the basis of an accompanying article by Hope et al. (3) in Diabetes Care, editorialists expressed concern regarding whether such an approach was applicable in diabetic subjects.
There are serious flaws in the report of Hope et al. (3) Their transfer function is different from that accepted by the FDA (2) and had been determined from use of conventional fluid-filled manometer systems of an unknown frequency response. From their original dataset (4), they had opined that different transfer functions were necessary to characterize the vascular properties of men and women and, subsequently from the same dataset (5), that their transfer function was unreliable for calculating the aortic augmentation index and other features of the synthesized aortic pressure wave. In their article, Hope et al. developed a “diabetes-specific” transfer function and showed that this could accurately generate the aortic from the radial pressure waveform. The problem with such an approach is that the same data used to validate were also used to generate the transfer function.
Other investigators (6) have found the approach used by Hope et al. to be problematic, but these issues are not taken into consideration. Other problems are also apparent. The “representative” (sic) synthesized aortic and measured aortic pressure values in their Fig. 1B show no evidence of the substantial (8 mmHg) diastolic pressure offset noted in Table 2. The method used for calibration (assuming mean and diastolic pressure are equal [4,7]) cannot give a diastolic pressure offset of 8 mmHg. In the system approved by the FDA, the difference was <1 mmHg (8). There are serious inconsistencies with other data published by the same authors (9); the time to inflection (144–163 ms) (their Table 2) is far higher than that described for the carotid artery (mean 81 ms), while the aortic augmentation index is much lower (13.1–21.3%) rather than higher than in the carotid (34 ± 13%).
Hope et al. (3) concluded that the use of their nondiabetic transfer function was inaccurate in generating aortic systolic and diastolic pressures in diabetic subjects. However, when the nondiabetic transfer function was used to calculate pulse pressure, as in the system approved by the FDA (2) (without the unexplained diastolic pressure offset), it was actually more accurate in diabetic (mean error 2 mmHg) than in nondiabetic (mean error 6 mmHg in Table 2) subjects.
These points cast doubt over previous publications from this group (4–6). Their transfer function differs markedly from that described by others (6) and those accepted by the FDA at frequencies above 3 Hz, which determines features of the pressure waveforms such as the aortic augmentation index (7).
The validity of the FDA-approved system is supported by studies of changes in heart rate and rhythm (8) and in arterial pressure during Valsalva maneuver (7) and with vasodilator challenge (8). It is also endorsed by modeling studies that have simulated changes in vascular properties in diabetic subjects (10). We believe that editorialists and readers of Diabetes Care can be reassured that no “diabetes-specific” (3) or “gender-specific” (4) transfer function is required to generate aortic pressure from the radial pressure waveform. An appropriately validated and approved generalized transfer function, however, is required.
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A.P.A. has received honoraria from, is a consultant for, and is a member of the advisory panel of Atcor Medical. M.F.O. is a founder and director of Atcor Medical and has received consulting fees from Atcor Medical, Philips, Pfizer, Lilly, and Servier.
