Author + information
- Bryan Williams, MD⁎ (, )
- Peter Lacy, PhD,
- Peter Yan, MBBS,
- Ngak Hwee Chua,
- Chen Liang, PhD and
- Choon Meng Ting, MBBS
- ↵⁎Department of Cardiovascular Sciences and Leicester NIHR Biomedical Research Unit in Cardiovascular Disease, University of Leicester, Clinical Sciences Wing, Glenfield Hospital, Groby Road, Leicester LE3 9QP, United Kingdom
We thank Dr. Wassertheurer and colleagues for their interest in our study (1) and note their agreement about the importance of developing simpler noninvasive methods for deriving central aortic systolic blood pressure (CASP) in man. They comment on the level of agreement between invasive and noninvasive measurements and suggest that analyzing multiple 10-s sampling windows using the same noninvasive initial calibration values is inappropriate and may have influenced our results.
Calibration of radial pressure waves to a single brachial blood pressure (BP) measurement in the noninvasive measurement of CASP is standard practice for many devices. However, the A-pulse tonometer (HealthSTATS International, Singapore) used in our study operates in a different way. After calibration of the initial 10-s radial waveform block to brachial BP, pulse wave height is subsequently scaled automatically to generate updated brachial BPs. This method allows the device to update brachial BP on a block-by-block basis across the sampling period, accounting for natural fluctuations in brachial BP over time (2). This technique potentially reduces scatter in the agreement between invasive and noninvasive measurements, largely accounting for the improved accuracy reported. Indeed, it is remarkable that a single brachial BP calibration of initial radial pressure waves yielded such good results in previous studies over extended sampling periods.
Mindful of the above, we analyzed the influence of calibrating all 10-s data blocks acquired during sampling to the initial brachial BP, which constitutes standard practice for all other devices. In this, we also processed radial waveforms using the algorithm of an established method (SphygmoCor, AtCor Medical, West Ryde, Australia). Bland-Altman comparison of such processed data revealed a wider degree of scatter (p < 0.01) compared with that reported in our study (1) (Fig. 1). This finding suggests that waveform calibration drift across the sampling period is likely to contribute to greater inaccuracy in deriving CASP in other studies.
With regard to data presentation for t test comparison in our study (1), we presented mean ± SE values for invasive and noninvasive data from our 20 subjects, both as mean per subject and for each individual data block. By contrast, data for Bland-Altman comparisons are frequently presented as mean ± SD. Accordingly, this format was followed in presenting the difference between invasive and noninvasive data.
In summary, we thank Dr. Wassertheurer and colleagues for their comments, which prompted us to better define the importance of automated waveform calibration updating in radial tonometry measurements. This, together with use of an N-point moving average, seems to provide improved accuracy for the noninvasive measurement of CASP in man.
- American College of Cardiology Foundation