Quantification of the Calibration Error in the Transfer Function-Derived Central Aortic Blood Pressures

Background The accuracy of the central aortic systolic (SBP-C) and pulse (PP-C) blood pressures estimated noninvasively by a generalized transfer function technique has been questioned. The purpose of the present study was to quantify precisely the impact of the input errors (differences between the oscillometric (SBP-O, DBP-O, PP-O) and invasive (SBP-B, DBP-B, PP-B) brachial systolic, diastolic, and pulse blood pressures) on the output errors (differences between the estimated and invasively measured SBP-C and PP-C). Methods Invasive high-fidelity right brachial and central aortic pressure waveforms, and SBP-O, DBP-O, and PP-O (=SBP-O - DBP-O) were obtained simultaneously in 40 patients during cardiac catheterization. A generalized transfer function was applied on the individual brachial pressure waveform to derive predicted SBP-C and PP-C. Results Observed input errors were -2.3 +/- 5.8 mm Hg from SBP-O, 8.1 +/- 5.3 mm Hg from DBP-O, and -10.4 +/- 7.1 mm Hg from PP-O, respectively. The output errors were -2.2 +/- 6.4 mm Hg and -10.3 +/- 8.0 mm Hg for SBP-C and PP-C, respectively, when the brachial pressure waveforms were recalibrated using SBP-O and DBP-O. The outputs were determined by the inputs according to the Equation (1): SBP-C error = 0.97 x SBP-O error + 0.03 ( r = 0.88, P < 0.01); and the Equation ( 2): PP-C error = 0.96 x PP-O error - 0.30 ( r = 0.86, P < 0.01). Conclusions Noninvasive application of the generalized transfer function techniques produces estimates of SBP-C and PP-C with errors equivalent to those of the oscillometric blood pressure monitor in the estimation of SBP-B and PP-B. The output errors can be predicted from input errors of SBP-O and DBP-O.