Peripheral Augmentation Index and Wave Reflection in the Radial Artery
To the Editor:
We were interested to read the recent article by Munir et al1 and congratulate them on making a valuable contribution to the field of pulse wave analysis. They and Payne and Webb, in their accompanying editorial,2 ponder why the late systolic shoulder (SBP2) corresponds so closely with central systolic blood pressure and suggest that selective modification of specific frequency components of the waveform may account for this. We agree with this (indeed, it is implicit in the widespread use of a transfer function to derive central blood pressure), but we consider that the physiological basis of this relationship is not readily apparent from this frequency-based interpretation. We concur with the belief by Payne and Webb2 that wave separation in the time domain may provide a clearer insight into the mechanisms involved, and we have undertaken such an analysis of the radial arterial waveform based on wave intensity analysis and data collected previously.3
Our findings are shown in the Figure. The data indicate that the characteristic peaked waveform seen in the radial artery is largely because of an early backward-traveling (reflected) compression wave that merges with the forward traveling compression wave attributable to cardiac ejection. This reflected wave elevates pressure and depresses flow. The early return of this wave is because of the relatively high wave speed in the radial and digital arteries and the proximity to the sites of impedance mismatching in the hand circulation.3 From the Figure, it is apparent that the SBP2 corresponds with a time in systole when the intensity of the backward compression wave has declined and before the arrival of a large forward expansion wave generated by deceleration of the rate of the ventricular contraction before closure of the aortic valve.4 The absence of significant wave intensity at the time of SBP2 means that pressure in the radial artery at this time should correspond relatively closely with aortic or central blood pressure, which is what Munir et al1 observed.
Our findings underline the value of wave intensity analysis and wave separation based on concurrent measurement of pressure and flow in understanding the hemodynamic events responsible for pressure wave morphology in the arterial tree and, we believe, provide a readily comprehensible explanation for the close correspondence between central systolic blood pressure and SBP2 reported by Munir et al.1
Sources of Funding
J.E.D. and D.F. are funded by fellowships from the British Heart Foundation.
Munir S, Guilcher A, Kamalesh T, Clapp B, Redwood S, Marber M, Chowienczyk P. Peripheral augmentation index defines the relationship between central and peripheral pulse pressure. Hypertension. 2008; 51: 112–118.
Payne RA, Webb DJ. Peripheral augmentation index: shouldering the central pressure load. Hypertension. 2008; 51: 37–38.
Zambanini A, Cunningham SL, Parker KH, Khir AW, McG Thom SA, Hughes AD. Wave-energy patterns in carotid, brachial, and radial arteries: a noninvasive approach using wave-intensity analysis. Am J Physiol Heart Circ Physiol. 2005; 289: H270–H276.