Confluence of incident and reflected waves interferes with systolic foot detection of the carotid artery distension waveform

OBJECTIVES: Local pulse wave velocity, a direct measure of arterial stiffness, can be measured using the systolic foot of the pressure waveform as the time reference point. The accuracy and precision of systolic foot identification, which may be disturbed by early wave reflections, heavily affects pulse wave transit time measurements. We investigated within subjects the existence of early wave reflections and their interference with systolic foot identification. METHODS: Fourteen ultrasound-derived distension waveforms, spaced over 16. 4 mm, were simultaneously recorded in the CCA 3 cm proximal of the bifurcation of 12 young subjects. The second derivatives of the distension waveforms were calculated to identify the systolic foot and an inflection point preceding systolic peak distension. Pulse wave transit time was calculated as the time difference between the most proximal and most distal time-point, using either the systolic foot or the inflection point. The time to reflection (DeltaTSFIP) was defined as the time difference between the systolic foot and the inflection point. RESULTS: Both transit times (TT SF and TT IP) could be determined with good intrasubject precision: 0. 7 and 1. 4 ms, respectively. The systolic foot is running forward, TT SF = 3. 1 +/- 0. 9 ms, whereas the inflection point appears to run backward, TT IP = -3. 9 +/- 1. 4 ms. DeltaTSFIP was 44. 3 +/- 8. 8 ms. CONCLUSION: Despite the good intrasubject reproducibility, confluence of incident and reflected waves disturbs identification and discrimination of the systolic foot and the inflection point, resulting in biased estimates. Therefore both points are unsuitable for local pulse transit time measurements in the common carotid artery.