Using multiple placements of accelerometers to measure cardiovascular pulse transit times

To measure pulse transit times (PTT) and calculate pulse wave velocities (PWV), tri-axial and uniaxial accelerometers were placed in groups of 2 to 4 over the manubrium, xiphoid process, forehead, wrist and ankle, and/or over the carotid, femoral, and posterior auricular arteries in 11 consented supine subjects. Signals were sampled at 1kHz and filtered. Radial vectors were calculated from the tri-axial measurements. A 3-lead ECG was simultaneously collected over the same 180s window, as well as respiratory rate. Ensemble averages (with ±S.D.) and raster plots were generated for each filtered time series from 200 ms before to 800 ms after the peak of each ECG R-wave. Lag times between the R-wave peak (taken as t=0) and one or more prominent peaks (or valleys if appropriate) of the various accelerometer signals were calculated, by using the signal from the axis (or the radial vector) with the best signal to noise ratio. PWV was calculated from the regression of the distance measured versus the PTT between pairs, especially of the clinically-relevant carotid-femoral PTT. A spectral analysis of each ensemble was taken, with the hypothesis that in young adults the measures at the periphery would have less energy at higher frequencies than those of an older adult because of age-related changes in arterial distensibility.