Benchtop Experimental Determination of PWV

PWV was calculated in each model using data acquired from a benchtop experiment (Figure 4). The pump, tubing length, in-vitro model test unit, and fittings were configured exactly as they were during the optics experiment. The compliance chamber and resistance valve were also adjusted in the same manner. Flow was acquired using a clip-on transit time ultrasonic flow probe (16PXL, Transonic, NY) attached to the 6-inch tygon tubing connected to the model. Measurements were performed at both ends of the tube separately and included ECG signal data for flow alignment. The ECG was generated by the PD-1100 pump system and was in sync with the piston movement. Data was acquired for 14 cycles and imported to Matlab for PWV calculation.

Schematic of bench top experimental setup. (a) Pulsatile pump; (b) 1.5 L compliance chamber; (c) 25 ft of PVC tubing; (d) PXL16 transit time ultrasonic flow probe; (e) luer-lock pressure transducer; (f) in-vitro model; (g) 25 ft of PVC tubing; (h) resistance valve

A Matlab program was created to calculate PWV from experimental data. The ECG signals acquired during the proximal and distal acquisition were used to align the flow waveforms as if they were obtained simultaneously (Figure 5). Flow waveforms were smoothed using a savitsky-golay filter. PWV was calculated using the equation, Δd/Δt, where Δd is the distance between flow measurement sites and Δt is the transit-time automatically calculated through the XCOR, TTM and TTF algorithms. This results in a “regional” PWV measurement, but is theoretically equal to the local PWV due to constant model material properties and geometry. This calculation is made once for each cycle for a total of 14 cycles. The experimental procedure was repeated 3 times for a total of 42 PWV measurements. PWV is expressed as the average of those calculations, plus or minus one standard deviation (SD).

(a) flow waveforms for proximal and distal locations are separately measured for 14 cycles; (b) each set of waveforms is examined individually; (c) proximal and distal waveforms are aligned using the ECG signal from each acquisition