A fluid dynamics study in a 50 cc pulsatile ventricular assist device: influence of heart rate variability

Abstract

Although left ventricular assist devices (LVADs) have had success in supporting severe heart failure patients, thrombus formation within these devices still limits their long term use. Research has shown that thrombosis in the Penn State pulsatile LVAD, on a polyurethane blood sac, is largely a function of the underlying fluid mechanics and may be correlated to wall shear rates below 500 s(-1). Given the large range of heart rate and systolic durations employed, in vivo it is useful to study the fluid mechanics of pulsatile LVADs under these conditions. Particle image velocimetry (PIV) was used to capture planar flow in the pump body of a Penn State 50 cubic centimeters (cc) LVAD for heart rates of 75-150 bpm and respective systolic durations of 38-50%. Shear rates were calculated along the lower device wall with attention given to the uncertainty of the shear rate measurement as a function of pixel magnification. Spatial and temporal shear rate changes associated with data collection frequency were also investigated. The accuracy of the shear rate calculation improved by approximately 40% as the resolution increased from 35 to 12 μm/pixel. In addition, data collection in 10 ms, rather than 50 ms, intervals was found to be preferable. Increasing heart rate and systolic duration showed little change in wall shear rate patterns, with wall shear rate magnitude scaling by approximately the kinematic viscosity divided by the square of the average inlet velocity, which is essentially half the friction coefficient. Changes in in vivo operating conditions strongly influence wall shear rates within our device, and likely play a significant role in thrombus deposition. Refinement of PIV techniques at higher magnifications can be useful in moving towards better prediction of thrombosis in LVADs.

Fig. 1

The 50 cc inlet and outlet valve orientations[13]

Fig. 2

(a) 5 mm measurement plane for PIV studies. (b) Schematic of pump body displaying areas of PIV data collection. The (i) blue box indicates the low PIV magnification and the (ii) red and (iii) green boxes the high PIV magnification areas of study. Wall shear rates are calculated along a 16 mm portion of the wall in the direction of the (iv) orange line. (c) 5 mm plane sectional cut exposing the (v) low wall shear zone with (vi) 16 mm orange line representing the region for the 10 and 50 ms wall shear rate comparison.

Fig. 3

Flow maps corresponding to the locations defined in Fig. 2(b). The low PIV magnification results are represented by the (a) blue box, and the high PIV magnification results by the (b) red and (c) green boxes. Flow similarities at the two different PIV magnifications exist for flow 300 ms from the onset of diastole.

Fig. 4

Contour plots displaying wall shear rate calculations along the (iv) orange line of Fig. 2(b) for the (a) low and (b) high PIV magnifications. Positive shear is a result of flow in the clockwise direction, with negative shear caused by flow in the counterclockwise direction. The shear rate was normalized over 500 s⁻¹ to observe areas between −1 and 1 that are prone to thrombus deposition.

Fig. 5

Shear rate calculations for 200, 350, and 500 ms of the cardiac cycle from Fig. 4 Dashed and solid lines represent results for the low and high PIV magnifications, respectively

Fig. 6

Contour plots displaying wall shear rate calculations along the orange line in Fig. 2(c) at the high PIV magnification. Data was collected every 10 ms in Fig. 6(a), and every 50 ms in Fig. 6(b).

Fig. 7

Spatial plots of wall shear rates along the (vi) orange line of Fig. 2(c), corresponding to regions of relative (a) low flow, 500–550 ms, and (b) high flow, 300–350 ms, into the cardiac cycle. Wall shear rates are shown every 10 ms for both time periods.

Fig. 8

Temporal plots of wall shear rates along the (vi) orange line of Fig. 2(c), corresponding to PIV data collection in (a) 10 ms intervals and (b) 50 ms intervals

Fig. 9

Wall shear rates, at the 5 mm plane, for 75, 115, and 150 bpm for a 6 mm section, 29.2 mm from the center of the inlet port and along the circumference of the device

Fig. 10

Contour plots displaying wall shear rate calculations for the 6 mm section, referenced in Fig. 9, at the high magnification. Shear rate distributions for (a) 75, (b) 115, and (c) 150 bpm, respectively. All images of wall shear rates are normalized with kinematic viscosity over the square of the average inlet velocity. The contour scale applies to this normalization. Dashed lines correspond to the normalized wall shear rate plots of Fig. 11.

Fig. 11

Normalized wall shear rates for 75, 115, and 150 bpm along the dashed lines of Fig. 10