Date on Master's Thesis/Doctoral Dissertation

7-2019

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Bioengineering

Committee Chair

Koenig, Steven C.

Committee Co-Chair (if applicable)

Monreal, Gretel

Committee Member

Monreal, Gretel

Committee Member

Roussel, Thomas

Committee Member

Williams, Stuart

Abstract

Background: Left ventricular assist devices (LVAD) are operated at constant speeds (rpm), consequently, pump flow is passively determined by the pressure difference between the LV and aorta. Since the diastolic pressure gradient (~70 mmHg) is much larger than the systolic gradient (~10 mmHg), the majority of pump flow occurs during systole. This limitation results in sub-optimal LV volume unloading, LV washing, and diminished vascular pulsatility that may be associated with increased risk for clinically-significant adverse events, including stroke, bleeding, arteriovenous malformations, and aortic insufficiency. To address these clinical adverse events, an intelligent control strategy using pump speed modulation was developed to provide dynamic LV unloading during the cardiac cycle to produce near-physiologic pulsatile flow delivery similar to that of the native heart. Materials and Methods: The objective of this study was to integrate a novel algorithm to dynamically control Medtronic HVAD pump speed and demonstrate proofof-concept by characterizing hemodynamic performance in a mock flow loop primed with a blood analog solution (glycerol-saline, 3 cP) and tuned to simulate class IV heart failure (HF). The intelligent LVAD control was operated a varying pump speeds (Dspeed = 0, 1000, 1500, 2000, 2500 rpm) and systolic durations (30%, 35%, and 40%); systolic duration correlates to the time spent at either the high or low pump speed setting. The intelligent LVAD control strategy modulates pump speed within a cardiac cycle triggered from an R-wave of an EKG waveform set to 80 BPM. This pump speed modulation control strategy allows for pulsatile operation of a continuous flow LVAD within a single cardiac cycle. Hemodynamic waveforms (LV pressure-volume, aortic pressure-flow, and pump flow) and intrinsic pump parameters (speed and current) were recorded and analyzed for each test condition. We hypothesize that pump speed modulation may be configured for optimal volume unloading (rest), vascular pulsatility (reloading), and/or washing. Results and Discussion: The intelligent LVAD control system successfully demonstrated the ability to rapidly increase and decrease HVAD pump speed within a single cardiac cycle to provide asynchronous, synchronous co-pulsation, and synchronous counter-pulsation profiles for all systolic durations (30, 35, 40%) and Drpm tested (D1000, D1500, D2000, D2500). Asynchronous support was achieved when pump speed increase (or decrease) was independent of the cardiac cycle, co-pulsation support was achieved when increase in pump speed was timed with beginning of systole corresponding with ventricular contraction (systole), and counter-pulsation support was when increase in pump speed was timed with the end of systole corresponding with ventricular filling (diastole). Ideally, the intelligent control would increase (or decrease) the HVAD pump speed instantaneously upon R-wave detection; however, two distinct time delays were observed: (1) a time delay from detection of the R-wave trigger and increase (or decrease) of pump speed for systolic durations of 35% and 40% (being 45 ± 3.0 ms and 82 ± 3.0 ms respectively and (2) a delay in LVAD flow when pump speed was increased which is hypothesized to be from the blood analog solution’s fluid inertia. Left ventricular stroke volume decreased for all LVAD pump speed modulation operating conditions compared to baseline (HF with LVAD off) indicating that the intelligent control strategy was able to reduce LV volume with increasing HVAD support. The highest flow was achieved with the HVAD operated at a fixed speed of 4000 rpm; however, co-pulsation pump speed modulation at the largest pump speed differential (low = 1500, high = 4000, Drpm = 2500, and systolic duration 30%) resulted in a mean pump speed 3,300 ± 1,200 rpm. By comparison, the forward flow at fixed pump speed of 4,000 rpm was 4.8 L/min compared to a mean co-pulsation rpm was 4.5 L/min. Additionally, all operating settings for the intelligent control during pulsatile function produced an average forward flow through the aortic valve, while in contrast at higher fixed speeds (3,500 and 4,000 rpm) the mean aortic flow was negative. Pulse pressure (DP) decreased with increasing mean pump speed (rpm) for all operating modes (fixed, asynchronous, co-pulsation, counter-pulsation). When operating at the same mean pump speed (rpm) copulsation has increased hemodynamic benefit for pulsatility when compared to counterpulsation and fixed speed at the same mean pump (rpm). Conclusion: The results of this study show the ability of the intelligent HVAD control strategy to increase and decrease pump speed within a single cardiac cycle. This study showed that asynchronous modulation with phases of co-pulsation can generate near physiologic pulse pressure and vascular pulsatility when compared to counterpulsation support, while counter-pulsation can generate greater ventricular volume unloading and diastolic augmentation when compared to co-pulsation. Furthermore, the clinical impact of this study is that through speed modulation adverse events of continuous flow LVADs may be reduced such as incidences of bleeding associated with decreased pulsatility and a decrease in the risk of thrombus formation from poor washing around the aortic valve.

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