Kevin J. Gillars

Characterization of an Adult Mock Circulation for Testing Cardiac Support Devices

A need exists for a mock circulation that behaves in a physiologic manner for testing cardiac devices in normal and pathologic states. To address this need, an integrated mock cardiovascular system consisting of an atrium, ventricle, and systemic and coronary vasculature was developed specifically for testing ventricular assist devices (VADs). This test configuration enables atrial or ventricular apex inflow and aortic outflow cannulation connections. The objective of this study was to assess the ability of the mock ventricle to mimic the Frank–Starling response of normal, heart failure, and cardiac recovery conditions. The pressure–volume relationship of the mock ventricle was evaluated by varying ventricular volume over a wide range via atrial (preload) and aortic (afterload) occlusions. The input impedance of the mock vasculature was calculated using aortic pressure and flow measurements and also was used to estimate resistance, compliance, and inertial mechanical properties of the circulatory system. Results demonstrated that the mock ventricle pressure–volume loops and the end diastolic and end systolic pressure–volume relationships are representative of the Starling characteristics of the natural heart for each of the test conditions. The mock vasculature can be configured to mimic the input impedance and mechanical properties of native vasculature in the normal state. Although mock circulation testing systems cannot replace in vivo models, this configuration should be well suited for developing experimental protocols, testing device feedback control algorithms, investigating flow profiles, and training surgical staff on the operational procedures of cardiovascular devices.

Physiologic control of rotary blood pumps: An in vitro study

Rotary blood pumps (RBPs) are currently being used as a bridge to transplantation as well as for myocardial recovery and destination therapy for patients with heart failure. Physiologic control systems for RBPs that can automatically and autonomously adjust the pump flow to match the physiologic requirement of the patient are needed to reduce human intervention and error, while improving the quality of life. Physiologic control systems for RBPs should ensure adequate perfusion while avoiding inflow occlusion via left ventricular (LV) suction for varying clinical and physical activity conditions. For RBPs used as left ventricular assist devices (LVADs), we hypothesize that maintaining a constant average pressure difference between the pulmonary vein and the aorta (ΔPa) would give rise to a physiologically adequate perfusion while avoiding LV suction. Using a mock circulatory system, we tested the performance of the control strategy of maintaining a constant average ΔPa and compared it with the results obtained when a constant average pump pressure head (ΔP) and constant rpm are maintained. The comparison was made for normal, failing, and asystolic left heart during rest and at light exercise. The ΔPa was maintained at 95 ± l mm Hg for all the scenarios. The results indicate that the ΔPa control strategy maintained or restored the total flow rate to that of the physiologically normal heart during rest (3.8 L/m) and light exercise (5.4 L/m) conditions. The ΔPa approach adapted to changing exercise and clinical conditions better than the constant rpm and constant ΔP control strategies. The ΔPa control strategy requires the implantation of two pressure sensors, which may not be clinically feasible. Sensorless RBP control using the ΔPa algorithm, which can eliminate the failure prone pressure sensors, is being currently investigated.

Hemodynamic and pressure-volume responses to continuous and pulsatile ventricular assist in an adult mock circulation

This study investigated the hemodynamic and left ventricular (LV) pressure–volume loop responses to continuous versus pulsatile assist techniques at 50% and 100% bypass flow rates during simulated ventricular pathophysiologic states (normal, failing, recovery) with Starling response behavior in an adult mock circulation. The rationale for this approach was the desire to conduct a preliminary investigation in a well controlled environment that cannot be as easily produced in an animal model or clinical setting. Continuous and pulsatile flow ventricular assist devices (VADs) were connected to ventricular apical and aortic root return cannulae. The mock circulation was instrumented with a pressure–volume conductance catheter for simultaneous measurement of aortic root pressure and LV pressure and volume; a left atrial pressure catheter; a distal aortic pressure catheter; and aortic root, aortic distal, VAD output, and coronary flow probes. Filling pressures (mean left atrial and LV end diastolic) were reduced with each assist technique; continuous assist reduced filling pressures by 50% more than pulsatile. This reduction, however, was at the expense of a higher mean distal aortic pressure and lower diastolic to systolic coronary artery flow ratio. At full bypass flow (100%) for both assist devices, there was a pronounced effect on hemodynamic parameters, whereas the lesser bypass flow (50%) had only a slight influence. Hemodynamic responses to continuous and pulsatile assist during simulated heart failure differed from normal and recovery states. These findings suggest the potential for differences in endocardial perfusion between assist techniques that may warrant further investigation in an in vivo model, the need for controlling the amount of bypass flow, and the importance in considering the choice of in vivo model.

Expanded Pediatric Cardiovascular Simulator for Research and Training

A mock circulation system has been developed to approximate key anatomic features and simulate the pressures and flows of an infant. Pulsatile flow is generated by 10 cc pulsatile ventricles (Utah infant ventricular assist device). Systemic vasculature is mimicked through the use of 3/8” ID bypass tubing with two flexible reservoirs to provide compliance. Vascular resistance, including pulmonary, aortic, and major branches, is controlled via a series of variable pinch clamps. The coronary branch has a dynamic resistor so that the majority of flow occurs during diastole. The system is instrumented to measure key pressures and flows. Right atrial pressure, left atrial pressure, pulmonary artery pressure, and mean aortic pressure are measured with high-fidelity pressure catheters (Millar Instruments, Houston, TX). Flows are measured by transit time ultrasonic flow probes (Transonic Systems, Ithaca, NY) in the pulmonary artery, aorta, coronary artery, and brachiocephalic artery along with assist device flow. The system can be tuned to create the hemodynamic values of a pediatric patient under normal or heart failure conditions. Once tuned to the desired hemodynamic conditions, the loop may be used to test the performance of various circulatory support systems including the intra-aortic balloon pump, left and right ventricular assist devices, or cardiopulmonary support systems such as extracorporeal membrane oxygenation.

Achieving physiologic perfusion with ventricular assist devices: comparison of control strategies

Rotary blood pumps (RBP) are currently being used as a bridge to transplantation as well as for myocardial recovery and destination therapy for patients with heart failure. Physiologic control systems for RBP that can automatically and autonomously adjust the pump flow to match the physiologic requirement of the patient while avoiding suction for varying clinical and physical activity conditions are needed to reduce human intervention and error, and to improve the quality of life. For RBP used as left ventricular assist devices (LVAD), we hypothesize that maintaining a constant average pressure difference between the pulmonary vein and the aorta (/spl Delta/Pa) or maintaining a constant average pressure difference between the left ventricle and the aorta (/spl Delta/P) would give rise to a physiologically adequate perfusion while avoiding LV suction. Using a mock circulatory system we tested the performance of the control strategy of maintaining a constant average /spl Delta/Pa and a constant average pump pressure head (/spl Delta/P) and compared it with the results obtained when constant rpm is maintained. The comparison was made for normal, failing, and asystolic left heart during rest and at light exercise. The /spl Delta/Pa was maintained at 95 /spl plusmn/ 1 mmHg and /spl Delta/P was maintained at 75 /spl plusmn/ 1 mmHg for all the scenarios. The results indicate that the /spl Delta/Pa control strategy maintained or restored the total flow rate to that of the physiologically normal heart during rest (3.8 l/m) and light exercise (5.4 l/m) conditions. The /spl Delta/Pa approach adapted to changing exercise and clinical conditions better than the constant rpm and constant /spl Delta/P control strategies. Our computer simulation studies indicate that the /spl Delta/P control strategy performs better than the constant rpm control strategy, especially at higher cardiac demand situations, which could not be tested experimentally due to the limitation of the mock circulatory system.

Mock circulatory system for testing cardiovascular devices

A need exists for a mock ventricle and vasculature that behaves in a physiological manner for testing cardiac devices in normal pathologic states. To this end, an integrated mock cardiovascular system consisting of a mock atrium, mock ventricle, and mock systemic and coronary vasculature was developed specifically for testing ventricular assist devices. This surgically-equivalent test configuration enables atrial or ventricular apex inflow and aortic outflow cannulation connections. The objective of this study was to evaluate the mock ventricle for its pressure-volume (PV) relationships under normal, heart failure, and partial recovery test conditions. The PV relationships were investigated by varying ventricular volume over a wide range via atrial (preload) and aortic (afterload) occlusions. The mock circulation was set-up to mimic physiologically-equivalent normal, heart failure, and partial recovery test conditions. Results showed that the mock PV loops and the end-systolic PV relationships were representative of the physiological behavior characteristics of the natural heart for these test conditions. Although mock circulations cannot replace in vivo models, this configuration should be well suited for developing experimental protocols, testing device feedback control algorithms, investigating velocity profiles, and training surgical staff with operational procedures of assist devices.

Intraaortic Balloon Pump Timing Discrepancies in Adult Patients

The objective of this clinical study was to quantify the incidence and magnitude of intraaortic balloon pump (IABP) inflation and deflation landmark discrepancies associated with the IABP catheter arterial pressure waveform. Cardiac surgery patients with an IABP inserted prior to surgery were recruited. Following cardiac exposure, a high-fidelity pressure catheter was inserted into the aortic root for digital recording. The radial artery pressure signal was simultaneously recorded from the patient monitor along with the arterial pressure and electrocardiogram waveforms from the IABP console while operating at 1:1 and 1:2 synchronization. In selected patients, recordings were obtained with the IABP timed to the high-fidelity aortic root waveform. In all 11 patients, inflation and deflation landmark delays were observed when comparing the aortic root waveforms to the IABP arterial pressure waveforms (inflation delay = 74 ± 29 [23-117] ms; deflation delay = 71 ± 37 [24-141] ms, mean ± standard deviation [min-max]). Delays were greater when compared to the radial artery waveform (inflation delay = 175 ± 50 [100-233] ms; deflation delay = 168 ± 52 [100-274] ms). In all cases, the landmark delays were statistically different from zero (P < 0.001). Diastolic augmentation and afterload reduction varied with waveform source. Conflicting indications of afterload reduction occurred in four patients. Timing to the aortic root waveform resulted in greater diastolic pressure augmentation and afterload reduction but mixed changes in stroke volume. Delay and distortion of the arterial waveform was consistently found when measured through the IABP catheter lumen. These delays can alter IABP efficacy and may be eliminated by using high-fidelity sensing of aortic pressure.

Hemodynamic and Left Ventricular Pressure-Volume Responses to Counterpulsation in Mock Circulation and Acute Large Animal Models

Alternative therapies for treating heart failure patients are being explored to provide effective options for patients with progressive heart failure. Cardiac assist devices that promote myocardial recovery may be a potential solution. Ventricular assist devices (VAD) have demonstrated long-term efficacy and intraaortic balloon pumps (IABP) have shown short-term successes. In this paper, testing of a hybrid counterpulsation device (CPD) that couples the attributes of device longevity (VAD) with less invasive surgery (IABP) is presented. Hemodynamic and ventricular pressure-volume responses to a 40 ml CPD and 40 ml IABP were evaluated in vitro in an adult mock circulation and in vivo in a large animal heart failure model. The CPD is a flexing diaphragm ventricle with a controlled stroke volume up to 85 cc through a single, valveless cannula. In this study, the CPD was cannulated to the brachiocephalic artery to provide 40 ml of counterpulsation support. The CPD effectively provided diastolic augmentation increasing coronary flow and afterload reduction. These results were comparable to IABP. These preliminary studies suggest that CPD may be an effective therapy for treating patients with early stage heart failure.

The Influence of Mock Circulation Input Impedance on Valve Acceleration During In Vitro Cardiac Device Testing

For a mechanical heart valve, a strong spike in pressure during closing is associated with valve wear and erythrocyte damage; thus, for valid in vitro testing, the mock circulation system should replicate the conditions, including pressure spikes, expected in vivo. To address this issue, a study was performed to investigate how mock circulation input impedance affects valve closure dynamics. A left ventricular model with polyurethane trileaflet inflow valve and tilting disc outflow valve was connected to a Louisville mock circulation system, which incorporates 2 adjustable flow resistors and 2 compliances. In the study, 116 cases matched zero frequency modulus well (982–1147 dyn · s/cm5), but higher harmonics were purposely varied. Acceleration measured at the outflow valve ring (42.4–89.4 milli-Gs) was uncorrelated with impedance error (74.1–237 dyn · s/cm5 relative to target impedance), but was correlated with end-systolic impedance (1082–1319 dyn · s/cm5) for cases with high zero frequency modulus, which exhibited just less than full ejection. These differences demonstrate that mock circulation response affects the magnitude of the closing spike, indicating that control of this parameter is necessary for authentic testing of valves. Correlation of acceleration to end-systolic impedance was weak for low zero frequency modulus, which tended toward full or hyperejection, reinforcing common laboratory observations that valve closing also depends on ventricular operating conditions.

Extracorporeal Membrane Oxygenation Versus Counterpulsatile, Pulsatile, and Continuous Left Ventricular Unloading for Pediatric Mechanical Circulatory Support*

Objectives: Despite progress with adult ventricular assist devices, limited options exist to support pediatric patients with life-threatening heart disease. Extracorporeal membrane oxygenation remains the clinical standard. To characterize (patho)physiologic responses to different modes of mechanical unloading of the failing pediatric heart, extracorporeal membrane oxygenation was compared to intra-aortic balloon pump, pulsatile-flow ventricular assist device, or continuous-flow ventricular assist device support in a pediatric heart failure model. Design: Experimental. Setting: Large animal laboratory operating room. Subjects: Yorkshire piglets (n = 47; 11.7 ± 2.6 kg). Interventions: In piglets with coronary ligation-induced cardiac dysfunction, mechanical circulatory support devices were implanted and studied during maximum support. Measurements and Main Results: Left ventricular, right ventricular, coronary, carotid, systemic arterial, and pulmonary arterial hemodynamics were measured with pressure and flow transducers. Myocardial oxygen consumption and total-body oxygen consumption were calculated from arterial, venous, and coronary sinus blood sampling. Blood flow was measured in 17 organs with microspheres. Paired Student t tests compared baseline and heart failure conditions. One-way repeated-measures analysis of variance compared heart failure, device support mode(s), and extracorporeal membrane oxygenation. Statistically significant (p < 0.05) findings included 1) an improved left ventricular blood supply/demand ratio during pulsatile-flow ventricular assist device, continuous-flow ventricular assist device, and extracorporeal membrane oxygenation but not intra-aortic balloon pump support, 2) an improved global myocardial blood supply/demand ratio during pulsatile-flow ventricular assist device and continuous-flow ventricular assist device but not intra-aortic balloon pump or extracorporeal membrane oxygenation support, and 3) diminished pulsatility during extracorporeal membrane oxygenation and continuous-flow ventricular assist device but not intra-aortic balloon pump and pulsatile-flow ventricular assist device support. A profile of systems-based responses was established for each type of support. Conclusions: Each type of pediatric ventricular assist device provided hemodynamic support by unloading the heart with a different mechanism that created a unique profile of physiological changes. These data contribute novel, clinically relevant insight into pediatric mechanical circulatory support and establish an important resource for pediatric device development and patient selection.