George M. Pantalos

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.

Modeling and control of a brushless DC axial flow ventricular assist device.

This article presents an integrated model of the human circulatory system that incorporates circulatory support by a brushless DC axial flow ventricular assist device (VAD), and a feedback VAD controller designed to maintain physiologically sufficient perfusion. The developed integrated model combines a network type model of the circulatory system with a nonlinear dynamic model of the brushless DC pump. We show that maintaining a reference differential pressure between the left ventricle and aorta leads to adequate perfusion for different pathologic cases, ranging from normal heart to left heart asystole, and widely varying physical activity scenarios from rest to exercise.

Intraoperative Evaluation of the HeartMate II Flow Estimator

BACKGROUND Direct measurement of blood flow output has been incorporated into ventricular assist devices (VADs), but long-term reliability of the additional device components has raised concerns regarding sensor drift and failure. As an alternative approach, the HeartMate II axial VAD (Thoratec Corp, Pleasanton, CA) estimates device flow output from power consumption and rotational speed of the device motor. This study evaluated the accuracy of HeartMate II flow estimation at the time of implantation. METHODS In 20 patients, intraoperative blood flow measurement of the HeartMate II flow estimator was compared with flow values obtained with an ultrasonic flow probe placed around the device outflow graft. Estimated and measured VAD flow data were simultaneously recorded and digitally stored while the device motor speed varied from 7,800 to 11,000 rpm and while achieving device flow outputs of 2 to 7 liters/min. Estimated and measured flows were compared using linear regression analyses and root mean square error. RESULTS HeartMate II flow estimation (FE) demonstrated a linear correlation with ultrasonic flow probe (FP) measurements: FE = 0.74 FP + 0.99 (R(2) = 0.56, p = 0.0001). A root mean square error of 0.8 liters/min was observed between flow estimation and direct flow measurement and suggests a 15% to 20% difference at flows of 4 of 6 liters/min. CONCLUSIONS These results suggest that HeartMate II flow estimation may be used to provide directional information for trend purposes rather than absolute values of device blood flow output. Patient management should include but not be limited to this information.

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.

Estimation of timing errors for the intraaortic balloon pump use in pediatric patients.

The use of the intraaortic balloon pump (IABP) for managing acute left ventricular failure in pediatric patients is less successful than in adults. It is often reported that rapid pediatric heart rates make accurate timing difficult to achieve. Traditional IABP theory requires that the balloon inflate during diastole (after aortic valve closure), for optimum coronary pressure and flow augmentation, and deflate just before the next systole for optimal ventricular afterload reduction. Errors in timing balloon inflation and deflation may result in the reduced IABP efficacy seen in children. To investigate timing errors when using the traditional IABP inflation and deflation markers in pediatric patients, six patients (age, 2.2+/-1.4 years; weight, 11.5+/-3.9 kg) were studied intraoperatively. Radial artery pressure (RAP) waveforms from a standard, fluid-filled pressure monitoring system were recorded on an FM data tape recorder simultaneously with high-fidelity, aortic root pressure waveforms, aortic root flow waveforms, and M-mode echocardiography. For each patient, a sequence of five recorded waveforms was analyzed. The mean +/- standard deviation of the time delay between aortic root and RAP markers and percentage delay of the corresponding part of the cardiac cycle were determined. When compared with aortic root waveforms, the RAP waveform consistently showed a delay in the IABP timing markers. A 107+/-23 msec (53+/-11%) delay in diastolic inflation and a 92+/-11 msec (40+/-4%) delay in presystolic deflation was found. If IABP timing to the RAP markers were to be used, the delay in IABP inflation would result in reduced diastolic augmentation, and the delay in IABP deflation into the systolic period would increase afterload. M-mode echocardiography provided timing markers that were identical to those provided by high-fidelity aortic root pressure waveforms. The combined effect of these delays on IABP function could substantially reduce the efficacy of the IABP in pediatric patients, indicating the need for more accurate indices for IABP timing in this patient group.

Long-term mechanical circulatory support system reliability recommendation: American Society for Artificial Internal Organs and The Society of Thoracic Surgeons: Long-term mechanical circulatory support system reliability recommendation

Jointly developed by members of the American Society for Artificial Internal Organs and the Society of Thoracic Surgeons along with staff from the Food and Drug Administration, the National Heart, Lung and Blood Institute and other experts, this recommendation describes the reliability considerations and goals for Investigational Device Exemption and Premarket Approval submissions for long-term, mechanical circulatory support systems. The recommendation includes a definition of system failure, a discussion of an appropriate reliability model, a suggested in vitro reliability test plan, reliability considerations for animal implantation tests, in vitro and animal in vivo performance goals, the qualification of design changes during the Investigational Device Exemption clinical trial, the development of a Failure Modes Effects and Criticality Analysis, and the reliability information for surgeons and patient candidates. The document will be periodically reviewed to assess its timeliness and appropriateness within five years.

How much of the intraaortic balloon volume is displaced toward the coronary circulation

Objective During intraaortic balloon inflation, blood volume is displaced toward the heart (Vtip), traveling retrograde in the descending aorta, passing by the arch vessels, reaching the aortic root (Vroot), and eventually perfusing the coronary circulation (Vcor). Vcor leads to coronary flow augmentation, one of the main benefits of the intraaortic balloon pump. The aim of this study was to assess Vroot and Vcor in vivo and in vitro, respectively. Methods During intraaortic balloon inflation, Vroot was obtained by integrating over time the aortic root flow signals measured in 10 patients with intraaortic balloon assistance frequencies of 1:1 and 1:2. In a mock circulation system, flow measurements were recorded simultaneously upstream of the intraaortic balloon tip and at each of the arch and coronary branches of a silicone aorta during 1:1 and 1:2 intraaortic balloon support. Integration over time of the flow signals during inflation yielded Vcor and the distribution of Vtip. Results In patients, Vroot was 6.4% ± 4.8% of the intraaortic balloon volume during 1:1 assistance and 10.0% ± 5.0% during 1:2 assistance. In vitro and with an artificial heart simulating the native heart, Vcor was smaller, 3.7% and 3.8%, respectively. The distribution of Vtip in vitro varied, with less volume displaced toward the arch and coronary branches and more volume stored in the compliant aortic wall when the artificial heart was not operating. Conclusion The blood volume displaced toward the coronary circulation as the result of intraaortic balloon inflation is a small percentage of the nominal intraaortic balloon volume. Although small, this percentage is still a significant fraction of baseline coronary flow.

Acute hemodynamic efficacy of a 32-ml subcutaneous counterpulsation device in a calf model of diminished cardiac function.

The acute hemodynamic efficacy of an implantable counterpulsation device (CPD) was evaluated. The CPD is a valveless single port, 32-ml stroke volume blood chamber designed to be connected to the human axillary artery using a simple surface surgical procedure. Blood is drawn into the pump during systole and ejected during diastole. The acute hemodynamic effects of the 32-ml CPD were compared to a standard clinical 40-ml intra-aortic balloon pump (IABP) in calves (80 kg, n = 10). The calves were treated by a single oral dose of Monensin to produce a model of diminished cardiac function (DCF). The CPD and IABP produced similar increases in cardiac output (6% CPD vs. 5% IABP, p > 0.5) and reduction in left ventricular external work (14% CPD vs. 13% IABP, p > 0.5) compared to DCF (p < 0.05). However, the ratio of diastolic coronary artery flow to left ventricular external work increase from DCF baseline (p < 0.05) was greater with the CPD compared to the IABP (15% vs. 4%, p < 0.05). The CPD also produced a greater reduction in left ventricular myocardial oxygen consumption from DCF baseline (p < 0.05) compared to the IABP (13% vs. 9%, p < 0.05) despite each device providing similar improvements in cardiac output. There was no early indication of hemolysis, thrombus formation, or vascular injury. The CPD provides hemodynamic efficacy equivalent to an IABP and may become a therapeutic option for patients who may benefit from prolonged counterpulsation.

Discerning aortic waves during intra-aortic balloon pumping and their relation to benefits of counterpulsation in humans

An explanation of the mechanisms leading to the beneficial hemodynamic effects of the intra-aortic balloon pump (IABP) is lacking. We hypothesized that inflation and deflation of the balloon would generate a compression (BCW) and an expansion (BEW) wave, respectively, which, when analyzed with wave intensity analysis, could be used to explain the hemodynamic benefits of IABP support. Simultaneous ascending aortic pressure (Pao) and flow rate (Qao) were recorded in 25 patients during control conditions and with IABP support of 1:1 and 1:2. Diastolic aortic pressure augmentation (Paug) and end-diastolic aortic pressure (ED Pao) reduction were calculated from Pao. Energies of the BCW and BEW were obtained by integrating the wave intensity contour over time. Paug was 19.1 mmHg (SD 13.6) during 1:2 support. During 1:1 support significantly higher Paug of 21.1 mmHg (SD 13.4) was achieved (P < 0.001). ED Pao decreased from 50.9 mmHg (SD 15.1) to 43.9 mmHg (SD 15.7) (P < 0.0001) during 1:1 assistance and the decrease was not statistically different with 1:2. During 1:1 support the energy of BCW was correlated positively to Paug (r = 0.83, P < 0.0001) and energy of the BEW correlated negatively to ED Pao (r = 0.78, P < 0.005); these relationships were not statistically different during 1:2. In conclusion, the energies of the BCW and BEW are directly related to Paug and ED Pao, which are the conventional hemodynamic parameters indicating IABP benefits. These findings imply a cause and effect mechanism between the energies of BCW and BEW, and IABP hemodynamic effects.