Pulsatile vs. continuous flow

Earlier left ventricular assist devices (LVADs) were volume displacement pumps (VDPs) that delivered pulsatile flow. However, due to improved survival rates of rotary blood pumps (RBPs), they are now the preferred device. Originally, RBPs operated at a constant speed and therefore delivered flow continuously with an absent or diminished pulse. Although RBPs were an improvement to the previous VDPs, the delivery of continuous flow has led to secondary complications, such as vascular and aortic valve dysfunction and gastrointestinal bleeding. Therefore, research has been made toward pulsatile RBPs by rapidly modulating pump speed. However, deriving pulsatile flow with RBPs has not been without controversy. Issues of debate have included the quantification of an adequate pulse and the influence of blood trauma and power consumption when generating a pulse with a RBP. Meanwhile, the pulsatility controversy has also expanded to total artificial heart and extracorporeal membrane oxygenator (ECMO) support. Nevertheless, commercial developments have been made toward combining the benefits of improved durability and survival rates of RBPs with a pulsing mechanism for mechanical circulatory support

In vitro evaluation of an immediate response starling-like controller for dual rotary blood pumps

Rotary ventricular assist devices (VADs) are used to provide mechanical circulatory support. However, their lack of preload sensitivity in constant speed control mode (CSC) may result in ventricular suction or venous congestion. This is particularly true of biventricular support, where the native flow-balancing Starling response of both ventricles is diminished. It is possible to model the Starling response of the ventricles using cardiac output and venous return curves. With this model, we can create a Starling-like physiological controller (SLC) for VADs which can automatically balance cardiac output in the presence of perturbations to the circulation. The comparison between CSC and SLC of dual HeartWare HVADs using a mock circulation loop to simulate biventricular heart failure has been reported. Four changes in cardiovascular state were simulated to test the controller, including a 700 mL reduction in circulating fluid volume, a total loss of left and right ventricular contractility, reduction in systemic vascular resistance (SVR) from 1300 to 600 dyne s=cm(5), and an elevation in pulmonary vascular resistance (PVR) from 100 to 300 dyne s=cm(5). SLC maintained the left and right ventricular volumes between 69214 mL and 29-182 mL; respectively, for all tests, preventing ventricular suction (ventricular volume=0 mL) and venous congestion (atrial pressures> 20 mm Hg). Cardiac output was maintained at sufficient levels by the SLC, with systemic and pulmonary flow rates maintained above 3.14 L= min for all tests. With the CSC, left ventricular suction occurred during reductions in SVR, elevations in PVR, and reduction in circulating fluid simulations. These results demonstrate a need for a physiological control system and provide adequate in vitro validation of the immediate response of a SLC for biventricular support

Mitral valve regurgitation with a rotary left ventricular assist device: the haemodynamic effect of inlet cannulation site and speed modulation

Mitral valve regurgitation (MVR) is common in patients receiving left ventricular assist device (LVAD) support, however the haemodynamic effect of MVR is not entirely clear. This study evaluated the haemodynamic effect of MVR with LVAD support and the influence of inflow cannulation site and LVAD speed modulation. Left atrial (LAC) and ventricular (LVC) cannulation was evaluated in a mock circulation loop with no, mild, moderate and severe MVR with constant speed and speed modulation (±600 RPM) modes. The use of an LVAD relieved pulmonary congestion during severe MVR, by reducing left atrial pressure from 20.5 to 10.8 (LAC) and 11.5 (LVC) mmHg. However, LAC resulted in decreased left ventricular stroke work (−0.08 J), ejection fraction (−7.9%) and higher MVR volume (+12.7 mL) and pump speed (+100 RPM) compared to LVC. This suggests that LVC, in addition to reducing MVR severity, also improves ventricular washout over LAC. LVAD speed modulation in synchrony with ventricular systole reduced MVR volume and increased ejection fraction with LAC and LVC, thus demonstrating the potential benefits of this mode, despite a reduction in cardiac output

Pulsatile operation of a continuous-flow right ventricular assist device (RVAD) to improve vascular pulsatility

Despite the widespread acceptance of rotary blood pump (RBP) in clinical use over the past decades, the diminished flow pulsatility generated by a fixed speed RBP has been regarded as a potential factor that may lead to adverse events such as vasculature stiffening and hemorrhagic strokes. In this study, we investigate the feasibility of generating physiological pulse pressure in the pulmonary circulation by modulating the speed of a right ventricular assist device (RVAD) in a mock circulation loop. A rectangular pulse profile with predetermined pulse width has been implemented as the pump speed pattern with two different phase shifts (0% and 50%) with respect to the ventricular contraction. In addition, the performance of the speed modulation strategy has been assessed under different cardiovascular states, including variation in ventricular contractility and pulmonary arterial compliance. Our results indicated that the proposed pulse profile with optimised parameters (Apulse = 10000 rpm and ωmin = 3000 rpm) was able to generate pulmonary arterial pulse pressure within the physiological range (9–15 mmHg) while avoiding undesirable pump backflow under both co- and counter-pulsation modes. As compared to co-pulsation, stroke work was reduced by over 44% under counter-pulsation, suggesting that mechanical workload of the right ventricle can be efficiently mitigated through counter-pulsing the pump speed. Furthermore, our results showed that improved ventricular contractility could potentially lead to higher risk of ventricular suction and pump backflow, while stiffening of the pulmonary artery resulted in increased pulse pressure. In conclusion, the proposed speed modulation strategy produces pulsatile hemodynamics, which is more physiologic than continuous blood flow. The findings also provide valuable insight into the interaction between RVAD speed modulation and the pulmonary circulation under various cardiovascular states

Rapid speed modulation of a totary yotal artificial heart impeller

Unlike the earlier reciprocating volume displacement–type pumps, rotary blood pumps (RBPs) typically operate at a constant rotational speed and produce continuous outflow. When RBP technology is used in constructing a total artificial heart (TAH), the pressure waveform that the TAH produces is flat, without the rise and fall associated with a normal arterial pulse. Several studies have suggested that pulseless circulation may impair microcirculatory perfusion and the autoregulatory response and may contribute to adverse events such as gastrointestinal bleeding, arteriovenous malformations, and pump thrombosis. It may therefore be beneficial to attempt to reproduce pulsatile output, similar to that generated by the native heart, by rapidly modulating the speed of an RBP impeller. The choice of an appropriate speed profile and control strategy to generate physiologic waveforms while minimizing power consumption and blood trauma becomes a challenge. In this study, pump operation modes with six different speed profiles using the BiVACOR TAH were evaluated in vitro. These modes were compared with respect to: hemodynamic pulsatility, which was quantified as surplus hemodynamic energy (SHE); maximum rate of change of pressure (dP/dt); pulse power index; and motor power consumption as a function of pulse pressure. The results showed that the evaluated variables underwent different trends in response to changes in the speed profile shape. The findings indicated a possible trade-off between SHE levels and flow rate pulsatility related to the relative systolic duration in the speed profile. Furthermore, none of the evaluated measures was sufficient to fully characterize hemodynamic pulsatility

Effect of varying pulmonary arterial compliance on the hemodynamics during speed modulation using pulsing profile C2.

<p>Effect of varying pulmonary arterial compliance on the hemodynamics during speed modulation using pulsing profile C2.</p

Investigation of the inherent left-right flow balancing of rotary total artificial hearts by means of a resistance box

With the incidence of end-stage heart failure steadily increasing, the need for a practical total artificial heart (TAH) has never been greater. Continuous flow TAHs (CFTAH) are being developed using rotary blood pumps (RBPs), leveraging their small size, mechanical simplicity, and excellent durability. To completely replace the heart with currently available RBPs, two are required; one for providing pulmonary flow and one for providing systemic flow. To prevent hazardous states, it is essential to maintain balance between the pulmonary and systemic circulation at a wide variety of physiologic states. In this study, we investigated factors determining a CFTAH’s inherent ability to balance systemic and pulmonary flow passively, without active management of pump rotational speed. Four different RBPs (ReliantHeart HA5, Thoratec HMII, HeartWare HVAD, and Ventracor VentrAssist) were used in various combinations to construct CFTAHs. Each CFTAH’s ability to autonomously maintain pressures and flows within defined ranges was evaluated in a hybrid mock loop as systemic and pulmonary vascular resistance (PVR) were changed. The resistance box, a method to quantify the range of vascular resistances that can be safely supported by a CFTAH, was used to compare different CFTAH configurations in an efficient and predictive way. To reduce the need for future in vitro tests and to aid in their analysis, a novel analytical evaluation to predict the resistance box of various CFTAH configurations was also performed. None of the investigated CFTAH configurations fully satisfied the predefined benchmarks for inherent flow balancing, with the VentrAssist (left) and HeartAssist 5 (right) offering the best combination. The extent to which each CFTAH was able to autonomously maintain balance was determined by the pressure sensitivity of each RPB: the sensitivity of outflow to changes in the pressure head. The analytical model showed that by matching left and right pressure sensitivity the inherent balancing performance can be improved. These findings may ultimately lead to a reduced need for manual speed changes or active control systems

Right ventricular PV loops for the mild and severe right heart failure (MRHF and SRHF) scenarios as well as at different pulmonary arterial compliance (PAC) levels.

<p>MRHF, mild right heart failure (ESPVR slope = 0.23 mmHg/mL); SRHF, severe right heart failure (ESPVR slope = 0.1 mmHg/mL); P_RV, right ventricular pressure; V_RV, right ventricular volume; PAC, pulmonary arterial compliance.</p

The PV loops produced through speed modulation with co- and counter-pulsation.

<p>CS: constant speed; C1: <i>A</i>_<i>pulse</i> = 13500 rpm and <i>ω</i>_<i>min</i> = 2000 rpm; C2: <i>A</i>_<i>pulse</i> = 10000 rpm and <i>ω</i>_<i>min</i> = 3000 rpm; C3: <i>A</i>_<i>pulse</i> = 6700 rpm and <i>ω</i>_<i>min</i> = 4000 rpm; P_RV, right ventricular pressure; V_RV, right ventricular volume.</p

Effect of varying right ventricular contractility on the hemodynamics during speed modulation using pulsing profile C2.

<p>Effect of varying right ventricular contractility on the hemodynamics during speed modulation using pulsing profile C2.</p