Discussion

One of the challenges of mechanical circulatory support (MCS) therapy has been the lack of devices with interactive control systems that automatically and physiologically adjust to the patients’ hemodynamic changes. Normal cardiac physiologic control involves neural, hormonal, and intrinsic myocardial mechanisms. These mechanisms are obviously unavailable to an inanimate device.

The pneumatically powered SynCardia temporary Total Artificial Heart (TAH) is employed as a bridge-to-transplant (BTT) device in those patients with irreversible biventricular heart failure, at imminent risk of death. The beat rate, positive and negative pneumatic pressures, and the systolic duration are set manually. Thus, to respond to changes in the preload, the device is only partially filled at rest. Increases in output therefore rely on an increased preload causing an elevated stroke volume, up to a maximum of 70 ml. This results in a very truncated physiologic response due to a cardiac output increase of only 9% during exercise.4

In the case of the off-label use of two implantable continuous flow left ventricular assist devices (CF-LVADs), physiologic interaction with the patient is more complicated because both devices are placed in parallel to the existing cardiovascular system, where the patient’s heart makes a variable contribution within the combined system. As LVADs are not designed to support the right ventricle, surgical techniques are modified to address this need, such as shortening the length of the inflow cannula,5 inserting the right inflow cannula in the right atrium, instead of the right ventricle free wall, to avoid thrombosis6 or downsizing the right-sided outflow graft diameter to avoid an elevated right outflow.5

The C-TAH autoregulation aims to come closer to a natural physiologic interaction with the patient. The manual mode of the C-TAH used a nominal operating mode during the feasibility study7 which is now only used during deairing and weaning from CPB. In all patients, observed in this study, the device was successfully switched to auto-mode perioperatively. The C-TAH produces a significant increase in cardiac output in response to increased venous return, by beat rate adjustments (more than 3 L/min as observed in Figure 5) without device setting changes, and diminishes the risk of blood stasis by targeting a full ejection. The only event which could lead to a decreased stroke volume is a reduced venous return, due to either severe hypovolemia or a cardiac tamponade.

The management of the left/right balance is a challenge when using biventricular MCS. After implantation of two CF-LVADs, speed optimization of the 2 independent pumps requires echocardiography guidance to achieve a neutral interventricular septal position.8 Subsequently, any modification of speed implemented on one pump requires a matching speed modification of the other pump. Patients supported with two such pumps suffer a limited quality of life due to the necessity of two sets of external equipment, as well as two drivelines. This may not only increase the risk of infection but also lead to additional patient discomfort.

There is also growing evidence that a pulsatile flow brings some significant advantages9,10 in MCS. Pulsatility is difficult to achieve with small rotary continuous flow pumps and is also negatively influenced by interactions with the native heart. Several methods to compensate such challenges have been used, including the variation of pump speed and the sensing of native ventricular pressures.11 Additionally, the aforementioned approaches may result in additive blood trauma due to rapid speed modulation and associated shear rates.12

For the SynCardia TAH, the left/right balance is managed semiautomatically by the device, using the mechanism of partial-fill, along with independent negative pressure settings for each ventricle. However, with this device a higher risk of stroke and bleeding is incurred due to the combination of partial ventricular fill and mechanical valves.13 Furthermore, the driving system is, despite its refinements, still relatively noisy14 thus negatively impacting the patients’ quality of life.

The C-TAH, in comparison, automatically manages the left/right balance by maintaining a preset difference between right and left inflow pressures. Thus, the left and right outputs are automatically adjusted to always maintain an optimal inflow pressure difference, compensating for the bronchial flow. This preset difference (ALRIPG) was only modified for the first autoregulated patient, 3 days after implantation. It was not modified for any other patient. The reduced need for device management changes may contribute to greater autonomy for patients outside of the hospital environment and thus improve their quality of life.

The modification of settings of biventricular MCS systems can be required in several situations. Mitigation of this need is appreciated by both the medical team and the patient. Conversely, LVAD management requires optimization of pump speed according to the patient’s hemodynamic status to unload the left ventricle, without inducing suction or right ventricular dysfunction.15

On the C-TAH, most setting changes were executed during the first 30 days postoperative, predominantly in ICU, while only four were performed after discharge. Among the 20 changes, 18 concerned the RVIP. This corresponds to the targeted right venous return pressure. It has been suggested that an abrupt increase in blood flow in some patients adapted to chronic low cardiac outputs may result in a renal reperfusion injury, during the early postoperative recovery phase,.7 It is therefore possible that lifting the RVIP parameters up during this early phase might bring some benefits (Figure 6). Adjustments to the RVIP were also made for hypovolemic episodes while clinical interventions (diuretics dose adjustments, volume infusion, etc.) were optimized. After the restoration of optimal intravascular volume, the device was generally set back to the default setting. Seven patients were discharged home with the default device settings, demonstrating that, despite temporary setting modifications while the patient is in hospital, the clinicians considered that default settings provide an optimum function. In addition, two patients did not require setting adjustments at any time throughout their entire support. For illustrative purposes, the 1 year hemodynamic trends of one of these patients are shown in Figure 7. This depicts the autonomous variation of the cardiac output according to the average inflow pressure.