Therapeutic potential of implantable replacement hearts

Devices in heart failure--the new revolution

Heart failure is a growing epidemic, with more patients living longer and suffering from this disease. There is a growing segment of patients who have persistent symptoms despite pharmacologic therapy. In an era when transplants are rare, the need for devices and interventions that can assist ventricular function is paramount. This review goes through the devices used in heart failure, including left ventricular reconstruction, aortic counterpulsation, short-term mechanical circulatory support, long-term mechanical circulatory support, and right heart interventions.

An insight into short- and long-term mechanical circulatory support systems

Cardiogenic shock due to acute myocardial infarction, postcardiotomy syndrome following cardiac surgery, or manifestation of heart failure remains a clinical challenge with high mortality rates, despite ongoing advances in surgical techniques, widespread use of primary percutaneous interventions, and medical treatment. Clinicians have, therefore, turned to mechanical means of circulatory support. At present, a broad range of devices are available, which may be extracorporeal, implantable, or percutaneous; temporary or long term. Although counter pulsation provided by intra-aortic balloon pump (IABP) and comprehensive mechanical support for both the systemic and the pulmonary circulation through extracorporeal membrane oxygenation (ECMO) remain a major tool of acute care in patients with cardiogenic shock, both before and after surgical or percutaneous intervention, the development of devices such as the Impella or the Tandemheart allows less invasive forms of temporary support. On the other hand, concerning mid-, or long-term support, left ventricular assist devices have evolved from a last resort life-saving therapy to a well-established viable alternative for thousands of heart failure patients caused by the shortage of donor organs available for transplantation. The optimal selection of the assist device is based on the initial consideration according to hemodynamic situation, comorbidities, intended time of use and therapeutic options. The present article offers an update on currently available mechanical circulatory support systems (MCSS) for short and long-term use as well as an insight into future perspectives.

Total artificial heart

End-stage heart failure represents a highly morbid condition for the patient with limited treatment options. From a surgical perspective, the treatment options for effective long-term survival are usually limited to heart transplantation, heart-lung transplantation or implantation of a destination mechanical circulatory support device. Assuming an advanced heart-failure patient is indeed deemed a candidate for transplantation, the patient is subject to shortages in donor organ availability and thus possible further decompensation and potential death while awaiting transplantation. Various extracorporeal and implantable ventricular-assist devices (VADs) may be able to provide temporary or long-term circulatory support for many end-stage heart-failure patients but mechanical circulatory support options for patients requiring long-term biventricular support remain limited. Implantation of a total artificial heart (TAH) currently represents one, if not the best, long-term surgical treatment option for patients requiring biventricular mechanical circulatory support as a bridge to transplant. The clinical applicability of available versions of positive displacement pumps is limited by their size and complications. Application of continuous-flow technology can help in solving some of these issues and is currently being applied in the research towards a new generation of smaller and more effective TAHs. In this review, we discuss the history of the TAH, its development and clinical application, implications for anaesthetic management, published outcomes and the future outlook for TAHs.

Vascular Tone Estimation in Patients Implanted with the AbioCor Implantable Replacement Heart

The AbioCor implantable replacement heart provides continuous hydraulic pressure data that are used for control purposes. The magnitude of the end systolic pressure spikes are compared to a preset threshold and used for controlling motor speeds to maintain a full stroke on every beat. Portions of the diastolic pressures of the left and the right side are averaged and can be used for left-right balance control. The mean right diastolic pressure may be used for beat rate (cardiac output) control. The systolic and diastolic hydraulic pressures of the left and right side are correlatable to the respective afterload and preload pressures. More importantly, with known hydraulic pump pressure-flow characteristics, cardiac output is derived. The slopes of the hydraulic pressure traces as a function of time are not used for control or monitoring purposes. However, the magnitudes and shapes of these slopes can provide information on the tones of a patient's vasculature. The ratio of the slope of the systolic pressure to that of the stroke volume yields the vascular tone given in mm Hg/cc. Due to pre-existing pulmonary complications, some AbioCor patients have pulmonary vascular tones that are substantially higher than their respective aortic tones by as much as a factor of four. Pulmonary tones as high as 2 mm Hg/cc, or approximately ten times the normal tone, are recorded. The ratio of averages of the aortic to the pulmonary tone range between 2.2 and 0.25 in patients implanted with the AbioCor compared with a normal value of 5 in subjects without pre-existing pulmonary diseases. The AbioCor implantable replacement heart's hydraulic drive system is capable of providing valuable physiological information, including estimates of physiological pressures, cardiac outputs, and vascular tones.