Prolonged mechanical support of the left ventricle.

An interdisciplinary group has developed a left ventricular assist pump system composed of a modified sac type pump, a pneumatic power unit, and a synchronizer. The pump fills from the left ventricle and discharges into the aorta. The system was employed for left ventricular assistance in a series of 12 normal calves, with an average pumping period of 70 +/- 8 days. The system was then evaluated in a series of calves in whom profound left ventricular failure had been produced. These studies indicate that the assist pump is effective in supporting the circulation and completely unloading the left ventricle. The assist system has now been employed in four patients who could not be weaned from cardiopulmonary bypass following cardiac valve replacement. The assist pump supported the circulation in three instances. In one patient, the assist pump was employed for 8 days until left ventricular function had improved sufficiently to permit pump removal; the patient was subsequently discharged from the hospital.

The artificial heart. Progress and promise.

A multidisciplinary group has designed, fabricated, and evaluated an artificial heart. The heart consists of two smooth-surfaced sac-type pumps, two pneumatic power units, and an electronic control system. The artificial heart has been employed in 22 calves. A variety of problems have been encountered and overcome and a significant improvement in pump design has been made. As a result, a gradual increase in survival times has occurred. The last two calves in which the heart was tested lived for 60 and 42 days respectively. These animals ate well and gained weight. The ability of the control system to balance the output of the two pumps over long periods of time and to automatically increase cardiac output with treadmill exercise has been confirmed. No insurmountable problems in the development of the artificial heart have been identified. The date that an artificial heart will be available for clinical use depends on the availability of funds and on the tenacity of the investigators.

A pressure-measuring system for long-term in vivo pressure measurements.

A new balloon-tipped catheter has been developed for the purpose of making long-term vivo intravascular pressure measurements. Among its advantages are its ability to electrically isolate the subject from the pressure transducer and be rebalanced and calibrated while in situ. Experimental results in calves indicate that the system provides a very reliable means of calibrating implanted catheter tip pressure transducers in situ and can be used as the primary long-term pressure measuring device when high frequency response is not required.

Automatic control of the artificial heart.

An automatic control system has been developed to balance and control the output of an artificial heart. The system consisted of 2 linked negative feedback loops. The left ventricle was controlled by a Servo-Stroke Optimizer, which insured complete filling and full stroke operation of the left pump with each beat and changed the beat rate in accord with changes in aortic pressure. The right ventricle was controlled by a Servo-Variable Systolic Duration unit, which changed the stroke of the right ventricle to maintain the left atrial pressure within a preset band. In the initial animal studies, control pressures (aortic and left atrial pressure) were obtained from implanted transducers. More recently, a method has been devised whereby the control pressures were obtained from specific points on the left air line pressure wave, obviating the need for implanted transducers. The control system has been evaluated in a mock circulatory loop and in 9 calves with implanted artificial hearts. The system has provided balance of the 2 implanted ventricles and changes in flow rate in response to changes in peripheral resistance. The need for an operator to make manual adjustment to the power units has been considerably reduced. Further studies are indicated to evaluate the function of the control system in animals performing moderate-to-maximal exercise. Furthermore, the ultimate benefits of full stroke operation vs fill limited mode operation remain to be delineated.

A comparison of three ventricles used for left ventricular bypass in the calf.

Long-term synchronized left ventricular bypass has been performed in calves using pneumatically powered pumps having a smooth lining fabricated of segmented polyurethane. Three different pump designs have been employed: a) sac pump, b) longitudinal tethered sac pump, and c) transverse tethered sac pump. Sizeable thrombi occurred in the apex of the sac and longitudinal tethered sac pumps. In the transverse tethered sac pumps, a considerably smaller thrombus was seen only at the flexion point on the inlet side of the pump. Thrombi occurred in areas of low flow velocity and were not prevented by anticoagulation or surface coating techniques. This study indicates the importance of ventricle design in the development of thrombus free pumps and suggests modifications to the present pumps to reduce or eliminate thrombus formation.

Complete left ventricular bypass with a paracorporeal pump: design and evaluation.

A multidiscipline group was established at The Pennsylvania State University to design and evaluate mechanical circulatory assist devices and the artificial heart. The group has designed a left ventricular to aortic assist system which consists of a sac-type pump, a synchronization unit, a pneumatic power unit, and appropriate monitoring apparatus. The assist system has been evaluated for long-term circulatory assistance in a series of ten calves. The assist pump was placed in the paracorporeal position. The longest period of continuous pumping was over eight months. The last four calves have had synchronized assist pumping which has permitted prolonged ventricular decompression and assist pump flow rates as high as 10 L/min. Three of these four calves had no evidence of thromboemboli. Additional animal studies will be required before clinical use of such an assist pump system can be safely undertaken.