PHYSIOLOGY OF THE NATIVE HEART AND THERMO-CARDIOSYSTEMS LEFT-VENTRICULAR ASSIST DEVICE COMPLEX AT REST AND DURING EXERCISE - IMPLICATIONS FOR CHRONIC SUPPORT

Studies of patients supported with a left ventricular assist device ha ve considered determinants of acute survival emphasizing the role of r ight heart function. In patients with refractory heart failure awaitin g heart transplantation, chronic left ventricular assist device implan tation may provide an opportunity for rehabilitation before surgery if hemodynamics are adequate at rest and during activities of daily life . For the assessment of the efficacy of the left ventricular assist de vice in this setting, four patients in whom the HeartMate pneumatic le ft ventricular assist device had been implanted were tested during gra ded supine bicycle exercise with Doppler echocardiography interrogatio n and central hemodynamic measurements. Patients with left ventricular assist device increased total left ventricular-left ventricular assis t device complex output with exercise as Fick cardiac output increased from 5.7 +/- 1.5 to 8.6 +/- 3.1 L/min (mean +/- standard deviation). In two patients, peak left ventricular assist device rate and output w ere either present at the start of exercise or reached at mid-exercise and were associated with abrupt increases in left ventricular filling pressures (pulmonary capillary wedge pressure = 9 to 27 mm Hg and 12 to 24 mm Hg, respectively). During exercise, left ventricular end-dias tolic size and pressure increased as right ventricular dimensions decr eased or remained the same (patients 1, 3, and 4: 1.7 to 1.8 cm, 4.7 t o 3.9 cm, and 2.6 to 1.8 cm, respectively) despite increased right atr ial filling pressures, implying a decrease in functional right ventric ular diastolic compliance. Although the left ventricular assist device functioned as a series pump at rest, Fick cardiac output exceeded lef t ventricular assist device output during exercise consistent with par allel ejection of the left ventricle through the native aortic valve. During exercise, residual left ventricular function may contribute to the hemodynamic response by (1) active filling of the left ventricular assist device to reduce filling time and to overcome left ventricular assist device inflow cannula impedance, (2) augmentation of total car diac output with parallel ejection out of the native aortic valve, or (3) reduction of ventricular interaction-related changes in functional right ventricular diastolic compliance. When residual left ventricula r function is sufficient, hemodynamics with exercise may be limited by peak left ventricular assist device rate. Although right ventricular function may affect acute postoperative survival, residual left ventri cular function and peak left ventricular assist device rate may be imp ortant determinants of exercise performance during chronic implantatio n. A preliminary model of factors affecting the ''left ventricular-lef t ventricular assist device complex'' performance at rest and during e xercise is presented.