Interim use of Jarvik-7 and Novacor artificial heart: blood rheology and transient ischemic attacks (TIA's)

Influence of Viscosity and Pressure on Prosthetic Valve Regurgitation

Blood viscosity varies during the course of artificial heart implants and is affected by pathological conditions. To gauge the potential effect of changing viscosity on valve performance, leakage rates were measured across a closed Medtronic-Hall valve with water, water/glycerol and fresh whole bovine blood for aortic and pulmonary pressure ranges. As might be expected from the low Reynolds numbers (< 140), losses across the valve were found to be primarily viscous. For the two Newtonian fluids, leakage was slightly less than linearly proportional to pressure. This is comparable with empirical data for orifice flow, which predicts three fifths power dependence on pressure. For blood, however, the greater than linear dependence on pressure found suggests that the pseudoplasticity (shear-thinning behavior) of blood is important. These data provide evidence that the viscous and non-Newtonian properties of blood must be taken into account in modelling prosthetic valve performance and may affect the test methods and flow regulation strategies for prosthetic blood pumps.

Cerebrovascular accidents in patients with a ventricular assist device

OBJECTIVE

A cerebrovascular accident is a devastating adverse event in a patient with a ventricular assist device. The goal was to clarify the risk factors for cerebrovascular accident.

METHODS

Prospectively collected data, including medical history, ventricular assist device type, white blood cell count, thrombelastogram, and infection, were reviewed retrospectively in 124 patients.

RESULTS

Thirty-one patients (25%) had 48 cerebrovascular accidents. The mean ventricular assist device support period was 228 and 89 days in patients with and without cerebrovascular accidents, respectively (P < .0001). Sixty-six percent of cerebrovascular accidents occurred within 4 months after implantation. Actuarial freedom from cerebrovascular accident at 6 months was 75%, 64%, 63%, and 33% with the HeartMate device (Thoratec Corp, Pleasanton, Calif), Thoratec biventricular ventricular assist device (Thoratec Corp), Thoratec left ventricular assist device (Thoratec), and Novacor device (WorldHeart, Oakland, Calif), respectively. Twenty cerebrovascular accidents (42%) occurred in patients with infections. The mean white blood cell count at the cerebrovascular accident was greater than the normal range in patients with infection (12,900/mm3) and without infection (9500/mm3). The mean maximum amplitude of the thrombelastogram in the presence of infection (63.6 mm) was higher than that in the absence of infection (60.7 mm) (P = .0309).

CONCLUSIONS

The risk of cerebrovascular accident increases with a longer ventricular assist device support period. Infection may activate platelet function and predispose the patient to a cerebrovascular accident. An elevation of the white blood cell count may also exacerbate the risk of cerebrovascular accident even in patients without infection. Selection of device type, prevention of infection, and meticulous control of anticoagulation are key to preventing cerebrovascular accident.