Platelet aggregation in patients taking anticoagulants after valvular surgery: evaluation by a laser light-scattering method

Abstract

The effects of anticoagulants in the early phase of platelet aggregation in patients with an artificial cardiac prosthesis are unclear. Newly developed particle-counting methods that employ light scattering (LS) can be used to quantify changes in the number of platelet aggregates of different sizes after administration of an aggregating stimulus. Using an LS method, we studied the effects of warfarin and aspirin on aggregation in patients with an artificial cardiac prosthesis. Forty-seven patients undergoing valvular surgery were divided into two groups: 25 patients were treated with warfarin alone (group 1), and 22 patients were treated with both warfarin and aspirin (81 mg/day) (group 2). The international normalized ratio (INR) value was controlled at 2.0–2.5 in both groups. Blood samples were obtained before surgery to obtain control values and on days 21 through 27 after surgery. Platelet aggregation was evaluated after administration of 2.0 μg/ml collagen: small (9–25 μm), medium (25–50 μm), and large (50–70 μm) aggregates were counted. Particle counting by the LS method showed no inhibition of platelet aggregation in group 1 (warfarin alone). The conventional optical density (OD) method yielded a similar result. In group 2 (warfarin plus aspirin), the LS method showed that generation of large aggregates after stimulation was significantly decreased versus presurgery (P = 0.046), whereas the quantity of small aggregates was increased (P = 0.003). The OD methods showed aggregation to be significantly decreased in this group (P < 0.001). Our findings suggest that warfarin does not appear to suppress platelet aggregation. Although aspirin appears to suppress platelet function, it does not affect the initial phase of aggregation. Platelet dysfunction caused by aspirin is due mainly to inhibition of the development of small aggregates into larger aggregates.