Measurement of growth rate of laser-induced intravascular platelet aggregation and the influence of blood flow velocity.

The growth rate of platelet microthrombi induced by laser injury in ear chamber arterioles on conscious rabbits was measured. A growth rate constant was obtained for each microthrombus by plotting its volume change on a semilogarithmic plot. In most instances each platelet microthrombus formed at any one injury site conformed to a similar growth rate pattern. There was a direct positive correlation between the mean growth rate constant and the number of emboli counted for each laser injury. Blood flow velocity had a variable effect on the number of emboli from sites of laser injury. At velocities above 2.5 mm sec^?1 the number of emboli remained relatively constant; between 1.0 and 2.5 cm sec^?1 the number of emboli tended to increase and there was a greater variation at this level; below 1.0 mm sec^?1 the number of emboli decreased. The number of platelets participating in formation of a microthrombus was expressed as a percentage of the number of available platelets to give a measure of platelet aggregability in vivo. Platelet aggregability varied with alterations in blood flow velocity. At blood flow velocities between 1.0 and 2.5 mm sec^?1 the percentage of platelets participating varied between 20 and 50%, but at velocities above 3.0 mm sec^?1 the percentage decreased to less than 20%. The results suggested that the number of emboli was an indirect measurement of the growth rate of microthrombi forming at sites of laser injury. Further, both the number of emboli and the percentage platelet aggregability in vivo were not significantly affected by changes in blood flow velocity over the range normally found in rabbit ear chamber arterioles.