Effect of shear stress on coagulation and inflammation in implantable artificial lungs

Excessive shear and blood contact with prosthetic surfaces is known to activate the coagulation and immune systems. The effect of shear is not known, however, in low-shear flow within implantable artificial lungs. Sterile, heparinized, recalcified pig blood was circulated within a test circuit containing a 150 ml reservoir, silicone tubing, and a miniature artificial lung (MAL). Each group of MALs (n=3 in each) had identical surface areas and transit times, but were designed to impose a different shear stress. Two circuits were tested simultaneously with blood from the same batch, one circuit with fiber in the MAL and a second, control circuit without fiber. Blood was sampled prior to addition to the circuit and at 15, 30, 90, and 240 minutes after initiation of flow. Complete blood counts were performed, and FXIIa, C3d, surface-expressed p-selectin, and lactoferrin concentrations were measured. The effect of the fiber, and thus shear, on these measures of activation was determined by subtracting control from fiber results. Average shear stresses for the three groups were 3.75±0.47, 8.45±0.60, and 12.7±0.30. Platelet and leukocyte counts showed no significant differences due to shear or time. Concentrations of all soluble factors increased progressively with time (1.4×10-6<p<0.014). P-selectin and lactoferrin were significantly higher in high vs. medium shear (p<0.02 and 0.01, respectively), while FXIIa is higher in medium vs. high shear (p<0.003). These low shear levels, therefore, have complex effects, but cellular activation levels do correlate with shear.