Extracorporeal circuits and autoregulation: effect of albumin coating

Autoregulation of muscle blood flow often is difficult to demonstrate when extracorporeal perfusion is used. This could be caused by contact of blood and foreign material. Accordingly, we tested whether autoregulation is preserved when the system is coated with albumin. Polyurethane tubing between the carotid and distal femoral artery of rats was partially occluded for 1-2 min. With the system uncoated (n = 6 rats) autoregulation was absent. With coated systems (n = 6 rats) the closed-loop gain (Gc) of the apparent autoregulation (0 < Gc < 1) for the pressure range from control (137 +/- 14) to 80 mmHg was 0.40 +/- 0.24 (mean +/- SD). In most cases autoregulation was preceded by a period of "superregulation": after a variable delay flow rose above control. When the distance between occluder and tip of femoral cannula was varied in another group of rats (n = 6), the delay (20-75 s) was linearly related to transit time (10-35 s) of blood. Flow increase thus seemed to be caused by a blood-borne vasodilator originating at the occlusion site and not by a myogenic or metabolic response to decreased pressure and flow. The vasodilator did not originate from the albumin coating. Partial occlusion of an extracorporeal shunt thus can increase flow to the perfused tissue when the system is coated with albumin. The reaction readily disappears when the system is not coated.