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A static column of contrast agent or saline in an angiographic catheter will passively exchange with blood during angiography. The authors investigated the time course of this exchange in 5.5- and 7-F polyethylene catheters inclined at various angles. Passive blood exchange occurred 2 cm into the catheters within 7-15 seconds at most catheter tip angles, except for those catheters oriented so that their tips were nearly horizontal. In a separate series of experiments, the effect of contrast agent on platelet function and blood clotting was analyzed. The agent was sufficiently diluted in blood so as to simulate an angiographic procedure. The studies were performed in 60 cylindrical polyethylene containers with both unheparinized and heparinized blood. Use of an ionic contrast agent, more than a nonionic agent, lengthened the time for platelet aggregation (mean increases for ionic vs nonionic agents were 46.4 and 37.1 seconds, for unheparinized and heparinized blood, respectively), platelet adhesion to polyethylene surfaces (mean increases, 46.0 and 64.2 seconds), and platelet-stimulated coagulation (mean increases, 38.5 and 43.9 seconds). Conventional, intermittent flushing with saline or filling the catheter with contrast agent may be insufficient to prevent blood from rapidly back-filling the catheter tips. Contrast agents (ionic more than nonionic) distributed in the patient's blood volume inhibit platelet coating of catheter lumens and/or blood clotting under such circumstances.