Patency of heparin-PVA coated tubes at low flow rates.

The patency of heparin-PVA coated 1.14 mm i.d. polyethylene tubes was greater than that of control tubes without heparin in a novel test section that enabled ex vivo evaluation in dogs at low flow and shear rates (2-10 ml/min). Low shear rates were achieved by diverting a small portion of a chronic A-V shunt flow through Y-connectors into the small diameter test tubing. For shear rates in the range of 200-650 s-1 half the heparin-PVA tubes remained patent for longer than 72 min whereas half the control tubes (PVA only) only remained patent for longer than 30 min. At higher shear rates (500-1000 s-1), the 50% patency times for heparin-PVA and PVA were 225 and 123 min respectively. The increased patency at the higher shear rates was attributed more to the effect of the connector than just shear rate. The higher shear rates were achieved by changing from a Y-connector in which the small diameter side-branch was cut flush to the larger diameter main line to one in which the small diameter test tubing protruded into the lumen of the main line; the latter 'protruding' connector was presumed to result in less platelet pre-activation than the other. This method has been found useful for assessing the thrombogenicity of heparinized tubes at low flow rates.