The invention relates to a vasodilation assembly for facilitating intravenous cannulation. The assembly comprises: (i) a flexible plastics sleeve (10); (ii) a heated air supply; and (iii) a conduit for conveying heated air from the air supply into the flexible plastics sleeve. The sleeve comprises an air inlet opening (12) for coupling to the conduit, and an opening (14) for accepting an appendage of a patient. The flexible plastics sleeve (10) is of a double-walled construction comprising transparent inner and outer sleeve layers (16, 18).