Venous spasm refers to a condition in which manipulation of the veins, or cold stimulation of the veins, causes the venous blood vessels to become thinner, which thickens the walls of the vessels and reduces blood circulation. This condition usually occurs when the infusion of fluids, the stimulation of the walls of the blood vessels, through the lancing of the needle can cause a spasmodic condition of the blood vessels. It may be due to the cold, in order to reduce the dissemination of heat, will lead to spasm of the venous blood vessels, all belong to the normal physiological reaction. Because of the spasm of the vein during lancing, the vein is further difficult to be lanced, so this can be done under ultrasound guidance.