Puncture of a peripheral vein is the most common invasive procedure performed in the Emergency Department. While some newer point-of-care testing techniques require only capillary blood, the vast majority of laboratory studies require venous blood. Cannulation of a peripheral vein is performed on a daily basis and is the cornerstone of circulatory resuscitation. It is an essential skill for all emergency personnel, from phlebotomists to nurses to the Emergency Physician. A variety of approaches for obtaining peripheral venous access are described in this chapter.
Veins and arteries are composed of a three-layered wall of internal endothelium surrounded by a layer of muscle then a layer of connective tissue (Figure 48-1).1 The muscular layer of a vein is much thinner and weaker than that of an artery. While veins can dilate and constrict somewhat on their own, they do so mostly in response to the pressure within them. Veins with high internal pressures become engorged and are easier to access. The use of venous tourniquets, dependent positioning, “pumping” via muscle contraction, and the local application of heat or nitroglycerin ointment all contribute to venous engorgement.2 These maneuvers can be used to aid in the identification of a peripheral vein.
Comparative anatomy of an artery and a vein. A. The generic blood vessel. B. A muscular artery. C. A vein. Note the vein's thinner wall with fewer myocytes and elastic fibers. This is indicative of the lower pressure within veins compared to arteries.
The connective tissue surrounding veins can be a help or a hindrance during attempts at gaining peripheral venous access. Deficient connective tissue permits the vein to “roll” from side to side and evade the needle. Tough connective tissue can impede the entry of a flexible catheter through the soft tissues and into the vein. This tissue also serves to stabilize the vein and prevent its collapse.
Venous valves are an important aspect of peripheral venous anatomy (Figure 48-2).1 Venous valves encourage unidirectional flow of blood back to the heart. Because of gravitational forces, they prevent blood from pooling in the dependent portions of the extremities. Valves can impede the passage of a catheter through and into a vein. Forcing a catheter past venous valves may damage them and contribute to later venous insufficiency. Valves are more numerous at the points where tributaries join larger veins and in the lower extremities. Valves are almost totally absent within the large central veins and the veins of the head and neck.
Venous valves. Cross section of converging veins demonstrating the valve leaflets that permit only forward flow, proximally, toward the right heart. The arrows represent the directional flow of blood.
Veins can be subdivided into central ...