The practice of Emergency Medicine frequently requires access to a patient's venous circulation. Venous access allows sampling of blood as well as administration of medications, nutritional support, and blood products. Devices such as cardiac pacing wires and pulmonary artery catheters can be introduced into the patient's central venous circulatory system.
Percutaneous, as opposed to surgical, venous access is usually rapid, safe, and well tolerated. An understanding of the various techniques available, the venous anatomy, and the indications for the procedure allows the Emergency Physician to choose the appropriate site and method of venous access.
Veins, like arteries, have a three-layered wall composed of an internal endothelium surrounded by a layer of muscle then a layer of connective tissue (Figure 47-1).1 The muscular layer of a vein is much smaller 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 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.3
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 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 47-2).1 They encourage unidirectional flow of blood back toward the heart. Venous valves prevent blood from pooling in the dependent portions of the extremities due to gravitational forces. 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, the veins of the head, and the veins of the neck.
Venous valves. Cross-section of converging veins demonstrating the valve leaflets that only permit forward flow, proximally, toward the right heart. The arrows represent the directional flow of blood.