Venous Cannulation and Drainage

Principles Of Venous Drainage

Venous Cannulas And Cannulation

Persistent Left Superior Vena Cava

Augmented Or Assisted Venous Return

Complications Associated With Venous Cannulation And Drainage

Causes Of Low Venous Return

PRINCIPLES OF VENOUS DRAINAGE

Venous blood usually enters the circuit by gravity or siphonage into a venous reservoir placed 40 to 70 cm below the level of the heart. The amount of drainage is determined by central venous pressure; the height differential; resistance in cannulas, tubing, and connectors; and absence of air within the system. Central venous pressure is determined by intravascular volume and venous compliance, which is influenced by medications, sympathetic tone, and anesthesia. Inadequate blood volume or excessive siphon pressure may cause compliant venous or atrial walls to collapse against cannular intake openings to produce "chattering" or "fluttering." This phenomenon is corrected by adding volume to the patient.

VENOUS CANNULAS AND CANNULATION

Venous cannulas are usually made out of flexible plastic, which may be stiffened against kinking by wire reinforcement. Tips are straight or angled and often are constructed of thin, rigid plastic or metal. Size is determined by patient size, anticipated flow rate, and an index of catheter flow characteristics and resistance (provided by the manufacturer). For an average adult with 60-cm negative siphon pressure, a 30F cannula in the superior vena cava (SVC) and 34F in the IVC or a single 42F cavoatrial catheter suffices. Catheters are typically inserted through purse-string guarded incisions in the right atrial appendage, lateral atrial wall, or directly in the SVC and IVC.

Three basic approaches for central venous cannulation are used: bicaval, single atrial, or cavoatrial ("two stage"). Bicaval cannulation and caval tourniquets are necessary to prevent bleeding and air entry into the system when the right heart is entered during CPB. Because of coronary sinus return, caval tourniquets should not be tightened without decompressing the right atrium. Bicaval cannulation without caval tapes is often preferred to facilitate venous return during exposure of the left atrium and mitral valve.

 
Single venous cannulation is adequate for most aortic valve and coronary artery surgery; however, usually a cavo-atrial cannula ("two-stage") is employed. This catheter is typically introduced via the right atrial appendage. Its narrowed distal end is threaded into the IVC while the wider proximal portion has side holes designed to rest within the right atrium. It tends to be more stable and provide better drainage than a single cannula, but proper positioning is critical.With single cannulas, elevation of the heart may kink the junction of the SVC with the atrium and partially obstruct venous drainage.

At times, venous cannulation is accomplished via the femoral or iliac vein. This either open or percutaneous cannulation is used for emergency closed cardiopulmonary assist, for support of particularly ill patients before induction of anesthesia, for prevention or management of bleeding complications during sternotomy, for reoperations, for certain types of aortic and thoracic surgery, and for applications of CPB that do not require thoracotomy. Adequate flow rates require using a cannula that is as large as possible and advancing the catheter into the right atrium guided by transesophageal echocardiography (TEE). Specially designed commercially manufactured long, ultrathin, wire-reinforced catheters are available for this purpose.

PERSISTENT LEFT SUPERIOR VENA CAVA

A persistent left superior vena cava (PLSVC) is present in 0.3% to 0.5% of the general population and usually drains into the coronary sinus; however, in about 10% of cases it drains into the left atrium. The presence of a PLSVC should be suspected when the (left) innominate vein is small or absent and when a large coronary sinus or the PLSVC itself is seen on baseline TEE.

A PLSVC may complicate retrograde cardioplegia or entry into the right heart. If an adequate-sized innominate vein is present (30% of patients), the PLSVC can simply be occluded during CPB, if the ostium of the coronary sinus is present. If the right SVC is not present (approximately 20% of patients with PLSVC), the left cava cannot be occluded. If the innominate vein is absent (40% of patients) or small (about 33%), occlusion of the PLSVC may cause venous hypertension and possible cerebral injury. In these patients a cannula is passed retrograde into the PLSVC through the coronary sinus ostium and secured. Alternatively, a cuffed endotracheal tube may be used as a cannula.

AUGMENTED OR ASSISTED VENOUS RETURN

Negative pressure is sometimes applied to the venous lines to provide assisted venous drainage using a roller pump or centrifugal pump, or by applying a regulated vacuum to a closed hard-shell venous reservoir (vacuum-assisted venous drainage, VAVD). This may permit use of smaller diameter catheters and may be helpful when long, peripheral catheters are used. Augmented negative pressure in the venous line increases the risk of aspirating gross or microscopic air and causing cerebral injury, hemolysis, or aspiration of air into the blood compartment of membrane oxygenators. Positive pressure in the venous reservoir can cause air to enter the venous lines and right heart. These potential complications require special safety monitors and devices and adherence to detailed protocols.

COMPLICATIONS ASSOCIATED WITH VENOUS CANNULATION AND DRAINAGE

These include atrial arrhythmias, atrial or caval tears and bleeding, air embolization, injury or obstruction due to catheter malposition, reversing arterial and venous lines, and unexpected decannulation. Placing tapes around the cavae may lacerate branches, nearby vessels (e.g., right pulmonary artery), or the cava itself. Before or after CPB, catheters may compromise venous return to the right atrium from the body. Venous catheters and/or caval tapes may displace or compromise central venous or pulmonary arterial monitoring catheters; conversely, monitoring catheters may compromise the function of caval tapes.

Any intracardiac catheter may be trapped by sutures, which may impede removal before or after the wound is closed. Any connection between the atmosphere and cannula intake ports may entrain air to produce an air lock or gaseous microembolism. Assisted venous drainage (AVD) increases the risk of air entrainment. Finally, improperly placed purse-string sutures may obstruct a cava when tied.

CAUSES OF LOW VENOUS RETURN

Low venous pressure, hypovolemia, drug- or anesthetic-induced venous dilatation, inadequate differential height between heart and reservoir, too small cannula size, cannula obstruction from any cause, air locks, or excessive flow resistance in the drainage system are possible causes of reduced venous return. Partial obstruction of the venous line may distend the right ventricle and impair later contractility.