Estimation of Venous Pressure

Systemic venous pressure is relatively easy to estimate by observing the filling of the jugular veins. The external jugular veins in the neck can be visualized where they run from the clavicle to the angle of the jaw. The clavicle is 10 to 15 cm above the heart, and normally the venous pressure is not enough to support that large a column of blood. As a result, the jugular veins in the neck of a healthy person should be collapsed in the upright posture. As venous pressure rises, the jugular veins are seen to be filled over more and more of their length. Systemic venous pressure can be accurately estimated by simply calculating the hydrostatic column from the level of the atrium to the level to which the jugular veins are filled.

Even mild congestion of the pulmonary veins can be detected with a chest x-ray and is seen as opacification of the hilar regions as fluid accumulates in the tissues. Pulmonary venous pressure is not so easy to measure because it is technically very difficult to position a catheter in the pulmonary veins; however, pulmonary venous pressure is an important indicator both of the degree to which the left ventricle has been compromised and of the possibility of a catastrophic pulmonary edema. Therefore, monitoring pulmonary venous pressure in the critically ill patient is often vital. Pulmonary venous pressure can be measured indirectly by insertion of a Swan-Ganz catheter and measuring the pulmonary wedge pressure. The Swan-Ganz catheter has an inflatable balloon near its tip and is inserted into a peripheral vein and advanced into the right atrium. When partially inflated, the balloon acts as a sail and the flowing blood literally pulls the tip of the catheter through the right ventricle and into the pulmonary arteries. The catheter is then advanced into a small pulmonary arterial branch. When the ballon is inflated it occludes the vessel, stopping the blood flow. If there is no flow through the occluded blood vessel, then there will be no pressure drop across it. Thus, the pressure at the tip of the catheter will suddenly drop to a pressure approximating that at the distal end of the occluded vascular bed, the left atrium.

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