(See also CATHETERS.) A flexible tube with a double lumen and a small balloon at its distal end. It is introduced into a vein in the arm and advanced until the end of the catheter is in the right atrium (see HEART). The balloon is then inflated with air through one lumen and this enables the bloodstream to propel the catheter through the right ventricle to the pulmonary artery. The balloon is deflated and the catheter can then record the pulmonary artery pressure. When the balloon is inflated, the tip is isolated from the pulmonary artery and measures the left atrial pressure. These measurements are important in the management of patients with circulatory failure, as under these circumstances the central venous pressure or the right atrial pressure is an unreliable guide to fluid-replacement.