Preparation of Vtyients
Using a sterile introducer (Vygon Desilet No. 8), a No. 7F Swan-Canz catheter (Edwards Laboratories) was inserted percutaneously into a femoral vein and placed in the pulmonary artery under continuous electrocardiographic monitoring and fluoroscopic control. The tip of the catheter was positioned in the pulmonary artery to sample mixed-venous blood and to measure pulmonary capillary wedge and arterial pressure (Bentley-Trantec transducer model 800). A Seldicath catheter Plastimed No. 4F was introduced into a radial or a femoral artery for sampljpg arterial blood and measuring systemic pressures. A cannula was placed in the most convenient vein in the arm. Samples of blood were then collected for measurement of the partition coefficient of each inert gas and the P, obtained for each patient by using the tonometer on two samples of blood with gases containing 2.5 percent or 5.5 percent oxygen, both with 5.5 percent carbon dioxide. read more
The patients were placed in a comfortable semirecumbent position, and they spontaneously breathed room air. The standard glucose solution of six inert gases (sulfur hexafluoride, ethane, cyclopropane, halothane, ether, and acetone), prepared using a sterile technique, was infused intravenously (through a brachial vein) at a continuous rate of 5 ml/min through a 0.5p. high-pressure Millipore filter. The patients were connected through a mouthpiece and a two-way valve to a heated flow-through expiratory system. After a 3Q-minute infusion, samples of mixed expired gas and arterial and mixed-venous blood were collected for measurement of the partial pressures of oxygen, carbon dioxide, and inert gases. Blood gas partial pressures were measured in arterial and mixed-venous blood using a blood gas analyzer (Corning 175). Inert gas concentrations were analyzed in expired gas and arterial and mixed-venous blood by chromatography with two detectors: (1) electron capture for SF6 (Fractovap 230); and (2) flame ionization for the other gases (Girdel). The Ve was measured with a Tissot spirometer and was corrected to BTPS. The systemic and pulmonary arterial pressures were recorded during the final phase of the infusion.
Values for R (arterial/mixed-venous concentration ratio) and E (mixed-expired/mixed-venous concentration ratio) were calculated from the measured solubility and the concentrations in arterial, mixed-venous, and mixed-expired gas of the six inert gases. The cardiac output was derived from these values and from the Ve using Ficks principle. The distributions of blood flow and ventilation as a function of Va/Q ratios were calculated from R and E values by computer (PDP 11/44; Digital Equipment Group) using the smoothing algorithm described by Evans and Wagner. The Va/Q distribution was combined with the measured mixed-venous blood gas levels, cardiac output, Ve, hemoglobin concentration, and P, using the model described by West and Wagner, in order to predict the Pa02 and ideal alveolar partial pressure of oxygen, assuming complete alveolar end-capillary equilibrium for each unit of lung.
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