Pulmonary venous wedge pressure closely reflects the pulmonarj' arterial mean pressure. Pulmonarj' arterial wedge pressure is not significantly different from the left atrial mean pressure. Neither the former nor the latter is an approxinmtion of the pulmonarj' capillary pressure.A PRESSURE recorded through a cardiac catheter wedged tightly in a small . pulmonary artery has been referred to as a "capillary," "venous" or "wedge" pressure. On the other hand, a pressure recorded through a cardiac catheter wedged into a pulmonary vein during thoracotomy and in intact animals has been referred to as a pulmonary venous wedge pressure' 1! .Some investigators have found good correlation between the mean pulmonary arterial wedge pressure and the mean left atrial pressure in man and other animals; others have found great variability. A thorough analysis of the data and methods of each group of investigators reveals differences in technics with which the data were obtained, and in methods of statistical analyses.The purpose of this study was to show the relationship between pulmonary arterial wedge pressure and mean left atrial pressure on the one hand, and the pulmonary venous wedge pressure and mean pulmonary arterial pressure on the other. Determinations were made in normal dogs with the thorax open so that catheters could be placed precisely in the desired position. METHODThis study was done in 24 normal dogs. Pulmonarj' arterial wedge pressures were obtained by wedging into a small pulmonary arterial branch a 7 F cardiac catheter with one hole in the tip and pulmonarj' venous wedge pressures by wedging a catheter into a small branch of a pulmonary vein in From the Department of Medicine of the Southwestern Medical School of the University of Texas and tho Veterans Administration Hospitals, McKinney, Texas and Minneapolis, Minnesota.the same lobe of the lung ( fig. 1). Seven-F catheters with three holes in the tips were placed into the left atrium through a small pulmonarj' vein and into the pulmonarj' arterj' through a small pulmonarj' arterial branch ( fig. 1). The catheters were attached to Lundy stopcocks so that pressures from the different vascular and cardiac chambers could be recorded simultaneouslj' or in rapid sequence. Pressures were recorded with the chest closed and open by calibrated Statham strain gauges on a 4-channel Sanborn recorder. The superior surface of the presenting left atrium was taken as the zero level. RESULTSPulmonary arterial wedge pressures ranged from 0.2 to 14.8 mm. Hg. and left atrial pressures from 0.6 to 15.4 mm. Hg. There was a highly significant correlation between the means of the pulmonarj' arterial wedge and left atrial pressures ( fig. 2); r = 0.9577, p < 0.01. There was no significant difference in the difference between the means of the left atrial and pulmonary arterial wedge pressure; t = 0.7700 and p > 0.4. A lobar pulmonary venous branch was ligated and the pressure recorded in the pulmonarj' arterial wedge was of the same order of magnitude as the pulmonary arterial mean ...

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