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Pulmonary vascular pressures were simultaneously measured by digital instrumentation and by analysis of graphic tracings at end-expiration. Three patients were prospectively studied to assess any discrepancies between the two methods and to assess the effect of variation in intrathoracic pressure on the digital output.In this study, it was found that spontaneous breaths tended to lower pulmonary capillary wedge pressure (PCWP) and that machine delivered positive pressure breaths tended to elevate PCWP by digital as compared to graphic analysis. The effect of variation of intrathoracic pressure on digital PCWP was variable, unpredictable, and of sufficient magnitude to make its use unreliable. Digital instrumentation recording systems have no advantage over graphic analysis.