Pulmonary artery wedge pressure (PAWP) will only reflect left atrial pressure (LAP) if continuity of fluid exists from the catheter tip to the left atrium. Either increased airway pressure or decreased hydrostatic pressure may lead to discontinuity of the fluid column and misinterpretation of PAWP. Simultaneous measurements of PAWP and LAP were made in 19 anesthetized dogs. Placement of the pulmonary artery wedge catheter above the left atrium (West Zone I) in combination with the incremental addition of 5 cm H2O of PEEP caused a 5 mm Hg gradient between PAWP and LAP in the normovolemic animal. Augmenting PEEP further or hypovolemia (i.e., decrease in LAP) increased the gradient. Hypervolemia (increase in LAP) diminished the gradient. Fluid continuity between the PAW catheter and LA is a prerequisite for monitoring LAP with the Swan-Ganz catheter. Increases in PEEP, placement of the catheter above the left atrium and hypovolemia may occlude the fluid column and cause artifacts in the PAWP obtained.