ESTIMATING LEFT-VENTRICULAR FILLING PRESSURE DURING POSITIVE END-EXPIRATORY PRESSURE IN HUMANS

In the critically III, accurate measurements of left ventricular (LV) filling pressure using pulmonary artery occlusion pressure (Ppao) are important for diagnostic and therapeutic purposes. In patients receiving positive end-expiratory pressure (PEEP), Ppao may not reflect LV filling pressure because of elevated pericardial pressure (Ppc). It has been proposed that in humans, Ppc and right atrial pressure (PRA) are equal, so that referencing Ppao to PRA may improve the assessment of LV filling pressure when Ppc is elevated. Similarly, it has also been shown in the dog that nadir Ppao immediately after airway disconnection from PEEP (nadir Ppao), accurately reflects LV filling pressure when LV filling pressure is less-than-or-equal-to 10 mm Hg. We examined methods of estimating LV filling pressure using Ppao measurements under conditions in which increases in Ppc were the primary determinants of differences in the two measurements. Using left atrial pressure (PLA) relative to Ppc, called transmural PLA (PLAtm), as LV filling pressure, we compared the accuracy of Ppao, nadir Ppao, and Ppao relative to PRA to reflect PLAtm in 15 postoperative cardiac surgery patients in whom an air-filled pericardial balloon catheter and a left atrial catheter were inserted during surgery. PEEP was sequentially increased from zero to 15 cm H2O. We found that PRA always exceeded Ppc (p < 0.01) and increased less with PEEP than did Ppc (p < 0.05). At less-than-or-equal-to 5 cm H2O PEEP, both Ppao and nadir Ppao were similar to each other and to PLAtm. At 10 and 15 cm H2O PEEP, Ppao exceeded PLAtm (p < 0.05), whereas nadir Ppao continued to reflect PLAtm. We conclude that in the postoperative cardiac patient PRA does not reflect Ppc or change similarly with PEEP-induced changes in Ppc. Similarly, Ppao accurately reflects PLAtm only during low levels of PEEP (less-than-or-equal-to 5 cm H2O), whereas nadir Ppao accurately reflects PLAtm to at least 15 cm H2O PEEP.