EFFECTS OF CONTINUOUS NEGATIVE EXTRATHORACIC PRESSURE VERSUS POSITIVEEND-EXPIRATORY PRESSURE IN ACUTE LUNG INJURY PATIENTS

Objective: To compare the effects of continuous negative extrathoracic pressure (CNEP) and positive end expiratory pressure (PEEP) at the sa me level of transpulmonary pressure. Design: Prospective analysis. Set ting: Medical intensive care unit of a university hospital. Patients: Nine consecutive acute lung injury patients.Patients with cardiac fail ure and patients with chronic lung disease were excluded from the inve stigation. Interventions: The patients were sedated and paralyzed whil e receiving mechanical ventilation and were studied in three different conditions: a) using a PEEP of 0 cm H2O (zero end expiratory pressure ); b) using a PEEP of 15 cm H2O; c) using CNEP, CNEP was applied to th e thorax and the upper abdomen and its level was chosen to obtain a tr anspulmonary pressure similar to the one observed at a PEEP of 15 cm H 2O, All patients had an arterial catheter, a pulmonary artery catheter , and a thermistor-tip fiberoptic catheter for thermo-dye dilution in the femoral artery. These catheters were connected to an integrated mo nitoring system. We also placed an esophageal catheter in each patient to detect esophageal pressure. Measurements and Main Results: For eac h step, we assessed the hemodynamic variations by measuring intravascu lar pressures (via a pulmonary artery catheter), transmural pressures (computed by subtracting esophageal pressure from intravascular pressu re), and blood, volumes (derived from the technique of double indicato r). The application of CNEP of -20 +/- 0.7 cm H2O produced a venous ad mixture and Pao(2)/Flo(2) improvement similar to that obtained with a PEEP of 15 cm H2O, This procedure is associated with a higher cardiac index (5.5 +/- 1.5 vs. 4.6 +/- 1.2 L/min/m(2); p<.05) coupled with low er central venous pressure, pulmonary artery occlusion pressure, and h igher transmural pressures and blood volume parameters. Conclusions: i n acute lung injury patients, a CNEP of -20 cm H2O has the capability to obtain transpulmonary pressure and lung function improvement simila r to a PEEP of 15 cm H2O, CNEP differs from the positive pressure by i ncreasing the venous return and the preload of the heart, and has no n egative effects on cardiac performance.