Background: Pleural effusion (PLEF) and intra-abdominal hypertension (IAH) cause restrictive effects on the lungs. But with a capacity for the diaphragm to descend caudally and the chest wall to expand outwardly, positive end expiratory pressure (PEEP) can re-expand the lungs when either PLEF or IAH are present alone. However, such accommodation may be limited when both conditions are present. In a swine model of PLEF and IAH we tested the ability of PEEP to increase FRC and reduce tidal recruitment.
Methods: Deeply anesthetized swine underwent tracheostomy, chest tube placement to instill liquid/create PLEF and laparostomy to apply CPAP/create IAH. Ventilation was kept constant at VT = 10 ml/kg, f=15, I:E=1:2 and PEEP of 1 or 10 cmH2O (Carestation, GE Healthcare, Madison, WS). The protocol included lung aeration measures (FRC and CT studies) at baseline, PLEF of 13 ml/kg, IAH of 24 cmH2O, and simultaneous PLEF/IAH at PEEP of 1 and 10 cmH2O. Tidal recruitment was calculated by determining the reduction in collapsed lung during tidal inspiration.
Results: In the fourteen swine studied (30-35 kg), PEEP of 10 cmH2O increased mean FRC by 368ml when PLEF was present and by 184 ml when IAH was present. When PLEF and IAH were simultaneously applied, PEEP was able to increase FRC by only 77 ml and tidal recruitment during ventilation was substantial at 12.73%.
Conclusions: When either PLEF or IAH were induced alone, PEEP effectively treated tidal de-recruitment and FRC reduction. Simultaneous PLEF/IAH - a common condition in critically ill patients - was not effectively treated by PEEP. We recommend decompression of IAH when both PLEF and IAH are present since PLEF is more amenable to treatment by PEEP. Support provided by HPRF and GE Healthcare.