HOW DOES POSITIVE END-EXPIRATORY PRESSURE DECREASE CO2 ELIMINATION FROM THE LUNG

Six chloralose-urethane anesthetized dogs (23 +/- 2 kg) underwent medi an thoracotomy (open pleural spaces) and constant mechanical ventilati on with O-2. We conducted measurements at baseline and during 25 min o f ventilation with 3.3 cmH(2)O positive end-expiratory pressure (PEEP3 ) or 10.7 cmH(2)O PEEP (PEEP11), including breath-by-breath values in the first 2 min after PEEP began. PEEP11 immediately decreased pulmona ry CO2 elimination per breath (VCO2,br, digital integration and multip lication of exhaled flow and F-CO2) from 8.4 +/- 2.0 to 4.5 +/- 1.6 ml (P < 0.05) by significantly decreasing alveolar ventilation (V over d ot A) (29% increase in anatomical dead space (VDana) and generation of high V over dot A/Q over dot regions) and by decreasing alveolar P-CO 2 (PA(CO2)) from 42.5 +/- 3.5 to 35.9 +/- 3.5 Torr (decreased CO2 tran sfer to the lung as electromagnetic aortic cardiac output (Q over dot T) decreased by 51%). The immediate dilution of alveolar gas and PA(CO 2) by fresh gas as PEEP increased functional residual capacity by 1152 +/- 216 ml was offset by simultaneous decreased expiratory volume and , hence, CO2 accumulation. Compared to baseline, the 17% reduction in V-CO2,V-br, was sustained at 25 min after addition of PEEP 11 because V over dot A remained depressed. Then, V-CO2,V-br could only be restor ed to baseline if PA(CO2) sufficiently increased. However, CO2 transpo rt was still in unsteady state at 25 min of PEEP. Peripheral tissue re tention of CO2 and the significant increase in mixed venous P-CO2 (P ( v) over bar(CO2), 62.4 +/- 6.2 Torr) were not enough to normalize CO2 transfer to the lung and to sufficiently increase PA(CO2), especially during the continued depression in Q over dot T that occurred at highe r PEEP. The sustained decrease in V-CO2,V-br during PEEP was not mirro red by changes in end-tidal P-CO2 (PET(CO2)).