Plasma volume expansion and PEEP in a canine model of acutePseudomonas pneumonia

Abstract

Four groups of anesthetized, ventilated dogs (n=6 in each group) inoculated endotracheally withPseudomonas aeruginosa were studied over 5 h as bilateral, hemorrhagic pneumonia developed. Groups I and II were ventilated with zero end-expiratory pressure (ZEEP) and groups III and IV with positive end-expiratory pressure (PEEP) (8 cmH₂O). Hetastarch (6%) in saline was infused intravenously to maintain similar transmural pulmonary wedge pressures (Pw_tm) in groups I and III (approximately 5 mmHg) and groups II and IV (approximately 10 mmHg) throughout the experiment. The effects of plasma volume expansion were analyzed by comparing groups I and III with groups II and IV and of PEEP by comparing groups I and II with groups III and IV. The number of lobes with gross consolidation was greater in groups II (4.8±1.2) and IV (5±0.9) than in groups I (2±1.1) and III (3.3±1). The mean lung wet weight/body weight ratio was greater in groups II (40±11 g/kg) and IV (48±12 g/kg) than in groups I (19±3 g/kg) and III (32±6 g/kg) and in groups III and IV than in groups I and II. Plasma volume expansion, in the absence of PEEP (group II vs. group I), dramatically increased intrapulmonary shunt (Qs/Qt 5 h after inoculation: group II, 62±13%; group I, 25±12%). However, overall gas exchange 5 h after inoculation was not significantly different between PEEP-treated groups and ZEEP-treated groups despite more extensive disease in the former. Despite maintenance of Pw_tm, cardiac output fell significantly over the 5 h study period in groups III (4.3±0.7 to 3.3±1.0 L/min) and IV (7.2±1.7 to 3.8±2.4 L/min) compared to groups I (3.8±1.0 to 3.5±1.2 L/min) and II (6.9±3.2 to 7.3±2.6 L/min). We conclude that plasma volume expansion, within the normal physiological range of Pw_tm, increases the extent of pneumonia. Positive end-expiratory pressure with maintenance of Pw_tm also independently increases pneumonia size, possibly by increasing pulmonary capillary hydrostatic pressure, but masks this effect by maintaining arterial oxygenation through recruitment of additional lung units for gas exchange. The data also suggest that PEEP adversely affects cardiac performance in this model of acute pneumonia.