ABSTRACT
The effects of a moderately elevated intra-abdominal pressure (IAP) on lung mechanics in acute respiratory distress syndrome (ARDS) have still not been fully analyzed. Moreover, the optimal positive end-expiratory pressure (PEEP) in elevated IAP and ARDS is unclear. In this paper, 18 pigs under general anesthesia received a double hit lung injury. After saline lung lavage and 2 h of injurious mechanical ventilation to induce an acute lung injury (ALI), an intra-abdominal balloon was filled until an IAP of 10 mmHg was generated. Animals were randomly assigned to one of three groups (group A = PEEP 5, B = PEEP 10 and C = PEEP 15 cmH2O) and ventilated for 6 h. We measured end-expiratory lung volume (EELV) per kg bodyweight, driving pressure (ΔP), transpulmonary pressure (ΔPL), static lung compliance (Cstat), oxygenation (P/F ratio) and cardiac index (CI). In group A, we found increases in ΔP (22 ± 1 vs. 28 ± 2 cmH2O; p = 0.006) and ΔPL (16 ± 1 vs. 22 ± 2 cmH2O; p = 0.007), with no change in EELV/kg (15 ± 1 vs. 14 ± 1 mL/kg) when comparing hours 0 and 6. In group B, there was no change in ΔP (26 ± 2 vs. 25 ± 2 cmH2O), ΔPL (19 ± 2 vs. 18 ± 2 cmH2O), Cstat (21 ± 3 vs. 21 ± 2 cmH2O/mL) or EELV/kg (12 ± 2 vs. 13 ± 3 mL/kg). ΔP and ΔPL were significantly lower after 6 h when comparing between group C and A (21 ± 1 vs. 28 ± 2 cmH2O; p = 0.020) and (14 ± 1 vs. 22 ± 2 cmH2O; p = 0.013)). The EELV/kg increased over time in group C (13 ± 1 vs. 19 ± 2 mL/kg; p = 0.034). The P/F ratio increased in all groups over time. CI decreased in groups B and C. The global lung injury score did not significantly differ between groups (A: 0.25 ± 0.05, B: 0.21 ± 0.02, C: 0.22 ± 0.03). In this model of ALI, elevated IAP, ΔP and ΔPL increased further over time in the group with a PEEP of 5 cmH2O applied over 6 h. This was not the case in the groups with a PEEP of 10 and 15 cmH2O. Although ΔP and ΔPL were significantly lower after 6 hours in group C compared to group A, we could not show significant differences in histological lung injury score.
ABSTRACT
INTRODUCTION: Intra-abdominal hypertension (IAH) is a well-known phenomenon in critically ill patients. Effects of a moderately elevated intra-abdominal pressure (IAP) on lung mechanics are still not fully analyzed. Moreover, the optimal positive end-expiratory pressure (PEEP) in elevated IAP is unclear. METHODS: We investigated changes in lung mechanics and transformation in histological lung patterns using three different PEEP levels in eighteen deeply anesthetized pigs with an IAP of 10 mmHg. After establishing the intra-abdominal pressure, we randomized the animals into 3 groups. Each of n = 6 (Group A = PEEP 5, B = PEEP 10 and C = PEEP 15 cmH2O). End-expiratory lung volume (EELV/kg body weight (bw)), pulmonary compliance (Cstat), driving pressure (ΔP) and transpulmonary pressure (ΔPL) were measured for 6 hours. Additionally, the histological lung injury score was calculated. RESULTS: Comparing hours 0 and 6 in group A, there was a decrease of EELV/kg (27±2 vs. 16±1 ml/kg; p<0.05) and of Cstat (42±2 vs. 27±1 ml/cmH2O; p<0.05) and an increase of ΔP (11±0 vs. 17±1 cmH2O; p<0.05) and ΔPL (6±0 vs. 10±1 cmH2O; p<0.05). In group B, there was no significant change in EELV/kg (27±3 vs. 24±3 ml/kg), but a decrease in Cstat (42±3 vs. 32±1 ml/cmH20; p<0.05) and an increase in ΔP (11±1 vs. 15±1 cmH2O; p<0.05) and ΔPL (5±1 vs. 7±0 cmH2O; p<0.05). In group C, there were no significant changes in EELV/kg (27±2 vs. 29±3 ml/kg), ΔP (10±1 vs. 12±1 cmH2O) and ΔPL (5±1 vs. 7±1 cmH2O), but a significant decrease of Cstat (43±1 vs. 37±1 ml/cmH2O; p<0.05). Histological lung injury score was lowest in group B. CONCLUSIONS: A moderate elevated IAP of 10 mmHg leads to relevant changes in lung mechanics during mechanical ventilation. In our study, a PEEP of 10 cmH2O was associated with a lower lung injury score and was able to overcome the IAP induced alterations of EELV.