The search for optimal PEEP in acute lung injury (ALI): correlation between intra-abdominal pressure (IAP) and the lower inflection point (Pflex). Results of a pilot study

It is well known that IAPs above 15-20 mmHg increase peak and plateau alveolar pressures. The rise in pressure on the diaphragm causes a pattern of restrictive lung disease with a drop in functional residual capacity and all other lung volumes. Finally this results in diminished chest wall compliance causing difficult ventilation and weaning. The respiratory system can be divided into the chest wall and the lung. Since the diaphragm is coupled to the abdominal wall any increase in IAP may therefore affect chest wall and lung compliance [1]. By calculation of static V-P curves it has been shown in animal and human studies that abdominal and subsequently chest wall compliance goes up after abdominal decompression and this correlates well with the volume recruited [1]. Recent studies looking at compliance in primary and secondary ARDS found that the latter presents with preserved lung but decreased chest wall compliance and PEEP allows to recruit lung units markedly [1,2]. In a previous study we found that in patients with secondary ARDS and raised IAP, PEEP-adjustment for IAP calculated at zero PEEP (ZEEP) resulted in significant better oxygenation at the expense of a significant increase in peak and plateau alveolar pressures but without the risk for early barotrauma [3]. In this pilot study we wanted to sort out if there is a correlation between IAP and Pflex.