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
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KeywordsAcute Lung Injury Functional Residual Capacity Restrictive Lung Disease Lung Unit Lower Inflection Point
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 . 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 . 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 . In this pilot study we wanted to sort out if there is a correlation between IAP and Pflex.
Over a 2 month period 115 measurements were performed in 11 patients. The IAP was calculated at ZEEP with the standardised intravesicle pressure recording method and Pflex with the super-syringe method. The M/F ratio was 6/5, age 67.1 ± 9.4, MODScore 6.5 ± 3.3, APACHE-II score 25.2 ± 8.5, SAPS-II score 54.4 ± 15.7, ICU-stay 11.3 ± 6.3 days. The number of measurements in each patient was 10.5 ± 7.5. There were three patients with primary and three with secondary ARDS, and three patients had secondary ALI according to the definitions given by the American-European consensus conference. Calculation of correlation was done with the Prism GraphPad- software (version 2.00 October 31 1995), values are mean ± SD.
The values for IAP (mmHg), IAP (cmH2O) and Pflex (cmH2O) were 14.9 ± 6.8, 19.4 ± 8.9 and 13.3 ± 5.5, respectively for the whole group of patients: 15.8 ± 7.6, 20.6 ± 9.8 and 13.2 ± 6.0, respectively in secondary ALI/ARDS and 12.6 ± 3.4, 16.4 ± 4.4 and 13.6 ± 3.9 respectively in primary ARDS. There was a very good correlation between IAP (cmH2O) and Pflex (cmH2O) for the whole group of patients (Fig. 1): Pflex = 0.552 × IAP+ 2.5146 (R2 = 0.808, P< 0.0001) and this correlation was even better in the patients with secondary ALI/ARDS; Pflex = 0.5745 × IAP+ 1.3227 (R2 = 0.888, P < 0.0001). As suspected the correlation was worse in patients with primary ARDS: Pflex = 0.7622 × IAP + 1.1624 (R2 = 0.7428, P < 0.0001).