An Objective Analysis of the Pressure-Volume Curve in the Acute Respiratory Distress Syndrome

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Abstract
To assess the interobserver and intraobserver variability in the clinical evaluation of the quasi-static pressure-volume (P-V) curve, we analyzed 24 sets of inflation and deflation P-V curves obtained from patients with ARDS. We used a recently described sigmoidal equation to curve-fit the P-V data sets and objectively define the point of maximum compliance increase of the inflation limb (P mci,i ) and the true inflection point of the deflation limb (P inf,d ). These points were compared with graphic determi- nations of lower Pflex by seven clinicians. The graphic and curve-fitting methods were also compared for their ability to reproduce the same parameter value in data sets with reduced number of data points. The sigmoidal equation fit the P-V data with great accuracy (R 2 5 0.9992). The average of Pflex determinations was found to be correlated with P mci,i (R 5 0.89) and P inf,d (R 5 0.76). Individual determinations of Pflex were less correlated with the corresponding objective parameters (R 5 0.67 and 0.62, respectively). Pflex 1 2 cm H 2 O was a more accurate estimator of P inf,d (2 SD 5 6 6.05 cm H 2 O) than Pflex was of P mci,i (2 SD 5 6 8.02 cm H 2 O). There was significant interobserver variability in Pflex, with a maximum difference of 11 cm H 2 O for the same patient (SD 5 1.9 cm H 2 O). Clinicians had difficulty reproducing Pflex in smaller data sets with differences as great as 17 cm H 2 O (SD 5 2.8 cm H 2 O). In contrast, the curve-fitting method reproduced P mci,i with great accuracy in reduced data sets (maximum difference of 1.5 cm H 2 O and SD 5 0.3 cm H 2 O). We conclude that Pflex rarely coin- cided with the point of maximum compliance increase defined by a sigmoid curve-fit with large dif- ferences in Pflex seen both among and within observers. Calculating objective parameters such as P mci,i or P inf,d from curve-fitted P-V data can minimize this large variability. Harris RS, Hess DR, Vene- gas JG. An objective analysis of the pressure-volume curve in the acute respiratory distress syndrome.