The remarkably good curve-fitting of our clinical data to the sigmoid curve is additional evidence that a gradual, rather than an abrupt, change in slope describes the compli- ance properties of the ARDS lung. However, usu- ally there is not a true linear segment in the P-V curve, but rather a gradual change from upward concavity to downward, and recruitment may be taking place all along the P-V curve (6,17,22). This has been interpreted as the pressure above which recruitment has been completed (1,21). has been described as the point where compliance suddenly changes to become "straight" or the "knee" of the curve (4, 15). Points of maximum compliance increase or decrease falling outside the range of data col- lected ( 0 cm H 2 O or the highest data point collected in cm H 2 O) were not included in the. The point of maximum compliance increase (P mci, where the rate of change of upward slope is maximal or where the second derivative of the function has a maximum) is: c -1.317 d (17). Initial guess co- efficients were a 0 L, b 3 L, c 20 cm H 2 O, d 10 cm H 2 O. The algorithm was set to run until the resulting sum of squared residuals changed by 0.0001, yielding estimates of the pa- rameters a, b, c, and d and the best-fit coefficient R 2. Using the program DeltaGraph (SPSS Inc., Chicago, IL), the equation was fitted to the P-V data using the Leven- berg-Marquardt iterative algorithm to minimize the sum of squared residuals. This equation has four fitting parameters: a, in units of volume, representing the lower asymptote b, in units of volume, representing the distance from a to the upper as- ymptote, or inspiratory capacity c, in units of pressure, representing the true inflection point (where concavity changes direction) d, in units of pressure, representing the distance from c of the zone of high compliance ( Figure 1).
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