INFLUENCE OF NONLINEARITIES ON ESTIMATES OF RESPIRATORY MECHANICS USING MULTILINEAR REGRESSION-ANALYSIS

To investigate the influence of nonlinearities on estimates of respira tory mechanics, differing patterns of mechanical ventilation patterns were analyzed from 8 puppies and 14 children. Respiratory mechanics we re calculated using multiple linear regression to fit a linear single- compartment model, a volume-dependent single-compartment model (VDSCM) , and a flow-dependent single-compartment model. The ratio of the comp liance of the last 20% of the dynamic volume-pressure (V-P) curve to t he total compliance (C20/C) and the contribution of a volume-dependent elastance to total elastance {%E2 = E2(VT)/[(E1 + E2)VT], where E1 E2 is total elastance, E2 is the volume-dependent component, and VT is tidal volume} were used as the indexes of overdistension. By position ing the dynamic loops on the static V-P curves, ventilation patterns w ere classified as overdistended or nonoverdistended. In the overdisten ded group, the C20/C was significantly lower (0.71 +/- 0.10 vs. 0.92 /- 0.16; P < 0.0001) and %E2 was significantly higher (43.4 +/- 15.0 v s. 0.51 + 18.02%, P < 0.0001) than in the nonoverdistended group. The mode of ventilation (pressure controlled vs. volume controlled) and th e resistive pressures that resulted in widening of the dynamic V-P loo p were found to alter C20/C but not %E2. When the respiratory system w as overdistended, i.e., ventilated up to the flattened portion of the V-P curve, the VDSCM gave more accurate estimates of respiratory mecha nics. Furthermore, %E2 calculated from VDSCM is a useful. parameter fo r estimating respiratory system overdistension that is not affected by resistive pressures.