Measurement of infant pulmonary mechanics: comparative analysis of techniques

In spontaneously breathing, intubated infants, lung elastance (E) and resistance (R) can be derived from the general equation of motion of the lung, Ptp = E.V + R.V + I.V + K, where P is the pressure applied to the system, E is the elastance (the reciprocal of the compliance), V is the volume, R is the resistance to airflow (V), I is the inertance, (V) is gas acceleration, and K is a constant representing alveolar pressure. However, differences in the methods of analysis may results in variations in the derived values of E and R. Eighteen non-sedated intubated infants, with a mean post-conceptual age of 32.3 weeks and mean weight of 1.51 kg, were studied to compare values in E and R derived by 7 different methods of analysis. For each infant, the same 7 consecutive breaths were analyzed by the following methods: 1) linear regression of Ptp on tidal volume (VT); 2) a Mead-Whittenberger analysis, 3) a 2-factor linear regression of Ptp with (V) and VT; 4) a 3-factor regression of Ptp with V, VT, and V; 5) a 3-factor linear regression model of Ptp with V, VT, and a (V)2 factor (to incorporate a turbulent flow factor); 6) a 4-factor regression of Ptp with V, VT, V, and (V)2; and 7) a transformation regression plot of Ptp/VT versus V/VT. In addition, the lung impedance was calculated from the values of E and R derived by each method. There was no significant difference between the measurements of E and R in methods 2-6 (by MANCOVA, with the subjects being covariates). Methods 1 and 7 had a significantly elevated E; in addition, Method 7 also had an elevated R. Furthermore, there were no differences between the impedance values derived from the different methods. We conclude that in this group of spontaneously breathing, intubated infants with a spectrum of pathology, the results of a 2-factor regression analysis are comparable to more complex models of the mechanics of the lung.