Referencing lung volume for measurements of respiratory system compliance in infants.

We propose a method for measurements of respiratory system compliance (Crs) in spontaneously breathing infants, which circumvents the potential problems introduced by the breath-by-breath oscillations in the end-expiratory level, i.e., functional respiratory capacity (FRC). Changes in lung volume (V) and pressure at the airway opening (P(ao)) were measured in 10 infants breathing through a face mask. A first brief occlusion was to establish a reference V and the corresponding static P(ao); a second occlusion was done at a different V, within the same expiration, or in the following breath. Both occlusions were sufficiently long for the establishment of a stable P(ao) value. From the V difference (delta V, where delta V was at least 20% tidal volume) and the corresponding difference in P(ao) (delta P(ao)) Crs was computed and averaged (Crs[REF.VOL.] = delta V/delta P(ao). Although, on average, the results were similar to those obtained by the traditional multiple occlusions technique with linear regression analysis of the P(ao)-V data points (Crs[MOT]), in several infants Crs[REF.VOL.] tended to be slightly higher than Crs[MOT]. Some possibilities for this discrepancy are discussed. It seems likely that breath-to-breath oscillation in FRC may potentially lower Crs[MOT] by introducing a bias on the V measurement at low P(ao). We conclude that referencing V for Crs measurements in spontaneously breathing infants is a simple approach, which does not require linear regression analysis and circumvents the effects of oscillation in FRC.