Effort and volume dependence of forced-deflation flow-volume relationships in intubated infants.

The application of negative pressure to the airway opening [called the forced-deflation (FD) technique] allows the examination of maximal expiratory flow-volume curves in intubated infants who are unable to generate a voluntary maximal expiratory maneuver. We explored the questions of effort and volume dependence of flows generated by FD in 18 intubated, sedated, and paralyzed infants [age 10.6 +/- 2.0 (SE) mo; weight 7.2 +/- 0.7 kg] with normal lungs. Effort dependence was assessed by isovolume pressure-flow curves that were constructed in 10 infants from repeated FD maneuvers from total lung capacity (defined as +40 cmH2O) by varying airway opening pressures from 0 (barometric pressure) to -100 cmH2O at intervals of 20 cmH2O. The effect of volume history was assessed by initiating FD maneuvers from different inspiratory volumes delineated by the inspiratory pressures +10, +20, +30, and +40 cmH2O. We compared maximal expiratory flows at isovolume points [50, 25, and 10% forced vital capacity (FVC) of the standard +40/-40 cmH2O FD maneuver] and found that flow limitation consistently occurred in all infants at and below 25% FVC with -40 cmH2O or greater airway opening pressure. We found no significant influence of volume history on maximal flows at and below 25% FVC. Under well-controlled study conditions, we demonstrated excellent reproducibility of maximal expiratory flows at low lung volumes, analogous to those of voluntary forced expiratory maneuvers in adults and older children. This information may be helpful in setting standards for performance and interpretation of FD maneuvers in intubated infants.