A computer-model analysis of the influence of the upper airway on passive flow-volume loops in infants

The present study was undertaken to determine the effects of upper airway resistance (Rua) and elastance (Eua) on the mechanical parameters calculated from the expiratory limb of the passive flow-volume curve in normal infants, using a computer model. The model included separate compartments for the respiratory system, upper airway, and measurement equipment. Inclusion of Rua in the model decreased the slope of the expiratory limb of the flow-volume loop, and the resistance calculated from the flow-volume loop accurately reflected the total resistance of the model. Inclusion of Eua in the model caused a decrease in the end-inspiratory (driving) pressure and a corresponding decrease in the peak expiratory flow but did not alter the slope of the linear portion of the expiratory limb of the flow-volume loop. Resistance calculated from the loop underestimated true model resistance by 14.5%. However, further small fluctuations in Eua did not cause significant variability in the value of resistance calculated. Total elastance was accurately calculated whether or not Rua and Eua were included in the model. The results of this analysis demonstrate that, while Eua causes the true resistance to be underestimated, fluctuations in Eua are not likely to be an important source of variability in the mechanical parameters calculated from the passive flow-volume loop; however, any changes in Rua will be reflected in the value of resistance calculated and therefore are likely to cause substantial variability in the calculated resistance.