EFFECTS OF CONTINUOUS NEGATIVE AIRWAY PRESSURE-RELATED LUNG DEFLATION ON UPPER AIRWAY COLLAPSIBILITY

Continuous negative airway pressure (CNAP) causes a decrease in lung volume, which is known to increase upper airway resistance by itself. We studied how this lung volume change could modify upper airway collapsibility with five normal awake subjects. In a first trial, pressure in a nasal mask (Pm) was progressively decreased in 3- to 5-cmH2O steps (CNAP). In a second trial, changes in lung volumes resulting from CNAP were prevented by applying simultaneously an equivalent level of negative extrathoracic pressure into a poncho-type respirator [isovolumetric CNAP (CNAP(isovol))]. For each trial, we examined the relationship between the maximal inspiratory airflow of each flow-limited inspiratory cycle and the corresponding Pm by least-squares linear regression analysis and determined the critical pressure. We also determined the Pm threshold corresponding to the first Pm value below which flow limitation occurred. Flow limitation was observed in each subject with CNAP but in only two subjects with CNAP(isovol). In these two subjects, the Pm threshold values were -20 and -9 cmH2O with CNAP and -39 and -16 cmH2O with CNAP(isovol), respectively. Critical pressures for the same two subjects were -161 and -96 cmH2O with CNAP and -202 and -197 cmH2O with CNAP(isovol), respectively. We conclude that CNAP-induced decreases in lung volume increase upper airway collapsibility.