EFFECTS OF CONTINUOUS NEGATIVE AIRWAY PRESSURE-RELATED LUNG DEFLATIONON UPPER AIRWAY COLLAPSIBILITY

Continuous negative airway pressure (CNAP) causes a decrease in lung v olume, which is known to increase upper airway resistance by itself. W e studied how this lung volume change could modify upper airway collap sibility with five normal awake subjects. In a first trial, pressure i n a nasal mask (Pm) was progressively decreased in 3- to 5-cmH2O steps (CNAP). In a second trial, changes in lung volumes resulting from CNA P were prevented by applying simultaneously an equivalent level of neg ative extrathoracic pressure into a poncho-type respirator [isovolumet ric CNAP (CNAP(isovol))]. For each trial, we examined the relationship between the maximal inspiratory airflow of each flow-limited inspirat ory 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 limi tation occurred. Flow limitation was observed in each subject with CNA P but in only two subjects with CNAP(isovol). In these two subjects, t he Pm threshold values were -20 and -9 cmH2O with CNAP and -39 and -16 cmH2O with CNAP(isovol), respectively. Critical pressures for the sam e two subjects were -161 and -96 cmH2O with CNAP and -202 and -197 cmH 2O with CNAP(isovol), respectively. We conclude that CNAP-induced decr eases in lung volume increase upper airway collapsibility.