A simplified method for monitoring respiratory impedance during continuouspositive airway pressure

The forced oscillation technique is useful in detecting changes in upper ai rway obstruction in patients,vith sleep apnoea undergoing continuous positi ve airway pressure (CPAP) ventilation. The aim of this study was to impleme nt and evaluate a method for estimating respiratory impedance (Zrs) from th e pressure and flow recorded at the inlet of the CPAP tubing. The method is based on correcting impedance measured at the inlet of the CP AP tubing (Zi) for the effect of the tubing and the exhalation port, The me thod was evaluated in mechanical analogues and in a healthy subject, Sinuso idal oscillation of 5, 10 and 20 Hz were superimposed on CPAP (5-15 cmH(2)O ). At 5 Hz, the changes in airflow obstruction were substantially underestimat ed by Zi. Furthermore, Zi exhibited a negative dependence on Zrs at 20 Hz. The assessment of Zrs was greatly improved after correcting Zi for the effe cts of the CPAP tubing and the exhalation port. Zrs was well estimated at l ow frequencies, reaching very high values during total occlusion (>60 cmH(2 )O . s . L-1 at 5-10 Hz). These results indicate that changes in airflow obstruction can be detected using the forced oscillation technique from pressure and flow recorded on t he continuous positive airway pressure device. This facilitates the clinica l application of the forced oscillation technique for monitoring upper airw ay patency during sleep.