HUMAN RESPIRATORY INPUT IMPEDANCE BETWEEN 32 AND 800 HZ, MEASURED BY INTERRUPTER TECHNIQUE AND FORCED-OSCILLATIONS

Respiratory input impedance (Zin) over a wide range of frequencies (f) has been shown to be useful in determining airway resistance (Raw) an d tissue resistance in dogs or airway wall properties in human adults. Zin measurements are noninvasive and, therefore, potentially useful i n investigation of airway mechanics in infants. However, accurate meas urements of Zin at these f values with the use of forced oscillatory t echniques (FOT) in infants are difficult because of their relatively h igh Raw and large compliance of the face mask. If pseudorandom noise p ressure oscillations generated by a loudspeaker are applied at the air way opening (FOT), the power of the resulting flow decreases inversely with f because of capacitive shunting into the volume of the gas in t he speaker chamber and in the face mask. We studied whether high-frequ ency respiratory Zin can be measured by using rapid flow interruption [high-speed interrupter technique (HIT)], in which we expect the flow amplitude in the respiratory system to be higher than in the FOT. We c ompared Zin measured by HIT with Zin measured by FOT in a dried dog lu ng and in five healthy adult subjects. The impedance was calculated fr om two pressure signals measured between the mouth and the HIT valve. The impedance could be assessed from 32 to 800 Hz. Its real part at lo w fas well as the f and amplitude of the first and second acoustic res onance, measured by FOT and by HIT, were not significantly different. The power spectrum of oscillatory flow when the HIT was used showed am plitudes that were at least 100 times greater than those when FOT was used, increasing at f> 400 Hz. In conclusion, the HIT enables the meas urement of high-frequency Zin data ranging from 32 to 800 Hz with part icularly high flow amplitudes and, therefore, possibly better signal-t o-noise ratio. This is particularly important in systems with high Raw , e.g., in infants, when measurements have to be performed through a f ace mask.