A NEW CORRECTION TECHNIQUE FOR MEASURING RESPIRATORY IMPEDANCE THROUGH AN ENDOTRACHEAL-TUBE

Measurement of respiratory impedance (Zrs) in intubated patients requi res corrections for now-dependent resistance and air compression insid e the endotracheal tube (ET). The purpose of this study was to test a new correction technique for these effects. We therefore studied 110 p atients in two conditions: breathing normally (C1), or breathing throu gh an ET placed at the mouth (C2), In C1, we measured pressure and flo w signals at the mouth, and in C2, at the ET inlet, during application of a pseudorandom forced excitation (4-32 Hz). In C1, respiratory imp edance was calculated directly as Z1, In C2, pressure data were first corrected for the flow-dependent resistance of the ET, and respiratory impedance was then corrected both for gas compression inside the set- up and ET inertance (impedance Z2). Strong linear relationships were f ound between the reference and corrected estimates of the resistance a t 6 Hz, the frequency dependence of resistance and the resonant freque ncy. The mean normalized distance between Z1 and 22 observed in the pa tients over the 4-32 Hz frequency range was about 14% for resistance a nd 12% for reactance (-9% and -4%, respectively, when considering the algebraic value of the distance), This slight underestimation of both components of impedance might be due to an overcorrection of pressure for the now-dependent resistance of the ET. We conclude that, in intub ated patients, newly tested corrections for the mechanical contributio n of the endotracheal tube may yield a fair estimate of respiratory im pedance when pressure is measured at the inlet of the endotracheal tub e.