Recording pressure at the distal end of the endotracheal tube to measure respiratory impedance.

To minimize the flow-dependent effects caused by an endotracheal tube during impedance measurements, we recorded pressure inside the tube at its distal end. We used a commercial endotracheal tube with a lumen built into its wall with the opening located near the outlet of the tube. We characterized the effect of the tube by means of an effective transfer function (H). We measured H from 0.25-32 Hz on a mechanical analogue by using pseudorandom excitation with different peak-to-peak flow amplitudes (Vpp). For an 8 mm internal diameter (ID) endotracheal tube the modulus of H measured with Vpp 0.2 l.s-1 was 1.00 at 0.25 Hz and increased with the frequency to 1.40 at 32 Hz. The phase factor was close to zero (less than 5 degrees) over the whole frequency band. The modulus of H changed less than 5% and the phase factor less than 3 degrees when Vpp was increased from 0.2 to 0.8 l.s-1. We evaluated the method on five mechanical analogues with increased resistance or elastance and with a different tracheal area. The mean normalized distance in the complex plane over the whole frequency band (dz) between the analogue impedance and the estimated value from intubation was always less than 5%. Finally, the method was tested on an active analogue which superimposed a high-amplitude (up to 1.4 l.s-1 peak-to-peak) low-frequency (0.25 or 0.33 Hz) sinusoidal flow onto excitation: dz was always less than 4.3%.