Respiratory mechanical impedances. Methodology and interpretation.

A synthetic approach to lung and chest mechanics is to apply pressure variations to the respiratory system over a range of frequencies and to study the amplitude and phase relationships between applied pressure and resulting gas or tissue flow. Such impedance measurements may be performed in different ways, which are not equivalent: the most usual are to measure gas flow at the mouth while pressure is varied either at the same place (input impedance) or around the chest (transfer impedance). Transfer impedance measurements were performed in healthy subjects from 3 to 70 Hz and tentatively interpreted with a model featuring tissue elasticity, resistance and inertance, gas compressibility, and airway gas resistance and inertance. The pressure-flow ratio was minimum at 5-10 Hz and increased very fast above 50 Hz. The phase angle was nil around 7 Hz, of + 90 degrees at 30-40 Hz and close to 180 degrees at 70 Hz. The validity of the model is supported by the quality of the fit to the data up to 50 Hz, the values of the coefficients and the results of experiments where the subject's mechanical properties were varied.