Dellaca defines respiratory mechanics as the relationship between atmospheric pressure (forces) and flow (displacements) of the respiratory system. Tidal EFL can be detected by measuring how respiratory mechanics change between inspiration and expiration during tidal breathing. Measuring the atmospheric pressure is more difficult because of the effect of respiratory muscles; these muscles are used to breathe in and out, and their effect needs to be negated.
The forced oscillation technique (FOT), in which pressure is applied from outside of the lungs, allows the study of structural and mechanical properties of the respiratory system deduced from its mechanical response to small time-varying forces. Loudspeakers are used to create the oscillation frequencies that are used in this technique.
In the resulting spectra, the high-frequency oscillation can be clearly identified from the quiet breathing of the patient via a digital filter. FOT measures respiratory system impedance and reactance. Impedance is the value that the forced oscillation produces. Reactance is the measurement of the compliance of the respiratory system, that is, its stiffness.
When FOT is applied during EFL, the forced oscillation can’t get past the choke points in the respiratory system, located primarily in the central airways. Oscillation can’t reach the alveoli, so only the mechanical pressure of the central airway is measured. The reactance becomes more negative because the system is much stiffer.
Dellacà’s group has found that using one continuous frequency, rather than several during FOT, and performing the analysis very quickly, means that resistance and reactance can be measured at the same time, within a single breath.