Measurement accuracy of oscillatory and whole body plethysmography determination of airway resistance. Study of a mechanical model

: The degree of accuracy of the plethysmographic and oscillatory method in determining respiratory resistance has been examined on a mechanical lung model. At this model different levels of the resistance could be reproducibly adjusted and exactly determined with sensitive measuring instruments. The plethysmographic method allows a precise estimation of the resistance. It was found that the absolute variation of the plethysmographically measured values is not greater than 5%. The Ros pointer scale of the Siregnost FD 5 yields systematically incorrect curve diagrams. In the lower range of the resistance the measured values are to high while the measured results of the resistance become progressively to low with an increasing resistance. The reason is the Ros pointer scale which does not show the real component of the impedance at a phase angle of 0 degree. The values of the real component of the respiratory impedance (Rreal) which yields the Siemens standard set show a great coincidence with the lung model resistance (R(aw)). The coincidence could be even improved by use of electronic data processing. With a computer program developed by us it is possible for the first time to indicate and registrate consecutively individual and average values of the real component (Rrealcomp) and the reactance of the respiratory impedance as well as the phase angle between the alternating pressure delta p and the oscillating flow (V). Thereby the accuracy of measurement is improved and the long winded analysis with the "phase diagram" is not necessary anymore. Further experimental and clinical investigation have to show whether the oscillatory method in the way described above will offer new possibilities for the assessment of the pulmonary function. The phase angle and its course during the respiration cycle is in this connection of special importance as a possible new parameter.