B. Daroczy

Generation of optimum pseudorandom signals for respiratory impedance measurements

Spontaneous breathing may impair the reliability of forced oscillatory impedance estimates at low frequencies, especially when the oscillatory power is distributed among many frequency values. Since the amplitude of the external forcing is limited to avoid non-linearities, it is suggested that the total energy of a composite electrical signal driving the loudspeaker be maximized at a given amplitude by finding the optimum phase relationships of the signal components, and that the low-frequency components increase in energy at the expense of the less disturbed high-frequency region. In healthy children and adults and in obstructed patients, the coherences and the coefficients of variation of the respiratory system impedance (Zrs) at 2 and 3 Hz were studied in the case of three test signals of 2-15 Hz bandwidth. Signals T1 and T2 had a flat power spectrum, whereas the components of T3 decreased sharply between 2 and 5 Hz; T1 was generated by simple random selection of phase angles, while optimization for maximum energy was done for T2 and T3. Optimization alone (T2) increased the reliability of the Zrs estimates at all frequencies, whereas enhancement of the low-frequency power (T3) resulted in a radical improvement of the estimates at 2 and 3 Hz, without loss in reliability at higher frequencies.

Computation of the equivalent airway resistance: a comparison with routine evaluations of plethysmographic measurements.

Equivalent (effective) values of airway resistance were determined for total breaths, inspiration and expiration in spontaneously breathing patients by means of an off-line processing system. An improvement in the alveolar pressure determination was achieved by subtracting thermal pressure drifts of the pressure plethysmograph and by taking into account instantaneous values of the intrathoracic gas volume. Equivalent resistance values were compared with the results of several simulated routine evaluations. It was found that the equivalent resistance is best approximated by the graphical procedure using average pressure points at +/- 0.5 liter/sec flow levels.

Computer-aided investigations of respiratory data

In the first part of this paper a computerised evaluation of plethysmographic signals is presented. The off-line processing described eliminates some erroneous simplifications that routine plethysmography is bound to involve. (1) A subtraction of the box pressure baseline permits measurements with perfectly sealed constant volume plethysmographs even in the case of thermal disequilibrium. (2) Alveolar pressure is computed from the box pressure signal, using instantaneous values of the intrathoracic gas volume. (3) This technique enables one to utilise all the measured data for determinations of airway resistance and viscous work, thus yielding more exact values as compared to the graphic evaluations. In Part 2 a three-parameter model of transpulmonary mechanics, applied to fit data of cat experiments, is described. The aim of this study is to demonstrate how transpulmonary parameters (static pleural pressure, compliance and resistance) are modified by the actual state of the pulmonary vascular bed.