Servo-Controlled Pneumatic Pressure Oscillator for Respiratory Impedance Measurements and High-Frequency Ventilation

Abstract

Abstract-The ability to provide forced oscillatory excitation of the respiratory system can be useful in mechanical impedance measurements as well as high frequency ventilation (HFV). Experimental systems currently used for generating forced oscillations are limited in their ability to provide high amplitude flows or maintain the respiratory system at a constant mean pressure during excitation. This paper presents the design and implementation of a pneumatic pressure oscillator based on a proportional solenoid valve. The device is capable of providing forced oscillatory excitations to the respiratory system over a bandwidth suitable for mechanical impedance measurements and HVF. It delivers high amplitude flows (>1.4 l/s) and utilizes a servo-control mechanism to maintain a load at a fixed mean pressure during simultaneous oscillation. Under open-loop conditions, the device exhibited a static hysteresis of approximately 7%, while its dynamic magnitude and phase responses were flat out to 10 Hz. Broad-band measurement of total harmonic distortion was approximately 19%. Under closed-loop conditions, the oscillator was able to maintain a mechanical test load at both positive and negative mean pressures during oscillatory excitations from 0.1 to 10.0 Hz. Impedance of the test load agreed closely with theoretical predictions. We conclude that this servo-controlled oscillator can be a useful tool for respiratory impedance measurements as well as HFV.

Tidal volume delivery during high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome*

“…Test lung compliance was validated across the full range of PAW used during the study. Using a forced oscillation technique (11), the resonant frequency of the test lung was determined to be between 1.2 and 5.5 Hz for all conditions studied.…”

Four methods of measuring tidal volume during high-frequency oscillatory ventilation

“…This approach obviated the need to make assumptions about the extent to which gas behavior was isothermal or adiabatic. It has previously been shown that in systems like ours, polytropic rather than purely isothermal or adiabatic conditions may exist at the frequencies we studied (28). We subsequently conducted a more extensive evaluation of the HW because of its favorable performance and relative ease of use.…”

Mechanical Properties of the Lung

“…Once pressure and flow have been recorded, digital signal processing techniques based around the use of the fast Fourier transform are used to calculate impedance simultaneously at all the different frequencies involved (Bates, 2009). There are a variety of different devices that have been used to apply multifrequency (broadband) oscillations in _ V to the lungs, the most common being loudspeakers (Landser et al, 1976) and piston pumps (Kaczka and Lutchen, 2004;Schuessler and Bates, 1995), but in all cases the basic principles of impedance calculation remain the same.…”