Annals of Biomedical Engineering, Vol. 32, No. 4, April 2004 (©2004) pp. 596–608 Servo-Controlled Pneumatic Pressure Oscillator for Respiratory Impedance Measurements and High-Frequency Ventilation DAVID W. KACZKA 1 and KENNETH R. LUTCHEN 2 1 Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, MD; and 2 Department of Biomedical Engineering, Boston University College of Engineering, Boston, MA (Received 3 November 2002; accepted 17 November 2003) 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). Ex- perimental systems currently used for generating forced oscilla- tions 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 ex- citations 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 magni- tude and phase responses were flat out to 10 Hz. Broad-band mea- surement 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 con- clude that this servo-controlled oscillator can be a useful tool for respiratory impedance measurements as well as HFV. Keywords—Proportional solenoid valve, Closed-loop pressure control, Forced oscillations, Harmonic distortion. INTRODUCTION The measurement of respiratory input impedance, the complex ratio of transrespiratory (or transpulmonary) pres- sure to flow at the airway opening as a function of frequency, is becoming an increasingly popular method for assessing the dynamic mechanical status of the lungs. When measured over low frequencies (0.1–10 Hz), respiratory impedance can be a sensitive indicator of serial and parallel airway heterogeneity, 19 provides insight into the locus of airway constriction, 18 and may be useful in partitioning the me- chanical properties of airways and lung tissues. 15 Address correspondence to David W. Kaczka, MD, PhD, Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Blalock 1412, 600 North Wolfe Street, Baltimore, MD 21287. Electronic mail: dkaczka1@jhmi.edu Several approaches have been developed to measure low frequency respiratory impedance in humans and large ani- mals. The most common is to excite the respiratory sys- tem with small amplitude pseudorandom noise using a loud-speaker. 9 While straightforward, this technique has several technical and clinical drawbacks. It requires high- performance subwoofer speakers relatively free of har- monic distortion. Moreover, only nonphysiologic flows can be generated (typically less than 0.2 l/s) that are often load-dependent unless a closed-loop design is employed. 3,5 Finally, this approach requires considerable subject coop- eration. Awake subjects require training to achieve the nec- essary prolonged periods of apnea and respiratory muscle relaxation, 9 which makes the method impractical for routine use in patients with impaired lung function. In anesthetized and paralyzed patients, this technique usually requires tem- porary interruption of artificial ventilatory support. 23 More recent studies have incorporated high amplitude broad-band flow forcings into waveforms that mimic phys- iological breathing maneuvers. 12,13,15,20 Specifically, Opti- mal Ventilator Waveforms (OVWs) 20 and Enhanced Ven- tilator Waveforms (EVWs) 14 concentrate flow spectral energy at specific frequencies to minimize nonlinear har- monic distortion in the resulting pressure waveforms. The phases of these waveforms are optimized to achieve tidal volume excursions sufficient for gas exchange, and thus are more clinically appropriate for awake and anesthetized patients. 12,13,15 Presently, these waveforms must be gener- ated by piston–cylinder arrangements actuated by servo- controlled linear motors, allowing for delivery of high amplitude and load-independent oscillatory flows. 15,20,27,28 Despite the ability of such systems to produce high fidelity flow waveforms, 15,27 they can be extremely inefficient due to mechanical friction and stick-slip effects between the pis- ton and cylinder. 1 Moreover, their use in humans or large animals require high-powered electrical driving amplifiers, capable of dissipating over 200 W. 28 Regardless of the method employed to acquire respira- tory impedance data, a more vexing problem is the abil- ity of either approach to make oscillatory measurements 0090-6964/04/0400-0596/1 C  2004 Biomedical Engineering Society 596