small airway compliance, Cp. IOS Rrs and Xrs at selected fre- quencies reflecting large and peripheral airway function. Model-derived parameters were ...
Respiratory System Model Parameters Track Changes in Lung Function after Bronchodilation E. Meraz1,3, H. Nazeran1, M. Goldman1, and B. Diong2 1
University of Texas at El Paso/Department of Electrical and Computer Engineering, El Paso, TX 7996, USA 2 Texas Christian University/Department of Engineering, Forth Worth, TX 76129, USA 3 Universidad Autónoma de Cd. Juárez/Departamento de Eléctrica y Computación, Cd. Juárez, Chihuahua, México Abstract— Impulse Oscillometry (IOS) provides respiratory resistance, Rrs, and reactance, Xrs, between 3 - 35 (Hz). We measured IOS parameters in 26 randomly selected Anglo children 6-19 years in El Paso TX. An expert pulmonologist classified Rrs and Xrs as: Normal (n = 5); Possible Small Airway Impairment, Possible SAI, (n = 4); Definite SAI (n = 6); or Asthma (n = 11). We used the extended RIC (eRIC) and augmented RIC (aRIC) models of the human respiratory system to derive Large (R) and Peripheral (Rp) airway resistance, and small airway compliance, Cp. IOS Rrs and Xrs at selected frequencies reflecting large and peripheral airway function. Model-derived parameters were compared between pre- and post-bronchodilator tests. IOS Xrs at low frequencies (AX) and model-derived Cp were the most sensitive parameters for detecting bronchodilator responses, which were significant in children with SAI and asthma. Model-derived Cp correlated closely with AX. We conclude that response to bronchodilator in children with SAI is sensitively measured by IOS. Electrical equivalent model parameters are as sensitive as primary IOS data in detecting bronchodilator response. Keywords— Respiratory Impedance, Respiratory Reactance, Respiratory system model, Impulse Oscillometry, asthma.
I. INTRODUCTION According to the American Academy of Allergy and Asthma & Immunology, Asthma and allergies strike 1 out of 4 Americans and approximately 20 million Americans have asthma. Nine million U.S. children under 18 have been diagnosed with asthma. Every day in America 40,000 people miss school or work, 30,000 have an asthma attack, 5,000 visit the emergency room, and 1,000 are admitted to the hospital and, although asthma is rarely fatal, 11 persons die due to asthma. Direct health care costs for asthma in the U.S. total more than $10 billion annually; and indirect costs (lost productivity) are $8 billion giving a total of $18 billion [1]. In Mexico, 10% (approximately 10 million people), of the population suffer from asthma. It is the most common cause of chronic illnesses and emergency hospitalizations in children according to the Mexican College of Allergy, Asthma and Pediatric Pulmonology [2].
Asthma can be properly controlled if it is accurately diagnosed and treated. Currently, spirometry is the most commonly used test to detect and monitor severity of asthma. Nevertheless it has been shown that some asthmatic patients do not improve spirometrically, regardless of clinical improvement with treatment [3]. This is of concern, because if asthma is not properly diagnosed and treated, it can lead to permanent airway damage. Moreover, spirometry has the disadvantage of requiring maximal inspiratory and expiratory efforts, which are difficult to achieve for children and older adults. The forced oscillation technique (FOT) superimposes small air pressure perturbations on the natural breathing of a subject to measure lung mechanical parameters. In contrast to spirometry, FOT requires only passive cooperation from subjects wearing a nose clip and breathing normally through the mouth. The Impulse Oscillometry System (IOS) is a commercially available method of forced oscillation that measures respiratory impedance by superimposing short pulses of air pressure 5 times each second. IOS has successfully been used to asses lung function in healthy and asthmatic children and adolescents [4]. IOS yields impedance (resistive and reactive) curves (frequency-dependent) that are visually analyzed to recognize changes in shape and magnitude of the curves and distinguish healthy respiratory function from disease. IOS provides respiratory impedance data that can be used to develop mechanical and equivalent electrical circuit models to evaluate and quantify the lung’s mechanical function. Estimates for these model parameters based on IOS data may possibly be used as baseline measures for better detection, diagnosis, and treatment of different respiratory diseases like asthma [5]. Previous efforts in our research group have resulted in the development of two different equivalent electrical circuit models for human respiratory impedance: Extended RIC (eRIC) and Augmented RIC (aRIC) models have been proven to show statistically significant differences between non-asthmatic and asthmatic children and between pre- and post-bronchodilaton IOS data in children [6-9].
A. McGoron, C. Li, and W.-C. Lin (Eds.): 25th Southern Biomedical Engineering Conference 2009, IFMBE Proceedings 24, pp. 319–322, 2009. www.springerlink.com
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1. Extended RIC Model (eRIC)
II. MATERIALS AND METHODS A. Subjects A total of 26 children were tested for this study before and after inhaling bronchodilator (BD) medicine. Anthropometric data for our study population are presented in table 1.
This model was developed as an improvement of the well known RIC model, by adding a peripheral resistance (Rp) in parallel with the peripheral airway compliance (Cp). This model is illustrated in figure 1. Rp
Table 1 Study Population
I
Variable
Range
Mean±SD
Age (years)
6-19
10.73±3.24
Height (cm)
121-185
146.63±19.71
Weight (kg)
21.8-93
45.33±20.65
B. IOS testing A Jaeger Master Screen IOS (Viasys/Cardinal Health, Yorba Linda, CA, USA) was utilized in this study. Children were asked to wear a nose clip, while breathing normally through a mouthpiece and were instructed to firmly close their lips around it to avoid air leakage. Children were tested three to five times, for 40-60 sec each replicate, both before and after inhaling, (for 5 to 6 min), nebulized bronchodilator (pre- BD and post-BD), this amount of recordings was done to ensure reproducible tests without artifacts caused by air leaks, swallowing, breath holding or vocalization. IOS data were carefully reviewed off line and quality-assured by our expert clinician to reject segments affected by airflow leak or swallowing artifacts. The IOS measures Respiratory Impedance (Z) by superimposing small pressure perturbations in the mouth of the subject. The IOS Respiratory Impedance has two components: Respiratory Resistance (R) and Respiratory Reactance (X). Respiratory Resistance is related to the resistance of the central and peripheral airways, lung tissue and chest wall. Respiratory reactance is associated with the mass-inertive forces of the moving air column represented in terms of Inertance (I) and the elastic properties of lung periphery (compliance) expressed as Capacitance (C). Other IOS parameters are the Resonant Frequency (Fres) and Reactance Area (AX). IOS parameters are obtained by FFT analysis of impulses of pressure and flow, resulting in a display of R and X at oscillation frequencies from 3 to 35 Hz. C. Respiratory Impedance models Two different respiratory impedance models were used for this investigation. In this section both models are briefly described.
R Cp
Fig. 1 Extended RIC model [9] 2.
Augmented RIC Model (aRIC)
This model was proposed as an improvement of the eRIC model, adding an additional capacitance called extrathoracic compliance (Ce). Figure 2 shows the aRIC model. Rp
R
I
Cp
Ce
Fig. 2 Augmented RIC model [9] D. Data Analysis The following IOS parameters were used in this study: Resistance at 3Hz (R3), R5, R10, R3-R20, R5-R20, Reactance at 3Hz (X3), X5, X10 and AX. Average values of resistances and reactances at 6 oscillation frequencies between 3 and 35 Hz were used to calculate the aRIC and eRIC model parameters: R (Resistance of the large airways), I (Inertance of the air column), Rp (Peripheral resistance), Cp (Compliance of peripheral airways), and only for aRIC, Ce (Extrathoracic compliance). The statistical significance of the IOS and model parameters was determined for pre- and post-BD tests performed on each child. The student t-distribution (Differences of Means) test was used and statistical significance was established at p
