Comparative observational study on diagnostic utility of impulse oscillometry vs spirometry in obstructive airway diseases

Introduction

Asthma and Chronic Obstructive Pulmonary Disease (COPD) are prominent constituents of obstructive airway diseases and are ranked as the fourth leading cause of death globally. In the context of obstructive airway diseases (OADs), pulmonary function measurement serves as a pivotal factor in diagnosing these conditions, gauging disease severity, predicting prognosis, and evaluating therapeutic response.¹

Patients exhibiting symptoms of dyspnea, cough, chronic expectoration, and exposure to harmful chemicals warrant suspicion of COPD. The diagnostic confirmation of COPD is achieved through spirometry, particularly when the FEV1/FVC ratio is less than 70% after bronchodilator administration. Notably, as the disease progressed, FEV1 demonstrated a decline.²

In the case of asthma, patients may present with recurrent chest wheezing, cough, chest tightness, or breathing difficulties, often accompanied by other allergic symptoms. The diagnosis of asthma, according to (Global Initiative for Asthma) guidelines, involves identifying airway reversibility after bronchodilator administration, specifically, a greater than 200 ml or 12% increase in FEV1 in spirometry. Asthma, a growing health concern, is diagnosed based on clinical history, physical examination, and pulmonary function testing following the GINA guidelines.³

Spirometry is the gold standard diagnostic test for OADs and is routinely performed as a pulmonary function test (PFT). However, it requires active patient participation and physical dexterity, which may be challenging for young children, the elderly, and individuals with physical or cognitive impairment. Additionally, the forced expiratory maneuver in spirometry can trigger volume-dependent closure of the small airways.⁴ Impulse Oscillometry (IOS) was developed as a modernized approach to traditional pulmonary function testing. It employs pressure flow oscillations at the mouth, superimposing over the patient’s tidal breaths at various oscillation frequencies to assess the respiratory system’s resistance and reactance.⁵

The IOS represents a variant of the forced oscillation technique (FOT), which investigates the mechanical properties of the respiratory system, including resistance and elasticity, across a frequency range of 5–20 Hz. Unlike FOT, which employs sinusoidal waves, IOS assesses the respiratory machine’s response to a triangular strain pulse.⁶ The key advantage of FOT/IOS lies in its use of simpler tidal breathing, requiring less cooperation and effort compared to spirometry. Furthermore, because IOS is conducted without forceful respiratory movements, it is less likely to affect airway smooth muscle tone.⁷

Several parameters were measured through IOS, including R5, representing the resistance at 5 Hz frequency, which reflects total airway resistance; R20, indicating resistance at a 20 Hz frequency, representing larger airway resistance; X5, denoting reactance at 5 Hz frequency, which reflects the elasticity of peripheral airways; and Ax, signifying the Area of Reactance, reflecting the degree of peripheral airway obstruction.⁸

Reversible blockage and bronchial hyperresponsiveness are key criteria for asthma diagnosis. The IOS exhibited sensitivity and accuracy in both aspects. A 30-35 percent decrease in R5 is considered a significant bronchodilator response. Asthmatic individuals typically present higher R5, fres, and AX values, whereas X5 values are lower. This pattern was also observed during bronchoprovocation testing, with an increase in R5, fres, and AX and a decrease in X5. Notably, a 20% reduction in FEV1 corresponds to a 50% decrease in X5, which is a more sensitive metric for detecting bronchial hyperreactivity.⁹

Gong et al. demonstrated that reactance parameters correlate more strongly with lung function than resistance parameters in COPD patients. They propose that changes in X5 over time could serve as an effective disease monitoring tool.⁵ For COPD diagnosis, even when spirometric values are within the normal range, patients with COPD-related complaints exhibit increased pulmonary resistance and decreased pulmonary reactance. Hence, IOS is highly sensitive in detecting subtle changes in pulmonary function, making it a more sensitive modality for this purpose.¹⁰

Methods

Discussion

In this study, we performed a comprehensive comparison of Impulse Oscillometry (IOS) and spirometry as diagnostic tools for the early detection of asthma and Chronic Obstructive Pulmonary Disease (COPD). In individuals with asthma and COPD, we expected to find elevated values of respiratory resistance indices R5 and R20, while reactance index X5 should exhibit a marked reduction.

A study conducted by Al-Mutairi et al.¹¹ in 2007 reported that the IOS demonstrated a sensitivity of 31.3% in diagnosing asthma, surpassing traditional pulmonary function tests, which had a sensitivity of 19.6%. Similarly, their research revealed that IOS had a sensitivity of 38.95% for diagnosing COPD compared to 47.4% sensitivity for conventional pulmonary function tests. Furthermore, the IOS exhibited a sensitivity of 45.8%, which was notably higher than that of pulmonary function tests (28.8%) for identifying individuals without any respiratory illnesses.¹¹

Importantly, reactance values have been used to gauge the progression of COPD. IOS resistance measurements have shown potential for identifying early stage COPD and subtle alterations in the small airways, particularly in response to bronchodilators and bronchogenic challenges.¹²

Building on the findings of prior research,¹³ our study anticipated a higher sensitivity of reactance measurements in asthmatic patients and a heightened sensitivity of resistance measurements in individuals with COPD. Our study aimed to highlight the exceptional sensitivity of IOS in detecting obstructive pulmonary diseases. We examined the comparative utility of Spirometry and IOS measures, including R5, R20, and X5, in patients with asthma and COPD. Our findings have the potential to elucidate the correlation between R5 stage and COPD severity. Prior research has also indicated strong associations between R5, X5, and FEV1.¹⁴

It is noteworthy that patients with advanced COPD tend to exhibit greater within-breath variations in Xrs5, as indicated by Kanda et al.¹⁵ Furthermore, recent studies suggest that the ability to differentiate between COPD and asthma patients using inspiratory-expiratory X5 analysis is more effective than relying solely on total respiratory IOS.¹⁶ In the context of IOS, it is worth highlighting that inspiratory assessments tend to offer superior accuracy compared to expiratory assessments.¹⁷ These insights contribute to a better understanding of the diagnostic potential of IOS and the nuanced nuances of its measurements in different respiratory conditions.