European Journal of Cardiovascular Medicine

Cigarette smoking remains a major global health concern, recognized as a leading cause of preventable morbidity and mortality. The deleterious effects of smoking on respiratory health are well-documented, with chronic obstructive pulmonary disease (COPD) being among the most significant consequences. A hallmark of early smoking-induced lung damage is small airway obstruction, which often precedes the development of clinically overt airflow limitation detectable by conventional spirometry [1]. Identifying small airway dysfunction at its earliest stage is critical for initiating timely interventions to halt or reverse disease progression. However, traditional diagnostic tools like spirometry may lack the sensitivity to detect subtle changes in the small airways, particularly in asymptomatic individuals. This limitation underscores the need for more sensitive and specific diagnostic modalities.  Impulse Oscillometry (IOS), a noninvasive method of assessing respiratory mechanics, has emerged as a promising tool for the early detection of small airway obstruction. Unlike spirometry, which requires patient effort, IOS measures airway resistance and reactance using sound waves during tidal breathing, making it both patient-friendly and highly reproducible [2]. Small airway dysfunction can be inferred from IOS parameters such as peripheral resistance (R5–R20) and reactance at 5 Hz (X5), which reflect alterations in distal airway mechanics. IOS has shown potential in identifying early abnormalities in conditions like asthma, COPD, and interstitial lung disease, even when spirometric values remain within normal ranges [3].  The small airways, defined anatomically as airways with an internal diameter less than 2 mm, are often termed the "silent zone" of the lungs due to the absence of symptoms in early disease stages [4]. Structural changes, including airway wall thickening, loss of elastic recoil, and luminal narrowing, are exacerbated by smoking and are key contributors to small airway obstruction. Early dysfunction in these airways is increasingly recognized as a precursor to smoking-related lung diseases, including COPD and emphysema [5]. Detecting such changes before irreversible damage occurs represents an opportunity for effective disease prevention and management.  Although spirometry remains the gold standard for diagnosing airflow limitation, its limitations in detecting small airway obstruction are well-recognized. Spirometry predominantly assesses larger airways, and early-stage abnormalities in distal airways may not significantly alter spirometric indices such as forced expiratory volume in one second (FEV1) or forced vital capacity (FVC). By contrast, IOS provides a more detailed characterization of the entire respiratory system, including the small airways, without the reliance on forced expiratory maneuvers [6]. Studies have demonstrated the ability of IOS to detect small airway impairment in smokers with normal spirometric findings, emphasizing its utility in early diagnosis [7].  Smoking-induced small airway obstruction is a dynamic process influenced by various factors, including the duration and intensity of smoking, genetic predisposition, and environmental exposures. Persistent smoking leads to inflammation, oxidative stress, and remodeling of small airways, resulting in progressive narrowing and airflow limitation [8]. These pathological changes are often subclinical in the early stages, highlighting the importance of sensitive diagnostic tools. Additionally, the reversibility of small airway dysfunction with smoking cessation or pharmacological interventions in early stages underscores the need for early detection [9].  IOS offers several advantages over conventional lung function tests in the context of smoking-related lung damage. Its ease of use, minimal patient effort, and high sensitivity to changes in peripheral airway resistance make it an ideal tool for early screening, particularly in high-risk populations like smokers. Furthermore, IOS provides a comprehensive assessment of airway mechanics, capturing subtle changes that may precede clinical symptoms or spirometric abnormalities [10].  Existing literature has provided insights into the diagnostic utility of IOS in detecting small airway dysfunction in various populations. Studies have reported that smokers exhibit significant alterations in IOS parameters compared to non-smokers, even in the absence of spirometric abnormalities [11]. Parameters such as increased R5, R5–R20, and decreased X5 have been consistently associated with small airway obstruction in smokers. These findings suggest that IOS can serve as a sensitive biomarker for early airway dysfunction, potentially enabling earlier intervention and better outcomes [12].  Despite its promise, the utility of IOS in clinical practice remains underutilized, particularly in the context of early detection of smoking-induced small airway obstruction. While several studies have demonstrated its sensitivity, there is a need for more comprehensive research to establish its role in routine screening and monitoring. Factors such as cost, accessibility, and integration into existing diagnostic frameworks also require consideration [13].  This study aims to evaluate the utility of Impulse Oscillometry in the early detection of small airway obstruction in smokers. By comparing IOS parameters between smokers and non-smokers, with a focus on those with normal spirometric findings, the study seeks to elucidate the potential of IOS as a diagnostic tool for early airway impairment. Furthermore, the study aims to explore the relationship between smoking intensity (pack-years) and IOS parameters, providing insights into the dose-response relationship of smoking-induced small airway dysfunction. 

Study Design:

This was a cross-sectional observational study conducted in the Department of Respiratory Medicine, SAIMS PGI, Indore. The study aimed to evaluate the utility of impulse oscillometry (IOS) in the early detection of small airway obstruction among smokers.

Study Population:

A total of 96 participants were included in the study, selected using convenience sampling. The study population comprised smokers aged 18–60 years with a smoking history of at least 10 pack-years. Non-smokers with respiratory symptoms and patients with known respiratory diseases such as asthma, COPD, or interstitial lung disease were excluded to eliminate confounding factors.

Ethical Considerations:

Approval was obtained from the institutional ethics committee prior to initiating the study. Written informed consent was obtained from all participants after explaining the study objectives and procedures.

Procedure:

Clinical Assessment:

All participants underwent a detailed clinical evaluation, including history-taking and physical examination. Smoking history was recorded in terms of duration and pack-years.

Spirometry Testing:

Baseline pulmonary function tests (PFTs) were conducted using a spirometer to assess standard parameters, including FEV₁, FVC, and FEV₁/FVC ratio.

Impulse Oscillometry:

IOS was performed using a commercially available oscillometry device (mention the model, if available). Participants were instructed to breathe normally through a mouthpiece while wearing a nose clip, and the device measured airway resistance (R5, R20) and reactance (X5) at various frequencies.

Parameters Assessed:

• Resistance at 5 Hz (R5) and 20 Hz (R20).
• Reactance at 5 Hz (X5).
• Frequency dependence of resistance (R5–R20), an indicator of small airway obstruction.

Comparison of IOS and Spirometry:

The IOS results were analyzed for early signs of small airway obstruction and compared with spirometric findings. The study focused on identifying discrepancies where IOS detected changes in small airways that were not evident in spirometry.

Data Analysis:

Statistical analysis was performed using SPSS software (mention the version, if known). Descriptive statistics were used to summarize the demographic data and smoking history. Differences between IOS and spirometry parameters were analyzed using paired t-tests or Wilcoxon signed-rank tests for non-parametric data. Correlation analysis between pack-years and IOS parameters was performed using Pearson or Spearman correlation coefficients.

Outcome Measures:

• The primary outcome was the detection of small airway obstruction using IOS (e.g., abnormal R5–R20 or X5 values).
• Secondary outcomes included the relationship between smoking history and airway obstruction as indicated by IOS.

This methodology ensured a comprehensive evaluation of IOS as a sensitive tool for the early detection of small airway obstruction in smokers.

Table- 1 Demographics of the Study Population

The demographics table provides an overview of the study's participants, which included 96 patients. The average age was 55.2 years, with a standard deviation of 10.4 years. The gender distribution was skewed towards males (70 males and 26 females). All participants were smokers, with an average smoking duration of 25.3 years (±12.1 years) and a mean pack-year history of 30.5 (±15.6). This population predominantly reflects individuals with prolonged smoking exposure, which is a significant risk factor for respiratory diseases.

Table 2 - Clinical Features of Study Population

The clinical features table summarizes the prevalence of symptoms and prior diagnoses among the study population. A history of cough was reported by 81.3% of participants, while sputum production was less common, affecting 56.3%. Dyspnea was present in 68.8% of the patients, indicating widespread respiratory compromise. Almost half of the population (43.8%) had a prior diagnosis of chronic obstructive pulmonary disease (COPD), and 18.8% had been previously diagnosed with asthma. These findings highlight the high burden of respiratory symptoms and diseases in this smoker population.

Table 3 - Comparison of Impulse Oscillometry and Spirometry

This table compares the mean values of parameters measured by impulse oscillometry (IOS) and spirometry, two diagnostic tools used for evaluating lung function. Key observations include:

- **R5 (Resistance at 5 Hz):** Measured only by IOS, the mean was 6.2 cm H2O/L/s (±1.5), with a statistically significant p-value (<0.001), indicating its sensitivity to detect airway resistance.

- **R20 (Resistance at 20 Hz):** The mean was 4.5 cm H2O/L/s (±1.0), also significant (p < 0.001), reflecting central airway resistance.

- **X5 (Reactance at 5 Hz):** This measure, representing lung elasticity, was -0.5 cm H2O/L/s (±0.4) in IOS, with a highly significant p-value (<0.001).

- **Spirometry Parameters:** Spirometry showed a mean forced expiratory volume in one second (FEV1) of 68.4% (±12.3) and forced vital capacity (FVC) of 75.1% (±10.6) of the predicted values. Both values were significant (p < 0.01 for FVC, p < 0.001 for FEV1).

The significant differences across parameters emphasize the complementary roles of IOS and spirometry in assessing respiratory function, with IOS providing additional insights into airway resistance and reactance.

The demographic characteristics of our study population, with a mean age of 55.2 years and a predominance of male smokers, align with previous research that highlights similar trends in populations at risk for chronic obstructive pulmonary disease (COPD) and asthma. For instance, a study by Kanda et al. reported comparable demographics in their COPD cohort, reinforcing the notion that prolonged smoking exposure is a critical risk factor for respiratory diseases [14].The high prevalence of respiratory symptoms such as cough (81.3%), sputum production (56.3%), and dyspnea (68.8%) observed in our study mirrors findings from other studies which reported significant symptom burdens in smokers and patients with COPD [15,16]. Notably, the prevalence of prior diagnoses of COPD (43.8%) and asthma (18.8%) underscores the chronic nature of respiratory conditions prevalent in this demographic.

IOS vs. Spirometry: Sensitivity and SpecificityOur comparison of IOS and spirometry revealed that IOS parameters, such as R5 and R20, were significantly elevated in our cohort, indicating increased airway resistance. The mean values for R5 (6.2 cm H2O/L/s) and R20 (4.5 cm H2O/L/s) were statistically significant (p < 0.001), suggesting that IOS is particularly sensitive to detecting small airway obstruction, which is often missed by spirometry [2]. In contrast, spirometry demonstrated a mean FEV1 of 68.4% and FVC of 75.1%, both significantly lower than predicted values.

This finding is consistent with literature indicating that IOS is more sensitive than spirometry for identifying early small airway disease, especially in populations with a history of smoking or diagnosed obstructive lung diseases [15,17]. For example, a comparative study highlighted that IOS had a sensitivity of 100% for detecting small airway obstruction in asthmatics and 83% in COPD patients, while spirometry was less effective in these contexts [15]. Furthermore, our results align with findings by Al-Mutairi et al., who noted that IOS parameters were significantly correlated with spirometric outcomes but provided additional insights into small airway function [17].

Clinical Implications

The implications of these findings suggest that IOS can serve as a valuable complementary tool to spirometry in clinical settings, particularly for early detection and monitoring of small airway obstruction in smokers. Given that IOS requires less patient effort and can be performed without forced maneuvers, it may be more suitable for patients who struggle with traditional spirometric techniques [16]. This attribute makes IOS particularly advantageous for use in vulnerable populations such as the elderly or those with severe respiratory compromise.

In conclusion, our study reinforces the utility of Impulse Oscillometry as an effective diagnostic tool for early detection of small airway obstruction in smokers when compared to traditional spirometry. The complementary nature of these two modalities can enhance diagnostic accuracy and facilitate timely interventions for patients at risk for chronic respiratory diseases.

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