What is the role of impulse oscillometry (IO) in diagnosing and managing respiratory diseases such as asthma or chronic obstructive pulmonary disease (COPD)?

Impulse Oscillometry in Respiratory Disease Diagnosis and Management

Impulse oscillometry (IOS) serves as a complementary diagnostic tool to spirometry, particularly valuable for detecting peripheral airway obstruction in asthma patients who cannot perform spirometry adequately, though it does not replace spirometry as the primary diagnostic standard for obstructive lung diseases. 1

Primary Role and Clinical Positioning

Diagnostic Utility

• IOS measures respiratory resistance during normal tidal breathing using sound waves, requiring minimal patient cooperation compared to forced expiratory maneuvers. 1, 2 This makes it especially useful in children, elderly patients, and those unable to perform adequate spirometry. 1, 3

• The European Respiratory Society guidelines state that resistance measurements (including forced oscillation technique/IOS) have no clinical advantage over FEV1 measurement in most COPD situations. 1 This represents the consensus view that spirometry remains the gold standard for diagnosis and severity assessment.

• IOS demonstrates superior sensitivity (45.8%) compared to spirometry (28.8%) in identifying normal subjects, though spirometry shows better sensitivity for detecting COPD (47.4% vs 38.95%). 4

Key Parameters Measured

• R5 (resistance at 5 Hz) represents total airway resistance, while R20 (resistance at 20 Hz) reflects large airway resistance. 3 The difference (R5-R20) indicates small airway resistance.

• X5 (reactance at 5 Hz) indicates elastic recoil of peripheral airways, and the area of reactance (AX) provides information about peripheral airway function. 5, 3

• Resonant frequency (Fres) returns toward lower, more normal values after bronchodilation in COPD patients, particularly as reactance (Xrs) increases. 1

Specific Applications

Asthma Diagnosis and Management

• IOS is particularly valuable for detecting peripheral airway obstruction in asthma patients with preserved spirometry (FEV1 ≥80% predicted). 5 In one study, 33.85% of asthma patients showed peripheral airway obstruction on IOS compared to only 9.23% of COPD patients (p=0.001). 6

• The area of reactance (AX) combined with FEV1 provides superior diagnostic accuracy (AUC 0.7437) compared to FEV1 alone (AUC 0.6758) for asthma with preserved lung function, with an optimal AX cutoff value of 0.51 kPa/L. 5

• For bronchodilator response in children with chronic respiratory symptoms, a decrease in Rrs0 of ≥27.8% (or >-1 SD score) demonstrates 69% sensitivity and 78% specificity compared to FEV1 increase of ≥10% predicted. 1

COPD Assessment

• There is no consensus regarding IOS sensitivity compared to spirometry for bronchodilation testing in COPD. 1 Some studies show IOS indices are markedly less sensitive than spirometry for detecting significant bronchodilation (requiring ~45% decrease in Rrs6), while others suggest equivalent performance. 1

• Post-bronchodilator changes in COPD typically show decreased Rrs (especially at low frequencies) and increased Xrs, with correlation to FEV1 changes dependent on the magnitude of bronchodilator response. 1

Connective Tissue Disease-Related ILD

• IOS has been identified as an easy alternative to HRCT for detecting early pulmonary involvement in systemic sclerosis and rheumatoid arthritis patients, though validation in large multicentric prospective studies is still needed. 1

Clinical Limitations and Caveats

When IOS Should NOT Replace Spirometry

• Spirometry with FEV1/FVC ratio <0.7 post-bronchodilator remains the diagnostic threshold for airflow obstruction per European Respiratory Society guidelines. 7 IOS cannot establish this fundamental diagnostic criterion.

• FEV1 percentage predicted determines COPD severity classification (Mild ≥70%, Moderate 50-69%, Severe <50%), which cannot be derived from IOS parameters. 1, 7

• IOS sensitivity for asthma diagnosis is only 31.3% compared to 19.6% for spirometry, with comparable specificity between methods. 4

Optimal Use Strategy

• IOS should be used as an adjunct when spirometry is normal but clinical suspicion for asthma remains high, particularly for assessing peripheral airway involvement. 6, 2, 5

• Consider IOS in patients unable to perform adequate forced expiratory maneuvers due to age, cognitive impairment, or acute respiratory distress. 1, 2

• IOS may detect early airway changes before spirometric abnormalities appear, potentially useful for monitoring asthma control and identifying early disease in smokers. 2

Practical Implementation

• IOS requires less patient cooperation than spirometry as measurements occur during normal tidal breathing, making it feasible in populations where spirometry cannot be performed. 1, 2

• The technique is suitable for bronchodilator response testing and bronchoprovocation challenges, though interpretation differs from spirometric criteria. 2

• Home-based IOS measurements appear feasible and may demonstrate advantages over peak flow for asthma monitoring, though diagnostic utility requires further study. 1