Impaired diffusion at submaximal lung inflation in asthma and copd patients

Area of Science:

• Pulmonary Medicine
• Respiratory Physiology
• Diagnostic Imaging

Background:

• Dissolved-phase ¹²⁹Xe MRI suggests gas diffusion impairment at submaximal lung inflation.
• Carbon monoxide transfer coefficient (Kco) is a key metric for assessing gas exchange.

Purpose of the Study:

• To investigate if the diffusion deficit at submaximal lung inflation, compared to total lung capacity (TLC), is detectable using Kco.
• To compare Kco at submaximal inflation (Kco_redux) with routine Kco at TLC in asthma and COPD patients.

Main Methods:

• Asthma and COPD patients underwent carbon monoxide diffusion tests at maximal inflation (for Kco, VA) and 30% reduced inflation (Kco_redux, VA_redux).
• Mixing efficiency (VA/TLC) was determined at both inflation levels to analyze Kco_redux/Kco behavior.

Main Results:

• Normal subjects (n=36) showed expected Kco recruitment (median Kco_redux/Kco = 130%).
• 60% of asthma patients (49/83) and 80% of COPD patients (44/55) exhibited reduced Kco recruitment (<122%) at submaximal inflation.
• In asthma, reduced Kco recruitment correlated with RV/TLC ratio (r=-0.53). In COPD, it correlated with lower FEV₁, higher RV/TLC, and lower VA/TLC.

Conclusions:

• A significant proportion of COPD and asthma patients demonstrate impaired Kco recruitment at submaximal lung inflation, associated with high RV/TLC.
• This finding suggests that hyperinflation at lung volumes below TLC can distort pulmonary capillaries and alveolar-capillary membranes, affecting diffusion capacity, even with normal Kco at full inflation in asthma.