Diffusing capacity at different lung volumes during breath holding and rebreathing.

Area of Science:

• Pulmonary Physiology
• Gas Exchange in Lungs

Background:

• The single-breath diffusing capacity of the lung for carbon monoxide (DLCO) increases with lung volume above functional residual capacity (FRC).
• The underlying physiological mechanism for this volume dependence is debated, with possibilities including changes in capillary blood volume, surface area, or regional distribution of inspired air.

Purpose of the Study:

• To investigate the physiological mechanism behind the volume dependence of DLCO.
• To determine if changes in inspired air distribution or effective gas exchange surface area account for the observed increase in DLCO at higher lung volumes.

Main Methods:

• Measured DLCO using two techniques: breath holding and rebreathing.
• Employed a rebreathing technique designed to homogenize gas distribution within the lungs, minimizing regional differences.
• Performed measurements at two lung volumes: 1,500 ml above FRC and near total lung capacity (TLC).

Main Results:

• DLCO measured during breath holding was 18% higher near TLC compared to 1,500 ml above FRC (P < 0.05).
• DLCO measured during rebreathing was 16% higher near TLC compared to 1,500 ml above FRC (P < 0.01).
• The similar results obtained with both breath-holding and rebreathing techniques indicate that changing distribution of inspired air is not the cause of DLCO's volume dependence.

Conclusions:

• The findings support the hypothesis that the effective surface area available for gas exchange increases as lung volume expands.
• Volume dependence of DLCO is not an artifact of uneven gas distribution but reflects a genuine physiological change in the lung's gas exchange capacity at higher volumes.