Density dependence of maximal flow is lung volume dependent during bronchoconstriction

Area of Science:

• Pulmonary Physiology
• Respiratory System Mechanics

Background:

• Histamine is a potent mediator of bronchoconstriction.
• Peripheral airway obstruction can be challenging to assess using standard pulmonary function tests.

Purpose of the Study:

• To investigate the impact of histamine-induced bronchoconstriction on maximum expiratory flow (Vmax) and its density dependence (delta Vmax).
• To evaluate the utility of delta Vmax in identifying the predominant site of airflow limitation during bronchoconstriction.

Main Methods:

• Histamine acid phosphate nebulization in six dogs to induce peripheral airway obstruction.
• Measurement of Vmax using air and an 80% He-20% O2 mixture (delta Vmax) at various lung volumes (51% to 23% vital capacity).
• Interpretation of findings using the wave-speed theory of flow limitation.

Main Results:

• Vmax decreased by approximately 30% across all lung volumes during bronchoconstriction.
• Histamine significantly increased peripheral airway resistance (threefold) with minimal changes in central airway resistance.
• Delta Vmax decreased at 44% vital capacity, indicating peripheral obstruction, but showed variable or increased changes at lower lung volumes (23% vital capacity).

Conclusions:

• Histamine-induced bronchoconstriction predominantly affects peripheral airways.
• Changes in delta Vmax at lower lung volumes may not accurately reflect the site of airflow limitation due to pre-existing differences in wave-speed variables.
• The study highlights limitations in using delta Vmax to pinpoint obstruction sites during dynamic airway narrowing.