The Use of the Internal Perimeter to Compare Airway Size and to Calculate Smooth Muscle Shortening

TLDR

The internal airway perimeter (Pi) defined by the folded epithelial surface has been reported to remain constant as airways narrow. This study tested that hypothesis by treating adjacent lung lobe slices with theophylline or carbachol and measuring airway dimensions. Transverse sections of contracted and relaxed airways were analyzed for Pi, lumen area, external perimeter, and external area; wall area was calculated, and smooth muscle shortening was quantified from the relaxed external perimeter and the measured external perimeter. Pi and wall area were unchanged between contracted and relaxed states, confirming Pi as a stable airway size marker and enabling calculation of smooth muscle shortening.

Abstract

Previous studies from our laboratory suggest that the internal airway perimeter (Pi) defined by the folded epithelial surface remains constant as the airways narrow. To test this hypothesis, we treated adjacent slices of resected lung lobes with either theophylline or carbachol and determined the dimensions of the airways in these lung slices. Transverse sections of contracted (n = 58) and relaxed (n = 55) airways were used to measure the Pi defined by the epithelial surface, lumen area (Ai), external perimeter (Pe) defined by the outer edge of the smooth muscle layer, and the external area (Ae). Wall area (WA = Ae − Ai) was calculated. The frequency distribution of internal perimeters was not significantly different for the contracted and relaxed airways, and when the square root of wall area was plotted against Pi, the regression lines for the contracted and relaxed airways were almost identical. The "relaxed" external perimeter was calculated Per = , and the percentage of muscle shortening (PMS) was determined: PMS = [(Per − Pe)/Per] × 100. We conclude that Pi and WA are constant in airways whether the smooth muscle is relaxed or contracted and that Pi can be used as a marker of airway size and, under controlled conditions, can be used to calculate the smooth muscle shortening present in a given airway.