Translation of Contractile Force to Constriction in Major Diameter Canine AirwaysIn Vivo

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Abstract
We studied the relationship between force generation and airway narrowing in the major diameter bronchi (Generations 1 to 5) in 14 mongrel dogs using tantalum bronchography in vivo. A characteristic tension-radius curve was described for each airway having two phases (different elastances). Substantial heterogeneity was demonstrated in the passive characteristics of these airways; compliance increased progressively with advancing airway generation. Changes in airway caliber elicited with intraveious infustion of methacholine (MCh) were used to calculate active force translated to airway wall tensions, and radius-tension (.apprxeq. length-tension) curves were calculated for each airway after correcting for wall thickness. The resting smooth muscle length at which maximal active tension (ATmax) was elicited for each generation (expressed at percent of the initial radius; %Ri) was similar for all generations (147 .+-. 7.32% for Generation 2 to 160 .+-. 8,82 for Generation 5). However, ATmax decreased with advancing airway generation from 6.58 .+-. 1.51 dyne/cm (Generation 2, extraparenchymal) to 1.60 .+-. 0.13 dyne/cm (Generation 5, intraparenchymal; p < 0.001). Relative change in airway diameter nonetheless was greater in smaller airways (28.2 .+-. 5.1% in Generation 5) than in larger airways (15.8 .+-. 3.5% in Generation 2; p < 0.001). These data indicate that increased narrowing of more distal airways in the dog results predominantly from mechanical advantage related to small Ri and/or greater compressability rather than to greater force translation from airway smooth muscle contraction, as suggested by prior investigations.