Hypocapnia does not alter collateral ventilation in sheep

Hypocapnic constriction has been proposed as a mechanism by which collateral pathways might rapidly alter ventilation to match perfusion. We studied the changes in response to hypocapnia with age in sheep, a species with collateral resistances (Rcoll) similar to those measured in humans. Measurements of Rcoll were made with either 5 or 10% CO₂ and with air (hypocapnia) in 29 anesthetized sheep, ages 6 mo to 10 yr, with the wedged bronchoscope technique. Rcoll was 0.42 ± 0.12, 0.58 ± 0.18, 0.32 ± 0.18, and 0.17 ± 0.04 (SE) cmH₂O · ml^-1 · min in 6-mo- and 1-, 2-, and 10-yr-old animals, respectively. These values were unchanged with hypocapnia. Despite the lack of a change in Rcoll with hypocapnia, administration of histamine aerosol (8 animals) through the bronchoscope increased Rcoll by 151 ± 35% (P < 0.05). These data suggest that although collateral pathways exist in sheep and are capable of constriction, they do not respond to hypocapnia. Furthermore, the response to hypocapnia is not influenced by age.