Effect of bronchomotor tone on work rate of breathing

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Abstract
The optimal airway caliber for minimizing the work rate of breathing in the lung (.ovrhdot.W) with different bronochomotor tones was studied in 6 normal subjects. The inhalation of methacholine contracted airway smooth muscle and the inhalation of salbutamol relaxed it. To calculate .ovrhdot.W at a given alveolar ventilation (.ovrhdot.VA), anatomical dead space (VDanat), pulmonary resistance (RL) and dynamic compliance were measured simultaneously, breath by breath, during various breathing maneuvers. VDanat increased and RL decreased with both increased breathing frequency and tidal volume, even at a given airway tone. This suggests that the airway caliber varied even at a given bronchomotor tone. The minimum .ovrhdot.W at a given .ovrhdot.VA increased in constricted airways, but there was no significant difference between control airways after saline inhalation and relaxed airways. Airway smooth muscle tones at both control and relaxed conditions may bring .ovrhdot.W to a minimum; the airway smooth muscle tone existing in the control state acts to keep the airway caliber optimal in order to minimize the .ovrhdot.W and stabilize the airway mechanics.