Airway volume, airway resistance, and work and force of breathing: theory

breathing at which mechanical work and mean inspiratory force are minimal and which assume dead space and resistance are constant ignore possible changes in airway caliber. We assumed an inverse relationship between resistance and dead space and plotted a series of curves relating mechanical work and mean inspiratory force to frequency of breathing for various dead spaces and corresponding resistances. As dead space in-creases, the levels of work and inspiratory force fall to minima, then rise; the frequencies at which work and force are minimal increase, then decrease. Adding a series resistance or dead space, changing the resistance/dead space relationship, de-creasing lung compliance, or increasing alveolar ventilation do not alter the general shapes of the curves, but change the values of optimal dead space. The optimal dead spaces for minimal work (0. I 25 liter) and for minimal inspiratory force (0.14 liter) lie within the normal range. Nervously mediated airway tone in healthy subjects may represent optimal adjustment of airway caliber.-dead space, optimal bronchial caliber I N NORMAL MAN the lower airways are tonically con-stricted. Anatomical dead space (VD) increases by 22-44 % after isoproterenol aerosol (35) and by 27-47 % after subcutaneous injection of atropine (33, 35). Airway resistance (RA) decreases by I 7-44 % after isoproterenol (6, 12, 23) and by 42 % after atropine (37). Vagotomy dilates bronchi observed through a bronchoscope (I 8). These last two studies indicate that the constriction is mediated by parasympathetic and cholinergic efferent pathways. Anesthetized animals also have tracheobron-chial smooth muscle tone which is abolished by va-gotomy, atropine, or isoproterenol (reviewed in 36, 38). This tonic constriction can be increased or decreased b