AEROSOL-DERIVED AIRWAY DIMENSIONS OF DOG LUNGS IN HISTAMINE-INDUCED BRONCHOCONSTRICTION

The single-breath aerosol recovery technique has been demonstrated to facilitate noninvasive assessment of effective airway diameters in ane sthetized mechanically-ventilated dogs with normal airway geometry. In order to test the sensitivity of the method for the detection of magn itude and site of airway constriction, effective airway diameters were determined in dogs with airways maintained in sustained constriction by intravenous histamine. Monodisperse di-(2-ethyl-hexyl) sebacate par ticles (mean aerodynamic diameter +/- SD, 1.17 +/- 0.13 mum) were appl ied by the single-breath technique in nine anesthetized, paralyzed and mechanically-ventilated dogs (mean body wt. +/- SD, 15.2 +/- 2.4 kg; mean total lung capacity +/- SD, 1102 +/- 346 ml) during control condi tions and steady airway constriction achieved by continuous infusion o f histamine (approximately 1.7 mg kg-1 h-1). The rate of aerosol recov ery during post-inspiratory breath-holding (0-10 s) was determined fro m instantaneous particle concentration continuously recorded at the ai rway opening by a miniature in-line aerosol photometer. For comparison between control and histamine-induced airway narrowing, airway diamet ers determined as function of volumetric lung depth were referred to t he actual volume of the dog's total lung capacity and expressed as % T LC. Narrowing of airway diameters by intravenous histamine was maximal (approximately 50%) in the proximal airways (corresponding to 8% TLC) and the effect was continually fading towards peripheral airways. For deeper lung regions, i.e. beyond a volumetric depth of 19% TLC, no si gnificant differences from control values were detectable. Airway resi stance during histamine challenge, determined simultaneously by an int errupter technique, was increased by a factor of 5.7 and would be expe cted to result mainly from constriction of proximal airways. The marke d reduction of TLC (-25%) and static (-26%) and dynamic (-38%) lung co mpliances (along with an increasing difference between the two) is int erpreted to suggest that a major fraction of peripheral airways was co nstricted but was not recovered by the aerosol technique. This finding may be attributed to presence of marked unequal distribution of perip heral airway constriction, in which case effective aerosol-derived air way diameters would represent not arithmetic means but volume-weighted averages according to the fractional contribution to total airway vol ume at a given volumetric depth.