EFFECT OF HYPERCAPNIA ON LARYNGEAL AIRWAY-RESISTANCE IN NORMAL ADULT HUMANS

Laryngeal airway resistance (Rlar) was measured in eight normal adult humans during progressive hyperoxic hypercapnia. In most subjects, the translaryngeal pressure-flow relationship appeared linear under normo capnic conditions. During hypercapnia, the pressure-flow relationship on inspiration and expiration was curvilinear with increasing translar yngeal pressure associated with progressively smaller increments in fl ow. Translaryngeal pressure-flow relationships at different CO2 levels were compared over their common flow ranges by performing a least-squ ares linear regression on data throughout inspiration and expiration. During normocapnia, the mean slope, i.e., mean Rlar, was 0.50 +/- 0.21 (SD) cmH(2)O.l(-1).s. A moderately significant decrease in Rlar was p resent at 9% end-tidal CO2 (P = 0.08). In a separate series of experim ents, subjects breathed oxygen- and helium-based gas mixtures through a face mask attached to a pneumotachograph. Data analysis over the flo w range present during normocapnia revealed no difference in Rlar betw een nose and mouth breathing and similar decreases in Rlar under hyper capnic conditions with the oxygen- and helium-based gas mixtures. The decrease in Rlar from normocapnic to hypercapnic conditions found over common, but relatively low, ranges of flow predicts that even greater increases in Rlar would occur at high flow rates in the absence of in creasing glottic aperture.