Abstract
Changes in the indices related to the slopes of the second (SII) and the third phases (SIII) of the expired CO2 curves are thought to reflect altered respiratory mechanics. However, the effects of various constrictor agonists acting mainly on the central conducting airways on these capnogram parameters have not been characterized. Thus, we aimed at relating the changes in SII and SIII to those reflecting airway mechanics when the constrictor stimulus is of cholinergic or histaminic origin. Forced oscillations were applied to measure airway resistance (Raw) in anaesthetized, mechanically ventilated rabbits (n=11). Mainstream capnography was performed to assess SII and SIII. Measurements were made under the baseline conditions (BL), and during increasing doses (D1-D4) of iv methacholine or histamine, administered in random sequence. The similar marked airway responses to methacholine (p<0.001) and histamine (p<0.001) were associated with opposite changes in SII (p<0.005), while the SIII exhibited no significant change to either of the agonists (p=0.36). These findings suggest the limited ability of SIII to detect constriction of the conductive airways, if no severe ventilation heterogeneities develop. While the level of the changes in SII is consistent with the severity of airway constriction, its alteration depends fundamentally on the mode of constrictor stimulus.
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