Authors: Bayat, Sam et al.
Anesthesiology November 2025 | DOI: 10.1097/ALN.0000000000005807
This investigation evaluated how the slope of phase III (SIII) of the volumetric capnogram reflects the balance between lung ventilation and perfusion (V̇A/Q̇) during acute bronchoconstriction. In a rabbit model, the researchers used methacholine to induce bronchoconstriction and synchrotron radiation K-edge subtraction (KES) imaging to quantify ventilation (V̇A) and perfused blood volume (VB) both before and after methacholine and following salbutamol administration.
Bronchoconstriction produced ventilation defects with significant decreases in both V̇A and VB and increased heterogeneity. Although salbutamol improved respiratory elastance, it reduced the V̇A/VB ratio and increased the shunt fraction, worsening gas exchange. SIII correlated significantly with V̇A (r = 0.49) and most strongly with the mean V̇A/VB ratio (r = –0.65), but not with perfusion alone. Multiple regression confirmed that ventilation exerted the greater influence on SIII.
These findings demonstrate that SIII is influenced not only by ventilation heterogeneity but also by V̇A/Q̇ imbalance and its regional variability. The study highlights the diagnostic potential of capnography in identifying ventilation–perfusion disturbances and interpreting the effects of bronchodilator therapy during acute bronchoconstriction.
What You Should Know
SIII on the volumetric capnogram is a sensitive indicator of V̇A/Q̇ mismatch. During bronchoconstriction, mechanical improvement after bronchodilator therapy may not guarantee better gas exchange, as redistribution of perfusion can increase shunt and worsen oxygenation.
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