Authors: Bayat S et al.
Anesthesiology, November 2025, DOI: 10.1097/ALN.0000000000005807
Summary:
This study examined how the capnogram phase III slope (SIII)—a bedside measure anesthesiologists often interpret as a marker of V̇A/Q̇ mismatch—actually corresponds to true ventilation and perfusion distribution in the lung. Using a rabbit model of acute methacholine-induced bronchoconstriction and synchrotron radiation K-edge subtraction imaging, the researchers directly measured regional ventilation (V̇A), perfused blood volume (VB), their ratio (V̇A/VB), and their heterogeneity. They compared these physiologic measures with changes in SIII before and after bronchoconstriction, and again after salbutamol.
Bronchoconstriction caused distinct ventilation defects, reduced both V̇A and VB, and significantly increased heterogeneity in both ventilation and perfusion. As expected, SIII increased. After salbutamol, mechanical lung properties improved, but gas exchange paradoxically worsened: V̇A/VB fell, shunt fraction increased, and more blood flowed to poorly ventilated regions. This dissociation between mechanical improvement and V̇A/Q̇ worsening reflects how bronchodilators can redistribute perfusion unfavorably in acute bronchoconstriction.
SIII correlated moderately with V̇A and strongly (negatively) with mean V̇A/VB, indicating it reflects combined effects of ventilation heterogeneity and V̇A/Q̇ mismatch. Multiple regression confirmed ventilation had the dominant influence on SIII, more so than perfusion changes.
Overall, the capnogram phase III slope is influenced by both V̇A heterogeneity and V̇A/Q̇ mismatch—not just ventilation distribution—and can worsen after bronchodilator therapy despite mechanical improvements.
What You Should Know:
• SIII is not a pure ventilation marker; it also reflects perfusion matching and V̇A/Q̇ heterogeneity.
• Acute bronchoconstriction increases both V̇A and VB heterogeneity—with SIII rising accordingly.
• Salbutamol improved mechanics but worsened V̇A/Q̇ by increasing perfusion to low-ventilation regions.
• A worsening SIII after bronchodilator therapy can reflect shunt and perfusion redistribution rather than airway obstruction.
• Capnography can reveal pathophysiologic changes not obvious from mechanics alone.
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