Authors: Bayat S et al.
Journal: Anesthesiology, December 12, 2025. DOI: 10.1097/ALN.0000000000005807
Summary
This experimental physiologic study examined whether the slope of phase III of the volumetric capnogram (SIII) truly reflects ventilation–perfusion (V/Q) mismatch during acute bronchoconstriction and how this relationship is altered by bronchodilator therapy. Using a rabbit model and high-resolution synchrotron radiation imaging, the investigators directly quantified regional ventilation and perfused blood volume to clarify the mechanisms underlying capnographic changes.
Acute bronchoconstriction induced by methacholine produced marked ventilation defects and reductions in both regional ventilation and perfused blood volume, with significant increases in their heterogeneity. These changes were accompanied by a steepening of SIII. Salbutamol administration improved respiratory mechanics, reflected by improved elastance, but paradoxically worsened gas exchange by decreasing the V/Q ratio and increasing shunt fraction due to preferential perfusion of poorly ventilated lung regions.
Statistical analysis showed that SIII correlated significantly with ventilation but not directly with perfused blood volume alone. The strongest association was observed between SIII and mean V/Q ratio, indicating that SIII reflects combined effects of ventilation magnitude, ventilation heterogeneity, and V/Q mismatch rather than ventilation inhomogeneity alone. Multiple regression analysis confirmed that ventilation-related factors exerted a stronger influence on SIII than perfusion alone.
These findings provide mechanistic validation for the clinical interpretation of volumetric capnography and explain why bronchodilation may improve airway mechanics without parallel improvements in oxygenation during acute bronchoconstriction.
Key Points
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Phase III capnogram slope reflects ventilation–perfusion mismatch, not ventilation heterogeneity alone.
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Acute bronchoconstriction increases heterogeneity of both ventilation and perfusion.
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Salbutamol improves respiratory mechanics but may worsen gas exchange by increasing shunt fraction.
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Increased perfusion of poorly ventilated lung regions explains paradoxical oxygenation changes.
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Synchrotron imaging provides direct physiologic confirmation of capnographic assumptions.
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Capnography offers valuable insight into V/Q physiology during acute bronchoconstriction.
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