Authors: Bayat S. et al.
Anesthesiology, October 20, 2025. DOI: 10.1097/ALN.0000000000005807
This experimental study investigated how the slope of phase III (SIII) of the volumetric capnogram relates to ventilation–perfusion (V̇A/Q̇) mismatch in acute bronchoconstriction and how this relationship changes after bronchodilator therapy. Using advanced synchrotron radiation K-edge subtraction imaging, the researchers directly visualized and quantified ventilation (V̇A) and perfused blood volume (VB) in anesthetized rabbits before and after methacholine-induced bronchoconstriction and following salbutamol administration.
Acute bronchoconstriction caused marked ventilation defects, reducing both airflow and perfusion while increasing their heterogeneity. After salbutamol treatment, mechanical lung compliance improved, yet the V̇A/VB ratio paradoxically decreased, indicating worsening gas exchange due to increased perfusion of poorly ventilated lung regions—essentially, a higher shunt fraction.
Statistical analysis revealed that SIII correlated significantly with mean V̇A (r=0.49) and most strongly with the mean V̇A/VB ratio (r=–0.65), but not with VB alone. Multiple regression confirmed that variations in ventilation contributed more to changes in SIII than perfusion did.
These findings demonstrate that SIII is influenced by both the magnitude and distribution of V̇A/Q̇ mismatch, not simply ventilation heterogeneity. Importantly, the study helps explain why patients with acute bronchospasm may experience improved airway mechanics after bronchodilator therapy without parallel improvement in oxygenation—the bronchodilator restores ventilation unevenly, leading to ventilation–perfusion imbalance and transient shunt.
What You Should Know
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The phase III slope (SIII) of the capnogram reflects V̇A/Q̇ mismatch, not just ventilation heterogeneity.
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Bronchodilators like salbutamol can improve lung mechanics but paradoxically worsen gas exchange by increasing shunt fraction.
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Direct imaging confirmed that acute bronchoconstriction alters both ventilation and perfusion distribution.
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SIII may serve as a valuable noninvasive indicator of V̇A/Q̇ imbalance during bronchospasm and mechanical ventilation.
Thank you to Anesthesiology for publishing this innovative study linking advanced imaging with capnographic monitoring in bronchoconstriction.