Authors: Mosing Martina, Böhm Stephan H, Rasis Anthea, Hoosgood Giselle, Auer Ulrike, Tusman Gerardo, Bettschart-Wolfensberger Regula, Schramel Johannes P
Journal: Frontiers in veterinary science
Summary
# Editorial Summary During general anaesthesia, arterial to end-tidal CO₂ difference (P(a-ET)CO₂) and alveolar dead space fraction (VDalvfrac) are commonly used to assess pulmonary function in horses, yet their interpretation as true dead space measures has remained poorly validated in equine anaesthesia. Mosing and colleagues investigated which physiological factors drive these CO₂ indices by anaesthetising six healthy horses in dorsal recumbency for three hours, collecting blood gases, capnography, and haemodynamic data every 30 minutes to model relationships between the CO₂ indices and potential explanatory variables including cardiac output, venous admixture, and airway dead space. Venous admixture and cardiac output together explained 75·2% of the variance in P(a-ET)CO₂ and 71·1% in VDalvfrac—substantially more than conventional dead space variables—whilst the alveolar ventilation-perfusion ratio component showed the strongest individual correlations (r = 0·899 and r = 0·938 respectively). These findings fundamentally reframe how practitioners should interpret these indices: rather than estimating true anatomical dead space, P(a-ET)CO₂ and VDalvfrac function as sensitive markers of ventilation-perfusion mismatch and pulmonary perfusion disturbances, making them more useful for detecting pulmonary dysfunction than for quantifying wasted ventilation during equine anaesthesia.
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Practical Takeaways
- •During equine anesthesia, CO2 capnography indices reflect pulmonary perfusion problems and shunting more than actual dead space—monitor cardiac output and venous admixture to interpret these values correctly
- •These indices are useful for detecting overall pulmonary dysfunction during anesthesia but should not be used alone to estimate anatomic dead space in clinical settings
- •Improvements in anesthetic management should focus on maintaining cardiac output and minimizing venous admixture to optimize gas exchange
Key Findings
- •Venous admixture and cardiac output best explained P(a-ET)CO2 variance (R²=0.752) and VDalvfrac variance (R²=0.711) in anesthetized horses
- •P(a-ET)CO2 and VDalvfrac correlated most strongly with alveolar V/Q ratios (r=0.899 and r=0.938 respectively), not true dead space
- •Mean pulmonary pressure and airway dead space provided only marginal improvement to predictive models
- •P(a-ET)CO2 and VDalvfrac should be interpreted as global indices of V/Q mismatch rather than measures of anatomic dead space