Accuracy of different oxygenation indices in estimating intrapulmonary shunting at increasing infusion rates of dobutamine in horses under general anaesthesia.
Authors: Briganti A, Portela D A, Grasso S, Sgorbini M, Tayari H, Bassini J R Fusar, Vitale V, Romano M S, Crovace A, Breghi G, Staffieri F
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Intrapulmonary shunting—the degree to which blood bypasses ventilated lung tissue—is a critical determinant of oxygenation during equine anaesthesia, yet clinicians typically rely on indirect surrogates rather than direct measurement. Briganti and colleagues investigated how accurately five commonly used oxygenation indices predict actual venous admixture (Qs/Qt) in isoflurane-anaesthetised horses receiving escalating dobutamine infusions, using invasive pulmonary artery catheterisation to obtain paired arterial and mixed venous blood samples across eight time points per animal. Whilst conventional oxygen tension-based ratios—including PaO2/FiO2, PaO2/PAO2, P(A−a)O2, and the respiratory index—all demonstrated weak correlation with true shunt (r² <0.2), the fractional shunt calculation (F-shunt), which incorporates measured oxygen content rather than tension alone, showed substantially stronger agreement with Qs/Qt (r² = 0.73), regardless of dobutamine dose. For practitioners managing anaesthetised horses, this work suggests that when simultaneous arterial and venous blood sampling is feasible, calculating F-shunt provides a more reliable assessment of pulmonary gas exchange than relying on readily available PaO2 values alone, potentially guiding more targeted ventilatory management during prolonged procedures or in horses with pre-existing respiratory compromise.
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Practical Takeaways
- •When assessing oxygenation adequacy in anaesthetised horses, F-shunt is a superior indicator of true intrapulmonary shunting compared to commonly-used oxygen tension ratios
- •Standard oxygenation indices (PaO2/FiO2 ratio, A-a gradient) may be misleading in horses under isoflurane anaesthesia and should not be relied upon as sole measures of gas exchange efficiency
- •F-shunt calculation requires mixed venous blood sampling via Swan-Ganz catheter, limiting practical field use but warranting consideration during critical care monitoring in referral settings
Key Findings
- •F-shunt showed strong correlation (r² = 0.73) with Qs/Qt (venous admixture), while all oxygen tension-based indices were weakly correlated (r² < 0.2)
- •F-shunt maintained substantial agreement with Qs/Qt across all dobutamine infusion rates tested
- •PaO2/FiO2, PaO2/PAO2, P(A-a)O2, and respiratory index were all poor estimators of intrapulmonary shunting in isoflurane-anaesthetised horses