Use of the SpO2/FiO2 Diagram to Assess Gas Exchange in Horses Under General Anaesthesia.

This study investigates the effectiveness of the SpO2/FiO2 diagram, a non-invasive method, in assessing gas exchange in anesthetized horses. The research uncovered a strong and significant correlation between this method and the traditional invasive measurements, proving its viability as a convenient tool in this context.

Study Design and Procedure

• The study was a prospective, controlled clinical research involving 10 horses that were placed under general anesthesia.
• The level of fractional inspired oxygen (FiO2), the proportion of oxygen in the inhaled air, was gradually lessened through four stages: 0.6, 0.4, 0.3, and 0.21.
• The oxygen saturation (SpO2), the proportion of oxygenated hemoglobin to total hemoglobin in the blood, was recorded at each FiO2 phase.
• Arterial blood samples were collected when FiO2 was at 1.0 and 0.21, and used to calculate intrapulmonary shunts using the Fshunt formula. Fshunt values at these two stages were dubbed “Fshunt 1.0” and “Fshunt 0.21”.

Data Analysis and Results

• The data points from FiO2 vs SpO2 were processed through a computer algorithm, which considered the hemoglobin level and a fixed value for the arterial-venous oxygen difference (3.5mL/dL).
• The algorithm determined the shunt value (SShunt) by getting the best fit between the collected data and an ideal SpO2/FiO2 curve.
• Correlations between the calculated SShunt and “Fshunt 1.0” and “Fshunt 0.21” were established using the Spearman Rank Correlation Coefficient test, regression curve analysis, and calculation of the coefficient of determination (r).
• A significant (P < 0.05) and strong correlation (P = 0.0069; r = 0.839; r = 0.6194) was found between SShunt and "Fshunt 1.0", indicating a strong relationship between the two measurements.
• A significant, though moderate correlation (P = 0.0443; r = 0.644; r = 0.2336) was found between SShunt and “Fshunt 0.21”, suggesting a moderate relationship between the two.

Conclusion

• The SpO2/FiO2 diagram was validated as a practical and non-invasive tool to assess gas exchange in horses under general anesthesia.
• This tool could significantly contribute to the handling of horses under anesthesia by providing a less invasive and more convenient method to monitor their respiration.