Comparison of different methods to calculate venous admixture in anaesthetized horses.

The research focuses on comparing different methods used to determine venous admixture (Q˙/Q˙) in anaesthetized horses. In particular, the research explores the influence of blood pressure and position on oxygen extraction, using regression analysis between jugular and mixed venous blood oxygen tension.

Objective of the Study

The research was premised on three main objectives:

• The first objective was to estimate venous admixture using jugular venous blood oxygen content (Q˙/Q˙), with a fixed value for the oxygen extraction, also referred to as F-shunt.
• The second objective was to assess the effect of both blood pressure and the positioning of the horse on oxygen extraction.
• Finally, the researchers aimed to perform regression analysis between jugular and mixed venous blood oxygen tension. This would give them a quantifiable relationship between these two variables.

Methodology

• The study involved seven warmblood horses, all of which were anaesthetized with a mixture of detomidine, butorphanol, ketamine, diazepam, and isoflurane in oxygen.
• Simultaneous arterial, jugular venous, and pulmonary arterial blood samples were collected under both normotensive (normal blood pressure) and hypotensive (low blood pressure) conditions in lateral (side) and dorsal (back) recumbency.
• The researchers then calculated arterial, mixed venous, and end-capillary oxygen content from these samples.

Results

• The study found significant correlation between Q˙/Q˙ and Q˙/Q˙, with the intraclass correlation coefficient (ICC) being 0.68, and a Bland-Altman analysis displaying a bias of -11.5%.
• The F-shunt significantly correlated with Q˙/Q˙ (ICC 0.88), with lower bias (-1.97) and narrower limits of agreement in Bland-Altman analysis.
• Both positioning and blood pressure had a significant impact on the extraction of oxygen.
• The regression formula between calculated mixed venous oxygen tension and jugular venous oxygen tension was determined as Y = 0.80X + 2.61 (where X represents the jugular venous oxygen tension and Y the calculated mixed venous oxygen tension).

Conclusion

The study concludes that the F-shunt is a reasonable method for estimating venous admixture. However, the process can be potentially improved with simple algorithms, thereby eliminating any need for pulmonary arterial catheterization. These algorithms employ blood pressure- and positioning-dependent oxygen extraction methods and regression analysis between jugular venous and pulmonary arterial oxygen tension. Yet, the validity of these algorithms necessitates further studies.