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

OBJECTIVE: The aim of this study was to compare different methods to determine venous admixture (Q˙s /Q˙t ) in anaesthetized horses. The first objective was to estimate Q˙s /Q˙t using jugular venous blood oxygen content (Q˙s /Q˙tjugular ), and a fixed value for the oxygen extraction (F-shunt). The second objective was to assess the influence of blood pressure and positioning on oxygen extraction. The third objective was to perform regression analysis between jugular and mixed venous blood oxygen tensions.

STUDY DESIGN: Prospective, experimental trial.

ANIMALS: The study was performed with seven warmblood horses that were anaesthetized with detomidine, butorphanol, ketamine, diazepam and isoflurane in oxygen.

METHODS: Multiple simultaneous arterial, jugular venous and pulmonary arterial blood samples were taken under normotensive and hypotensive conditions in lateral and dorsal recumbency. Arterial, mixed venous, and end-capillary oxygen content were calculated.

RESULTS: A significant correlation between Q˙s /Q˙t and Q˙s /Q˙tjugular was found [intraclass correlation coefficient (ICC) = 0.68, p < 0.001], and Bland-Altman analysis showed a bias of -11.5% and wide limits of agreement (-27.7% to 4.6%). F-shunt significantly correlated with Q˙s /Q˙t (ICC = 0.88, p < 0.001), and Bland-Altman analysis showed a lower bias (-1.97) and narrower limits of agreement (-13.8% to 9.9%). Positioning and blood pressure significantly influenced oxygen extraction. The regression formula was Y = 0.80X + 2.61 (where Y is the calculated mixed venous oxygen tension and X is the jugular venous oxygen tension) when outliers were excluded (ICC=0.82, p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE: This study shows that F-shunt provides reasonable estimates of Q˙s /Q˙t but can possibly be improved by using simple algorithms without the need for pulmonary arterial catheterization. These algorithms use blood pressure- and positioning-dependent oxygen extraction and regression analysis between jugular venous and pulmonary arterial oxygen tension. Although promising, the validity of these algorithms needs to be determined in future studies.