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_t jugular), 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_t jugular was found [intraclass correlation coefficient (ICC) ¼ 0.68, p < 0.001], and BlandeAltman analysis showed a bias of e11.5% and wide limits of agreement (e27.7% to 4.6%). Fshunt significantly correlated with Q_s=Q_t (ICC ¼ 0.88, p < 0.001), and BlandeAltman analysis showed a lower bias (e1.97) and narrower limits of agreement (e13.8% to 9.9%). Positioning and blood pressure significantly influenced oxygen extraction. The regression formula was Y ¼ 0.80X þ 2.61 (where Y is the calculated mixed venous oxygen tension and X is the jugular venous oxygen tension) when outliers were excluded (ICC¼0.82, 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