Impact of Trendelenburg (head down) and reverse Trendelenburg (head up) position on respiratory and cardiovascular function in anaesthetized horses.

Objective To describe the cardiorespiratory effects of a change in table position in anaesthetized horses. Study design Prospective, crossover, randomized, experimental study. Animals Six adult horses (mean body weight 621 +/- 59 kg, aged 13 +/- 4 years). Methods The horses were anaesthetized twice in dorsal recumbency. They were either placed in the Trendelenburg position (head down; HD) followed by reverse Trendelenburg position (head up; HU) or in reverse order. Every position was maintained for 90 minutes. The order of positions was randomly assigned at initial anaesthesia. Extensive cardiorespiratory monitoring was performed. Statistical analysis consisted of a mixed model with horses as random effect and time, position, section of anaesthesia and interaction between those as fixed effects (p < 0.05). Results When HU was applied during the first section of anaesthesia, PaO2, (p = 0.012), oxygen saturation (SaO(2), p < 0.01) and oxygen content (CaO2, p < 0.01) were significantly higher, while venous admixture (Qs/Qt, p < 0.01), mean arterial (p = 0.039), right atrial (p < 0.01) and mean pulmonary arterial pressure (p < 0.01) were lower than in HD. After changing from HU to HD, PaO2 and SaO(2) remained higher and Qs/Qt lower compared to the inverse order. Independent of the order, in the HD position Qs/Qt (p = 0.019) increased while PaO2 (p < 0.01), SaO(2) (p = 0.011), CaO2 (p < 0.01), venous PO2 (PvO(2); p = 0.019), venous saturation (p = 0.004) and venous oxygen content (p = 0.010) decreased over time. No significant differences were found for cardiac output, oxygen delivery, oxygen consumption and dobut-amine requirement between the two positions. Conclusions and clinical relevance Gas exchange is better preserved in HU compared to HD, especially if applied from the start of the anaesthesia.