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, PaO, (p = 0.012), oxygen saturation (SaO, p < 0.01) and oxygen content (CaO, p < 0.01) were significantly higher, while venous admixture (Q˙s/Q˙t, 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, PaO and SaO remained higher and Q˙s/Q˙t lower compared to the inverse order. Independent of the order, in the HD position Q˙s/Q˙t (p = 0.019) increased while PaO (p < 0.01), SaO (p = 0.011), CaO (p < 0.01), venous PO (Pv¯O; 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 dobutamine 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.