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 (SaO₂, p < 0.01) and oxygen content (CaO₂, p < 0.01) were significantly higher, while venous admixture ($\dot{\text{Q}}$s/$\dot{\text{Q}}$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, PaO₂ and SaO₂ remained higher and $\dot{\text{Q}}$s/$\dot{\text{Q}}$t lower compared to the inverse order. Independent of the order, in the HD position $\dot{\text{Q}}$s/$\dot{\text{Q}}$t (p = 0.019) increased while PaO₂ (p < 0.01), SaO₂ (p = 0.011), CaO₂ (p < 0.01), venous PO₂ (P$\overline{\text{v}}$O₂; 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.