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

The study explores how changing a horse’s position during anaesthesia – specifically, moving its head either up or down – affects its cardiovascular and respiratory systems. The research reveals that horses have better oxygen absorption when their heads are elevated (reverse Trendelenburg position), especially if this position is used from the start of anaesthesia.

Research Objectives and Methods

• The primary purpose of this prospective, crossover, randomized, experimental study was to determine how the cardiovascular and respiratory systems respond when an anaesthetized horse’s position is changed.
• The research was carried out on six adult horses, with an average weight of around 621 kg and aged 13 ± 4 years.
• Each horse was anaesthetized twice in dorsal recumbency, lying flat on the back. They were positioned either head down (Trendelenburg position) or head up (reverse Trendelenburg position), with each position maintained for 90 minutes. The position order at initial anaesthesia was randomly assigned.
• The research team implemented extensive cardiorespiratory monitoring during the experiment. They used a mixed model for statistical analysis, considering time, position, section of anaesthesia, and interaction between these elements as fixed effects.

Research Findings

• The study found significant differences in several cardiorespiratory parameters when the horse was placed in a head-up position during the first section of anaesthesia. These include higher partial pressure of oxygen in arterial blood (PaO), oxygen saturation levels (SaO), and oxygen content (CaO). Conversely, parameters such as venous admixture (Q˙s/Q˙t), mean arterial, mean pulmonary arterial, and right atrial pressures were lower in head-up position compared to head down.
• Even when changing from a head-up to a head-down position, both PaO and SaO levels remained higher, whilst Q˙s/Q˙t was lower compared to when the process was reversed.
• Regardless of the order of positions, in the head-down position, Q˙s/Q˙t increased over time while PaO, SaO, CaO, venous partial pressure of oxygen, venous saturation, and venous oxygen content decreased.
• No significant differences were observed in cardiac output, oxygen delivery, oxygen consumption, and dobutamine requirement between the two positions.

Conclusions

• The results of this study suggest that maintaining an anaesthetized horse in a head-up position can result in improved gas exchange compared to a head-down position. This is particularly applicable if the head-up position is used from the beginning of the anaesthesia process.