It is important to ensure adequate ventilation in horses undergoing surgery in dorsal recumbency. Researchers compared the use of oxygen ventilation with a mixture of oxygen (30%) and helium (70%)— heliox—in horses under isoflurane anesthesia [Hopster K.; Duffee, L.R.; Hopster-Iversen, C.C.S.; Driessen, B. Efficacy of alveolar recruitment maneuver for improving gas exchange and pulmonary mechanics in anesthetized horses ventilated with oxygen or a helium-oxygen mixture. AJVR Oct 2018, vol. 79, no. 10; pp. 1021-1027].
The authors reported that it is not unusual for a horse in dorsal recumbency to develop hypoxemia in spite of ample oxygen-rich gas. They noted that mismatch of ventilation-perfusion along with atelectasis formation in dependent areas of the lung results in right-to-left shunting of blood. Considering that helium is one-seventh as dense as air, when added to inspired gas it reduces turbulent flow and airway resistance to improve ventilation and gas exchange. Its low density compared to air or oxygen allows it to move easily through narrow airways to reach the alveoli for diffusion. In addition, a horse experiences less breathing effort with less airway resistance, and carbon dioxide eliminates rapidly.
This study examined six horses in dorsal recumbency undergoing isoflurane anesthesia with mechanical ventilation. The horses ventilated with heliox experienced improved gas exchange, less airway resistance and less alveolar collapse than horses ventilated with pure oxygen.