A preliminary study on the role of the equine guttural pouches in selective brain cooling

The equine guttural pouch is a large, air-filled diverticulum of the auditory tube whose function is not clear. Since the horse does not possess a known, well-developed brain-cooling mechanism that could satisfy cerebral thermoregulatory demands, an hypothesis is proposed that respiratory air enters the guttural pouches, when needed, to ventilate and cool the internal carotid arteries (ICA). Experiments were initially carried out on nine cadavers, where blood flow was mimicked with warmed saline propelled by peristaltic pumps. Subsequent experiments were conducted on an anaesthetized horse where the guttural pouch was ventilated and ICA temperatures were measured. Results showed that whenever the guttural pouch was ventilated with cooled or warmed environmental air, or warmed 100% humidified air, temperatures within the ICA dropped significantly in cadavers (0.4-5 degrees C) and in the anaesthetized horse (1-3 degrees C), depending on conditions. Simulated respiration trials also resulted in ICA temperature drops of 0.9-2.3 degrees C in two of five cadavers tested, indicating that the wide 3-5 cm pharyngeal orifices of the guttural pouches have the capacity to allow enough respiratory air to ventilate the pouch. Despite the fact that a single, unbranching 13 cm portion of the ICA is exposed on the wall of each guttural pouch, the results of this investigation suggest that during heavy exercise, horses could utilize their guttural pouches to cool ICA blood destined for the brain.