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Abstract

A hypothetical, aetiological relationship between the horse's bit, nasopharyngeal asphyxia and negative pressure pulmonary oedema.

Abstract

Tentative explanations for the aetiology of soft palate instability and dorsal displacement at exercise are based on the proposition that it is caused by one or more, as yet unidentified, neuromuscular or inflammatory diseases. As judged by reviews, none of the treatments based on these assumptions are entirely satisfactory. Most researchers agree that their cause is unknown. In 1999, Cook proposed that they were caused by the bit breaking what should be an airtight lip seal. The present article describes a development of this hypothesis - loss of oral compartment subatmospheric pressure. The feral horse is an obligate nose-breather and runs with a closed mouth and no air in the oral cavity and oropharynx. Evacuation of air from these compartments is proposed as the primary mechanism, strengthened by the cohesive force of surface tension, whereby the soft palate resists the elevating force of negative pressure on inspiration. It is suggested that subatmospheric pressure in the oral compartment is generated by a single swallow before running. Poll extension supplements the rigidity of the nasopharyngeal tube and reinforces the ostium intrapharyngium seal. In the domesticated horse, the lip seal is broken by a bit and bit-induced poll flexion, jaw and tongue movement often breaks the ostium seal. Oral atmospheric pressure is proposed as the mechanism for nasopharyngeal asphyxia. It is considered that negative pressure pulmonary oedema - currently known as exercise-induced pulmonary haemorrhage and conventionally thought to be a separate problem - is one of its clinical signs. It is suggested that the bit is the most common cause of nasopharyngeal asphyxia and that this, in turn, results in a cascade of dynamic obstructions in the larynx and cervical trachea, leading ultimately to pulmonary oedema and 'bleeding.' It is concluded that these are sequelae to rules in many disciplines that mandate bit usage.