Effect of upper airway carbon dioxide on ventilation and blood gases in the awake pony

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Abstract
CO2 concentrations were increased during expiration in the upper 1/2 of the trachea, pharynx and nasal sinuses to determine if elevation of upper airway CO2 would alter breathing or arterial blood gases in the awake pony. CO2 (100%) was injected into the midcervical trachea via a chronically implanted transcutaneous cannula during the 1st part of the animal''s expiration. This maneuver elevated upper airway expiratory CO2 concentrations but prevented any exogenous CO2 from entering the lung and being absorbed into the arterial blood. Twelve experiments were performed on 6 ponies in which upper airway CO2 was elevated 2, 4 and 6% above the normal expired CO2 concentrations. Tidal volume increased in a dose dependent manner during upper airway CO2 exposure, but total ventilation was unchanged from base-line measurements made while the animal breathed room air. Arterial PO2 [partial pressure of O2] also increased during upper airway CO2 administration, reaching a mean value 6 torr (1 torr = 133.322 Pa [pascal]) greater than the base-line values at the + 6% CO2 exposure. Upper airway CO2 exposure alters breathing pattern slightly (increases tidal volume) and increases arterial PO2 in the awake pony.

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