DEPENDENCE OF MIDDLE-EAR GAS-COMPOSITION ON PULMONARY VENTILATION

The middle ear (ME) steady state gas composition resembles that of mix ed venous blood. We changed arterial and venous blood gases by artific ially ventilating anesthetized guinea pigs and measured simultaneous M E gas changes during spontaneous breathing, hyperventilation, and hypo ventilation. During hyperventilation, PaCO2 and PvCO2 (a = arterial, v = venous) decreased from 46.0 and 53.0 mm Hg to 17.9 and 37.5 mm Hg, respectively, while PaO2 and PvO2 (85.6 and 38.2 mm Hg) did not change . This was accompanied by an ME PCO2 decrease from 70.4 to 58.8 mm Hg and a PO2 decrease from 36.8 to 25.4 mm Hg. During hypoventilation, Pa CO2 and PvCO2 increased to 56.8 and 66.4 mm Hg, while PvO2 decreased t o 21.8 mm Hg. The ME PCO2 increased simultaneously to 88.8 mm Hg and t he ME PO2 decreased to 25.4 mm Hg. The ME PO2 decrease during hyperven tilation may be explained by a 33% decrease in ME mucosa perfusion, ca lculated from the ME ventilation-perfusion ratio. This study shows tha t ME gas composition follows fluctuations of blood gas levels and thus may affect total ME pressure.