Enriched inspired C02 stimulates respiration despite hypocapnia in hyperthermic exercising sheep

Previously we have shown a negative correlation (R2-0.68) between rectal temperature (Tre) and arterial carbon dioxide tension (PaCO₂) during exercise in sheep. We hypothesized that progressive hypocapnia during exercise in sheep reflects a decreasing set point for PaCO₂ governed by mean body temperature, as opposed to failed adherence to a static set point. To test this hypothesis, we infused CO₂ at 3-4 L/min for 3 minutes into the tracheae of 4 sheep at 6, 14.5, and 23.5 min of a 30 min moderate intensity exercise bout. As controls, we infused air in the same manner during separate exercise trials, and both air and CO₂ at 1, 2, and 3 L/min into the sheep at rest. Tre and minute ventilation (VE) were measured continuously and PaCO₂ was measured at 1.5 and 3 min of CO₂ or air infusion and 5 min after the end of infusion. Air infusions caused no systematic changes in PaCO₂ or VE at rest or during exercise. CO₂ infusion at 1, 2, and 3 L/min at rest caused a variable increase in PaC02 and a proportionate increase in VE of 13±4, 19±2, and 29±6 L/min (n-4). CO₂ infusion during exercise resulted in a transient increase in PaCO₂, although often not to pre-exerclse levels, and an increase in VE despite hypocapnia. CO₂ infusion did not alter the slope of the negative relationship between Tre and PaCO₂ during exercise (p>0.1). The results support the hypothesis that the set point for PaCO₂ decreases as hyperthermia increases in exercising sheep.