The respiratory response of the new-born lamb to inhaled CO2with and without accompanying hypoxia*

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Abstract
The respiratory response to inhaled CO2 was measured in 20 unanaesthetized new-born lambs aged 4 hr-10 days. Measurement of resting arterial pH, PCO2 and plasma bicarbonate showed a non-respiratory acidosis immediately after birth which was corrected in the first 24-28 hr.: thereafter, the acid-base pattern was of a compensated respiratory alkalosis. When CO2 was added to the inspired gases and resting arterial oxygen tension (Pa,O2) was controlled the average increase In minute ventilation (V)was 0.0751. min.-1. kg.mm Hg, Pa CO2 -1 [arterial CO2 pressure] and duplicate responses in the same lamb differed by 6-22.5%. The slope of the V/Pa,CO2 line (S) varied inversely with Pa, O2. In one lamb, severe hypoxia (Pa O2 = 21 mm Hg) caused a marked depression of the slope. Neither the slope S nor the horizontal intercept B of the lines was related to the age of the lamb. B was not related to pHa and only slightly affected by acute hypoxia. B was related to arterial [HCO3-] and values for both were reduced with the acid-base disturbances seen in the first 10 days after birth. Evidence was given which suggested that the response of the new-born lamb to inhaled CO2 was similar to that of man acclimatized to a Pa,O2 of 70-75 mm Hg. In the lightly anaesthetized lamb, bilateral section of the sinus nerves caused a small reduction in the sensitivity to inhaled 5% CO2 in air, an increase in the respiratory lag and a reduction in the rate at which V increased. It was concluded that, in the new-born lamb, the carotid chemoreceptors are involved in the response to inhaled CO2 and that hypoxia potentiates this response.