Respiratory and circulatory effects of breathing 100% oxygen in the new-born lamb before and after denervation of the carotid chemoreceptors*

Abstract

Methods are described for measuring tidal volume and frequency, end-tidal CO2, blood pressure and heart rate, and arterial gas tensions in the unanaesthesized new-born lamb. The resting values of minute ventilation (V)/kg body wt. and arterial oxygen and carbon dioxide tension, (Pa, O2) and Pa, CO2) were similar to those which have been reported in the new-born baby. There was a direct and significant relation between Pa, O2 and Pa, CO2 and the age of the lamb. Thirty-five unanaesthetized lambs aged 40 mln. to 10 days breathed 100% oxygen; minute ventilation fell by an average of 19% of control, end-tidal CO2 increased and the ratio of change in tidal volume ([DELTA] Vf) to change in pressure ([DELTA] P) ([DELTA] Vt/[DELTA] P) remained constant. In a proportion of lambs, a small decrease in blood pressure and heart rate was observed. The effect of breathing 100% O2 on lung compliance was variable. These changes in ventilation were virtually abolished after both sinus nerves had been cut. The results therefore suggest that a significant hypoxic drive to ventilation exists in the new-born lamb and that this drive is mediated by functioning and mature peripheral chemoreceptors. Preliminary evidence suggested that, on 100% O2, the sensitivity of new-born lambs to inhaled CO2 was reduced.