SOURCE OF RESPIRATORY DRIVE DURING PERIODIC BREATHING IN LAMBS

In order to investigate the mechanisms underlying periodic breathing ( PB), we studied the initiation of breathing after passive hyperventila tion in 14 anaesthetised 10-20 day old lambs. Eight of the lambs exhib ited PB following post-hyperventilation apnea (PHA), with an epoch dur ation of 82.4 +/- 14.2 sec (mean +/- SEM), a cycle duration of 9.7 +/- 0.7 sec and a ratio of ventilatory duration to apnea duration (V-A ra tio) of 1.24 +/- 0.32. The remaining lambs showed stable breathing pat terns following PHA. The ventilatory response to isocapnic hypoxia was significantly greater in the group that had PB (-7.2 +/- 1.0 ml min(- 1)%Sa(O2)(-1) kg(-1)) than in the animals that did not (-2.5 +/- 1.0 m l min(-1) %Sa(O2)(-1) kg(-1)). Using experimentally determined ventila tory response curves to O-2 and CO2 we calculated that the swings in S a(O2) and Pa-CO2 during PB generated chemical drive that accounted for only 16.2% of the ventilatory oscillations observed during PB. Much o f the remaining drive appeared to originate in the 'switch-on' charact eristics of the respiratory controller; in lambs that exhibited period ic breathing, when breathing began after PHA ventilation jumped abrupt ly from zero to 55.1% of the eupneic ventilation. The magnitude of thi s jump in ventilation accounted for 51.9% of the amplitude of ventilat ory oscillations that occur during PB. We speculate that this previous ly unrecognised feature of the respiratory controller, together with a n elevated sensitivity to hypoxaemia, play crucial roles in generating PB in the infant.