Exponential and diphasic ventilatory response to hypoxia in conscious lambs

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Abstract
This study was undertaken to test the hypothesis that in the neonate the hypoxic chemoreflex drive adapts to steady-state hypoxia but not to progressive hypoxia. First we have compared the ventilatory (.ovrhdot.VE) response of 2-day-old conscious lambs to steady-state hypoxia with their response to progressive hypoxia. Second, we have quantified the chemoreceptor excitatory function operating at the end of each period of hypoxia by studying the immediate .ovrhdot.VE response to the withdrawal of the hypoxic stimulus. Lambs responded to steady-state hypoxia [fractional concentration of inspired O2 (FIO2) = 0.08] by a diphasic .ovrhdot.VE response but responded to progressive hypoxia (FIO2 0.21-0.08) by an exponential .ovrhdot.EV increase. Hyperventilation in steady-state hypoxia was transient; .ovrhdot.VE increased immediately from 532 to a mean peak response of 712 ml .cntdot. kg-1 .cntdot. min-1 and decreased to 595 ml .cntdot. kg-1 .cntdot. min-1 within 10 min. With progressive hypoxia, .ovrhdot.VE increased within 13 min from 514 to 705 ml .cntdot. kg-1 .cntdot. min-1. At the end of steady-state and progressive hypoxia the abrupt withdrawal of the hypoxic drive caused an instantaneous .ovrhdot.VE decrease to 390 and 399 ml .cntdot. kg-1 .cntdot. min-1, respectively; the .ovrhdot.VE decrease was respectively 306 and 205 ml .cntdot. kg-1 .cntdot. min-1 (P < 0.05). This demonstrates that during steady-state hypoxia the lambs had suffered a loss of one third of the chemoreceptor excitatory function. It is concluded that 1) the same newborn lamb is capable of a diphasic .ovrhdot.VE response to steady-state hypoxia and an exponential response to progressive hypoxia and 2) in the lamb the .ovrhdot.VE damping to steady-state hypoxia is largely due to a loss of the excitatory stimulation of the chemoreceptors presumably by adaptation of the chemoreceptors.

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