PROTECTIVE VENTILATORY RESPONSES TO HYPOXIA

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The protective response to hypoxia is organized in two layers. With initial exposure, eupneic ventilation increase. In severe hypoxia, eupnea is replaced by gasping, which promotes "autoresuscitation." The eupneic ventilatory response to hypoxia changes with development. In the newborn, ventilation rises and then falls. With age, the hypoxia- induced augmentation becomes more sustained. We hypothesize that hypoxia- induced ventilatory depression results from activation of a mesencephalic- pontine "central oxygen detector." Piglets will be studied. We will identify the region containing the "central oxygen detector." Hypoxia-induced depressions of ventilation will be attenuated if neurons of this central oxygen are destroyed. Neuronal activities of this detector will be characterized. These activities are hypothesized to increased during depressions of phrenic activity in hypoxia or during localized hypoxia by applications of sodium cyanide. We will then examine the role of neurons in the ventral medulla in hypoxia-induced ventilatory depressions and in the neurogenesis of gasping. These ventral medullary neuronal activities may provide a generalized tonic input for ventilatory activity. Yet gasping will not be altered since the mechanisms underlying the neurogenesis of eupnea and gasping differ fundamentally. In unanesthetized piglets, ventilatory activity will be recorded during wakefulness and sleep. We hypothesize that apneic episodes will be recorded following ablation of neurons in the medullary gasping center and those in the ventral medulla. Our results will have profound implications as to the mechanisms of apnea and the sudden infant death syndrome. Mechanisms of central apnea may be elucidated by our studies of the "direct" influence of hypoxia on the brainstem ventilatory control system, of the role of ventral medullary mechanisms in this hypoxia-induced depression and the role of medullary gasping mechanisms in the control of eupnea.