Effect of prior O-2 breathing on ventilatory response to sustained isocapnic hypoxia in adult humans

Sixteen healthy volunteers breathed 100% O(2) or room air for 10 min in random order, then their ventilatory response to sustained normocapnic hypoxia (80% arterial O(2) saturation, as measured with a pulse oximeter) was studied for 20 min. In addition, to detect agents possibly responsible for the respiratory changes, blood plasma of 10 of the 16 subjects was chemically analyzed. 1) Preliminary O(2) breathing uniformly and substantially augmented hypoxic ventilatory responses. 2) However, the profile of ventilatory response in terms of relative magnitude, i.e., biphasic hypoxic ventilatory depression, remained nearly unchanged. 3) Augmented ventilatory increment by prior O(2) breathing was significantly correlated with increment in the plasma glutamine level. We conclude that preliminary O(2) administration enhances hypoxic ventilatory response without affecting the biphasic response pattern and speculate that the excitatory amino acid neurotransmitter glutamate, possibly derived from augmented glutamine, may, at least in part, play a role in this ventilatory enhancement.