Interaction of hypoxic and hypercapnic stimuli on breathing pattern in the newborn rat

We aimed to investigate whether newborn rats respond to acute hypoxia with a biphasic pattern as other newborn species, the characteristics of their ventilatory response to hypercapnia, and the ventilatory response to combined hypoxic and hypercapnic stimuli. First, we established that newborn unanesthetized rats (2-4 days old) exposed to 10% O2 respond as other species. Their ventilation (VE), measured by flow plethysmography, immediately increased by 30%, then dropped and remained around normoxic values within 5 min. The drop was due to a decrease in tidal volume, while frequency remained elevated. Hence, alveolar ventilation was about 10% below normoxic value. At the same time O2 consumption, measured manometrically, dropped (-23%), possibly indicating a mechanism to protect vital organs. Ten percent CO2 in O2 breathing determined a substantial increase in VE (+47%), indicating that the respiratory pump is capable of a marked sustained hyperventilation. When CO2 was added to the hypoxic mixture, VE increased by about 85%, significantly more than without the concurrent hypoxic stimulus. Thus, even during the drop in VE of the biphasic response to hypoxia, the respiratory control system can respond with excitation to a further increase in chemical drive. Analysis of the breathing patterns suggests that in the newborn rat in hypoxia the inspiratory drive is decreased but the inspiratory on-switch mechanism is stimulated, hypercapnia increases ventilation mainly through an increase in respiratory drive, and moderate asphyxia induces the most powerful ventilatory response by combining the stimulatory action of hypercapnia and hypoxia.