This study was designed to test 1) suitability of the rat as a model of the biphasic ventilatory response to sustained hypoxia and 2) the role of hypoxia-induced increase in end-expiratory lung volume (EELV) in the decline of ventilation during this response. Ventilatory responses to three hypoxic levels (16, 12, 10 % O2 in N2) together with changes of EELV were tested in 10 male rats (Wistar strain) anesthetized by Thiopental (50 mg/kg i.p.). Tidal volume (VT), frequency of respiration (f) and minute ventilation (V ˙ E) were measured for 35 minutes by the plethysmographic method. The hypoxic mixtures were administred between the 10th and 25th min. EELV was measured before and at the 10th min. of hypoxia. Biphasic ventilatory response was seen in both 12 and 10% O2 in N2. During breathing 12% (10%) O2 the ventilation initially increased to 220 % (247 %) and than declined to 196 % (221 %) of its control value. The increase in EELV correlated with decreasing O2 concentration in the inspired gas mixture. Individual changes of ventilation during its secondary decline did not correlate with the hypoxia-induced EELV enlargements. The results indicate that the rat is an appropriate model for studies of biphasic ventilatory response to sustained hypoxia.

        Key words: hypoxia, biphasic ventilatory response, rat