The effect of anemia on the ventilatory response to transient and steady state hypoxia

The effects of anemia upon the ventilatory responses to transient and steady state hypoxia were studied in unanesthetized goats. Responses to transient hypoxia (inhalation of several breaths of nitrogen) were considered to reflect peripheral chemoreceptor mediated reflexes, whereas responses to steady state hypoxia were considered to reflect both the chemoreceptor and nonchemoreceptor influences of hypoxia upon ventilatory control. In all goats, severe anemia (hemoglobin 3.1 to 4.8 g/100 ml) markedly heightened the responses to transient hypoxia (from a mean of 0.27 to a mean of 0.75 liter/min/percentage fall in SaO₂). This phenomenon was substantially reversed by alpha adrenergic blockade (phenoxybenzamine, 5 mg/kg). In contrast, the ventilatory responses to steady state hypoxia were unaffected by severe anemia. These data suggest that severe anemia enhances the peripheral chemoreceptor mediated response to hypoxia through a mechanism involving the alpha adrenergic system. It also appears that a ventilatory depressant effect of hypoxia which is not mediated by the peripheral chemoreceptor is also enhanced by severe anemia, thereby preventing an increase in the steady state ventilatory response to hypoxia. Finally, experiments involving variation in oxygen affinity of hemoglobin suggested that O₂ tension rather then O₂ availability in arterial blood is the major determinant of peripheral chemoreceptor activity.