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

Abstract

A B S T R A C T 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 3.1-4.8 g/100 ml) markedly heightened the responses to transient hypoxia (from a mean of 0.27 to a mean of 0.75 liter/ min/percent 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 chemoreceptors 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 O tension rather than 02 availability in arterial blood is the major determinant of peripheral chemoreceptor activity.

Mechanism of the ventilatory response to carbon monoxide.

“…Similar findings have been recorded by Lloyd and co-workers (16), who reported no ventilatory stimulation after administration of single breaths of carbon monoxide to human volunteers. We and others (8,17,18) Paintal (6) and by Mills and Edwards (7), reported increases in chemoreceptor nerve fiber discharge after inhalation of carbon monoxide. In both of these studies, however, the authors reported a time of onset of per- ipheral chemoreceptor responses of several minutes.…”

“…A saturation for each period of inhalation of the test gas. The rationale and details of this method have been previously described (8).…”

“…In a recent study of a related problem, the ventilatory response to hypoxia in severe anemia (8), we pointed out the complexities inherent in experiments of this nature. Our data indicated that while diminished 02 content of arterial blood did not seem to be a ventilatory stimulus per se, it could substantially alter both peripheral chemoreceptor-and central nervous system-mediated responses to hypoxic hypoxia.…”

Hypoxic ventilatory response in subjects with normal and high oxygen affinity hemoglobins.

“…In addition to the difficulties inherent in the model (14), factors related to phlebotomy are known to influence chemoreceptor function and could affect the results obtained. These include sympathetic nervous activity (1), baroreceptor/chemoreceptor interactions (15), catecholamine effects (16), and possibly altered local blood flow (17).…”

“…A satisfactory model was more closely approximated in the study of goats with varying P50s (14). Unfortunately, the animals were made anemic, thus adding to the objections above those of iatrogenic alterations of oxygen delivery such as decreased end-capillary Po2 (18), altered cardiac output, and a possible shift towards anaerobic metabolism in low-P50 animals (19).…”