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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Effects of human pregnancy on the ventilatory chemoreflex response to carbon dioxide

¹School of Physical and Health Education; Departments of ²Physiology, ³Obstetrics and Gynaecology, and ⁴Medicine; ⁵Division of Respirology and Critical Care, Queen's University, Kingston General Hospital, Kingston, Ontario, Canada

Submitted 23 December 2004 ; accepted in final form 24 January 2005

This study examined the effects of human pregnancy on the central chemoreflex control of breathing. Subjects were two groups (n = 11) of pregnant subjects (PG, gestational age, 36.5 ± 0.4 wk) and nonpregnant control subjects (CG), equated for mean age, body height, prepregnant body mass, parity, and aerobic fitness. All subjects performed a hyperoxic CO₂ rebreathing procedure, which includes prior hyperventilation and maintenance of iso-oxia. Resting blood gases and plasma progesterone and estradiol concentrations were measured. During rebreathing trials, end-tidal PCO₂ increased, whereas end-tidal PO₂ was maintained at a constant hyperoxic level. The point at which ventilation (E) began to rise as end-tidal PCO₂ increased was identified as the central chemoreflex ventilatory recruitment threshold for CO₂ (VRTCO₂). E levels below (basal E) and above (central chemoreflex sensitivity) the VRTCO₂ were determined. The VRTCO₂ was significantly lower in the PG vs. CG (40.5 ± 0.8 vs. 45.8 ± 1.6 Torr), and both basal E (14.8 ± 1.1 vs. 9.3 ± 1.6 l/min) and central chemoreflex sensitivity (5.07 ± 0.74 vs. 3.16 ± 0.29 l·min^–1·Torr^–1) were significantly higher in the PG vs. CG. Pooled data from the two groups showed significant correlations for resting arterial PCO₂ with basal E, central chemoreflex sensitivity, and the VRTCO₂. The VRTCO₂ was also correlated with progesterone and estradiol concentrations. These data support the hypothesis that pregnancy decreases the threshold and increases the sensitivity of the central chemoreflex response to CO₂. These changes may be due to the effects of gestational hormones on chemoreflex and/or nonchemoreflex drives to breathe.

human gestation; hyperoxia; chemoreflex sensitivity; ventilatory recruitment threshold