Chemical and mechanical adaptations of the respiratory system at rest and during exercise in human pregnancy

Human pregnancy is characterized by significant increases in ventilatory drive both at rest and during exercise. The increased ventilation and attendant hypocapnia of pregnancy has been attributed primarily to the stimulatory effects of female sex hormones (progesterone and estrogen) on central and peripheral chemoreflex drives to breathe. However, recent research from our laboratory suggests that hormone-mediated increases in neural (or non-chemoreflex) drives to breathe may contribute importantly to the hyperventilation of pregnancy. This review challenges traditional views of ventilatory control, and outlines an alternative hypothesis of the control of breathing during human pregnancy that is currently being tested in our laboratory. Conventional wisdom suggests that pregnancy-induced increases in central respiratory motor output command in combination with progressive thoraco–abdominal distortion may compromise the normal mechanical response of the respiratory system to exercise, increase the perception of exertional breathlessness, and curtail aerobic exercise performance in otherwise healthy pregnant women. The majority of available evidence suggests, however, that neither pregnancy nor advancing gestation are associated with reduced aerobic working capacity or increased breathlessness at any given work rate or ventilation during exhaustive weight-supported exercise.

Aerobic working capacity of Eskimos

The aerobic working capacity was measured in eight young Eskimos. Their maximal oxygen uptake averaged 2.6 liter/min. This is considerably lower than that found in young Caucasian men. The heart rate-oxygen uptake relationship also indicates a lower physiological capacity of the Eskimos. Expressing maximal oxygen uptake on a body weight basis brings about substantial agreement with results of experiments on sedentary-living Scandinavian students. The heart rate taken at the end of the maximal work averaged 173 min for the Eskimos which is about 10% lower than found in Caucasian men, indicating that the work test (bicycling) did not activate the circulatory system of the Eskimos maximally. By assuming a linear correlation between heart rate and O2 consumption up to its maximal value, the maximal O2 uptake would have to be 20% greater to give a maximal heart rate similar to that observed in Caucasian men. The pulmonary ventilation efficiency during exercise was found to be lower in Eskimos than in Caucasian men. Submitted on February 7, 1963