Serial Physiological Changes DuringPregnancy

Abstract

0225 Only a few studies have examined the time course and mechanisms of increases in heart rate and breathing at rest and during exercise in early human pregnancy. This information is urgently needed to provide guidelines to safely prescribe and monitor physical activity throughout pregnancy. PURPOSE: To accurately describe the time sequence of changes in heart rate, ventilation and gas exchange at rest and during exercise in healthy pregnant women. METHODS: Serial testing was conducted on healthy, moderately-active women (n = 11) between the 7th and 20th weeks of pregnancy. Subjects were studied at rest and during steady state cycling at a work rate corresponding to approximately 60% of aerobic working capacity. Measurements included heart rate, breath-by-breath alveolar gas exchange, pH, and circulating levels of progesterone (a known respiratory stimulant). Changes between the 7th and 20th week of gestation were analyzed using a mixed-model linear regression procedure. Comparisons were also made with pre-conceptual data from the same subjects and with measurements in late gestation from earlier studies in this laboratory (J Appl Physiol 88:149, 2000; MSSE 34:S119, 2002). RESULTS: The findings support the hypothesis that the primary changes in autonomic control of heart rate occur prior to the 7th week, with gradual moderate increases thereafter. A partly compensated respiratory alkalosis is also established prior to the 7th week. Further increases in pulmonary ventilation appear to be driven primarily by increases in resting metabolic rate. Plasma progesterone reached only 21% of the level observed in late gestation by the 20th week. CONCLUSIONS: The most important changes in cardio-respiratory control are established prior to the 7th week of pregnancy in preparation to support fetal growth. Factors other than increased circulating progesterone should be investigated as potential causes of resting and exercise hyperpnea during pregnancy. Supported by Ontario Thoracic Society, OTS Block Term Grant Funding and NSERC.