An otherwise healthy 31-year-old woman, G3P2, presents to the obstetrics clinic for a scheduled evaluation at 18 weeks gestation. She mentions occasionally feeling "out of breath" even when resting. She has no history of asthma or cardiac disease. Her vital signs are within normal limits, with a respiratory rate of 15/min and oxygen saturation of 99% on room air. Physical examination of the chest and heart is unremarkable. An arterial blood gas would most likely reveal a mild, compensated respiratory alkalosis. Which of the following normal physiologic changes of pregnancy is primarily responsible for this patient's expected acid-base status?
- Increased tidal volumeAnswer
- BIncreased respiratory rate
- CDecreased functional residual capacity
- DIncreased pulmonary vascular resistance
- EDecreased vital capacity
Answer
Increased tidal volume
During pregnancy, elevated levels of circulating progesterone directly stimulate the medullary respiratory centers. This stimulation increases minute ventilation primarily through a 30% to 50% increase in tidal volume. The respiratory rate, however, remains essentially unchanged. This progesterone-induced hyperventilation leads to an expected, mild chronic respiratory alkalosis. This physiological adaptation is beneficial as it creates a favorable concentration gradient that facilitates the transfer of carbon dioxide from the fetal circulation to the maternal circulation.
Step-by-Step Solution
Key Concept
Normal pregnancy is characterized by a progesterone-mediated increase in minute ventilation, which is driven by an increased tidal volume with an unchanged respiratory rate, resulting in a mild chronic respiratory alkalosis.