Arterial oxygen tension during sleep in the third trimester of pregnancy

Changes during pregnancy include reduced functional residual capacity (FRC) and residual volume (RV), increased alveolar difference for oxygen and, in the supine position, reduced cardiac output. In conjunction with sleep-related disturbances, these changes could lead to maternal oxygen desaturation during sleep.

Because of conflicting data from respiratory sleep studies in pregnancy, we performed complete polysomnography on 21 pregnant women at the 36th week of gestation and again postpartum. We also measured the partial pressure of oxygen in the arterial blood (PaO2) in the supine and sitting positions.

We tested 21 healthy pregnant women at the 36th week of gestation. Arterial samples were taken in the sitting position. Complete polysomnography was performed in all of the pregnant women. Before the polysomnography arterial samples were taken in the supine and sitting positions and then every 2 h until termination of the study.

We did not find any correlation between SaO2/PaO2 levels and apnea, hypopnea or percent of rapid eye movement (REM) sleep. The frequency of apnea and hypopnea was significantly lower during pregnancy (5.81 +/- 2.1 apneas or hypopneas per hour of sleep) than postpartum (12.1 +/- 2.7 apneas or hypopneas per hour of sleep) (p < 0.001), which may be due to the raised level of progesterone. The PaO2 levels in the supine position were significantly lower than in the sitting position at 36 weeks of gestation (p < 0.001). No differences were found between PaO2 levels in the sitting and supine positions postpartum (p < 0.5).

According to our results we conclude that 1) the frequency of apnea and hypopnea in pregnancy was significantly lower than postpartum, and 2) a significant difference in PaO2 levels in the sitting and supine positions was observed at 36 weeks of gestation.