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OBJECTIVE: We sought to study the effects of gestational age and maternal position on peak expiratory flow rates.

METHODS: Peak expiratory flow rates were measured in the standing, sitting, and supine positions in 38 healthy pregnant women at 4-week intervals starting at less than 10 weeks until delivery and again at 6 weeks postpartum. The highest reading of 3 consecutive peak expiratory flow rate measurements for each encounter and position was used in the analysis. Repeated measures analysis of covariance was performed with subjects, gestational age, position, and gestational age times position as the model effects. Least squares mean peak expiratory flow rates were compared among positions at different gestation ages using Bonferroni-adjusted least significant difference t tests.

RESULTS: Peak expiratory flow rate declined significantly throughout gestation in all positions (P < .001) with mean rate of decline of 0.65 L/min per week). The slopes of linear trends were not statistically different between positions (P = .222). However, the rate of decline for the supine position was higher than for standing and sitting positions (0.86 compared with 0.46 and 0.57 L/min per week), respectively. On average, the postpartum peak expiratory flow rate returned to 71.9% of its measurement in early gestation. Nomograms depicting mean and the 5th and 95th percentiles of peak expiratory flow rates were constructed for each position.

CONCLUSION: Peak expiratory flow rate measurements are affected by maternal position and advancing gestational age, especially in the supine position. Adjustment of patient's flow rate in relation to gestational age and maternal position is recommended, especially in pregnant women with asthma.

LEVEL OF EVIDENCE: III

(C) 2005 The American College of Obstetricians and Gynecologists