Comparison of helium dilution and plethysmographic lung volumes in pregnant women

Abstract

The multibreath helium equilibration method is the technique recommended for routine measurement of static lung volumes in normal subjects. However, pregnancy could be an exception to this general rule, due to airway closure during the second half of gestation. The aim of this study was to compare the measurements of lung volumes by plethysmography and helium dilution during pregnancy.Twenty three healthy women were studied at 12, 24 and 36 weeks of pregnancy, and 4 months postpartum. Total lung capacity (TLC), functional residual capacity (FRC) and residual volume (RV) were measured by multibreath helium equilibration (TLCHe, FRCHe and RVHe) and by plethysmography (TLCbox, FRCbox and RVbox).Only at 36 weeks were there differences between the two methods. RVbox was significantly larger than RVHe We conclude that using the multibreath helium equilibration method to measure lung volumes in at-term pregnant women results in underestimation of functional residual capacity and total lung capacity.

Pre‐oxygenation in pregnancy: an investigation using physiological modelling

“…2). However, when the number of breaths required to achieve the same P É O 2 as after 95% complete tidal pre-oxygenation was analysed, seven breaths were required [range [5][6][7][8][9][10] in pregnant subjects, compared to six breaths [range 4-9] in nonpregnant subjects. Vital capacity breathing accelerated pre-oxygenation to produce a similar rate in pregnant and non-pregnant women.…”

“…Firstly, an 'average' pregnant and an 'average' non-pregnant subject were generated from published mean physiological values [4][5][6][7][8][9]. An additional four subjects, predicted to undergo slow or rapid equilibration during pre-oxygenation were then created (Table 1), with baseline physiological parameters from within the ranges found in published studies.…”

“…Pre-oxygenation during tidal breathing proceeded more rapidly in pregnancy, the median [range] time to achieve 95% of the maximum change in P É O 2 being 1 min 37 s [1:23-1:52] in pregnant subjects, compared to 2 min 51 s [2:28-3:15] in non-pregnant subjects. Vital capacity pre-oxygenation required seven breaths [5][6][7][8][9][10] in pregnant subjects, compared to six breaths [4][5][6][7][8][9] in non-pregnant subjects, to achieve the same P É O 2 as after 95% complete tidal pre-oxygenation. We recommend 2 min of tidal breathing for pre-oxygenation in pregnancy.…”

“…Pregnancy is associated with considerable changes to the cardiovascular and respiratory systems [4][5][6][7][8][9]. Reduced functional residual capacity, coupled with increased minute ventilation, predicts more rapid preoxygenation.…”

Apnoea in pregnancy: an investigation using physiological modelling

“…Significant changes to the cardiovascular and respiratory systems occur during pregnancy [2][3][4][5][6][7], and these predict differing effects on the physiology of preoxygenation and apnoea. During apnoea, reduced functional residual capacity, together with increased oxygen consumption, predicts quicker onset of hypoxaemia and desaturation.…”