Validation of non-invasive measurements of cardiac output: using whole body bio-impedance versus inert gas rebreathing in healthy women undergoing in vitro fertilisation

Background: Haemodynamic assessment in and before pregnancy is becoming increasingly important in relation to pregnancy complications and outcomes. Different methodologies exist but there is no gold-standard technique for non-invasive measurement of cardiac output (CO). We sought to assess two methods of CO measurement in healthy women undergoing In Vitro Fertilisation Cycles (IVF). This was a prospective longitudinal study of 71 women aged 18-44yrs planning IVF undergoing CO measurements obtained via Inert Gas rebreathing (IGR) using InnocorTM and whole-body bio-impedance (WBI) using NicasTM in order to assess the reproducibility between the methods. Four visits occurred at which both techniques were used: initial assessment, embryo transfer, day of pregnancy test and 4 weeks post transfer (regardless of whether conception occurred). Cross-sectional agreement of the methods was assessed using the calculation of bias, percentage error and Limits of Agreement (LOA) via the Bland-Altman analysis. Longitudinal agreement of the methods was assessed using a 4-quadrant plot with concordance rate, angular bias and radial limits of agreement (%). Results: 113 measurements from 44 participants were suitable for cross-sectional (Bland-Altman) analysis. IGR (InnocorTM) Mean CO was 4.61 L/min and 5.05 L/min with WBI (NicasTM). The bias was 0.44L/min. The percentage error was 76% and intra-correlation coefficient was 0.135 (95%CI -0.43 – 0.306). 59 measurements from 28 participants were suitable for longitudinal (4Q-plot) analysis. The concordance rate was 64.4 %, angular bias -0.14, radial limits of agreement +- 13.25°. Conclusions: There was poor cross-sectional and longitudinal agreement between inert gas rebreathing and whole-body bio- impedance techniques. These techniques cannot be used interchangeably when measuring CO in women undergoing IVF, and these results may be more generalizable