Vinayagam, D; Thilaganathan, B; Stirrup, O; Mantovani, E; Khalil, A; (2017) Maternal Hemodynamics in Normal Pregnancies: Reference ranges and the Role of Maternal Characteristics. Ultrasound in Obstetrics & Gynecology 10.1002/uog.17504.

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Abstract

Objectives The main aim of this study was to construct reference ranges of the maternal central hemodynamic parameters during pregnancy. The second aim was to determine the maternal and pregnancy characteristics, which influence these hemodynamic parameters. Methods This was a prospective cohort study of low-risk pregnant women attending for routine antenatal care at St George's Hospital, London. The exclusion criteria included any medical disorder present at the time of study recruitment, or development of hypertension or intrauterine fetal growth restriction following study recruitment. Stroke volume (SV), cardiac output (CO) and systemic vascular resistance (SVR) were obtained using non-invasive cardiac output monitoring (USCOM-1A®). USCOM-1A® utilises a non-imaging probe in the suprasternal notch to obtain velocity-time integrals of transaortic blood flow at the left ventricular outflow tract. Once the distribution of the data had been determined with respect to the gestational age (GA), maternal characteristics were added to the model to test whether they provided a significant improvement in the prediction of the median value. Results The analysis included 627 singleton pregnancies. The estimated median CO was constant for a maternal age above 32 years, but was around 0.5 L/min higher for women aged 25 or younger (p < 0.001). Maternal weight (p < 0.001) and height (p < 0.001) affected CO and there was a significant interaction (p = 0.002). In women with a height less than 1.60 m, there was no association between median CO and weight. In those with a height exceeding 1.60 m, an increase in weight was associated with an increase in CO. SV was primarily associated with height (p < 0.001), although some positive association with weight (p < 0.001) can also be observed within the normal body mass index range. Greater height (p < 0.001) was associated with lower median values of SVR with an estimated difference of around 120 dynes · sec · cm5 between 1.60 m and 1.80 m. Advancing maternal age was associated with higher median SVR with an estimated difference of around 50 dynes · sec · cm5 between 25 and 35 years. Smokers had a lower SVR of 73.5 (95% CI, 8.6 – 138.4) dynes · sec · cm5. Conclusion Maternal hemodynamics are significantly influenced by maternal age, height and weight. We provide USCOM-1A®-specific reference ranges and calculator for SV, CO and SVR in uncomplicated pregnancies that correct for maternal age, height and weight. This will enable clinical application and comparison in both uncomplicated and pathological pregnancies.

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