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• W4297177527 abstract "Postpartum haemorrhage continues to be a leading cause of morbidity and mortality in the obstetric population worldwide, especially in patients at extremes of body weight. Quantification of blood loss has been considered extensively in the literature. However, these volumes must be contextualised to appreciate the consequences of blood loss for individual parturients. Knowledge of a patient's peripartum circulating blood volume is essential to allow accurate interpretation of the significance of haemorrhage and appropriate resuscitation. Greater body weight in obesity can lead to overestimation of blood volume, resulting in inappropriately high thresholds for blood product transfusion and delays in treatment. The most recent Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE-UK) surveillance report demonstrated the risk to this population, with more than half of all maternal mortality recorded in parturients who were either overweight or obese. Current linear calculations used to estimate circulating blood volumes based on patients’ weights could be contributing to this phenomenon, as blood volume increases at a disproportional rate to body composition. In this review, we summarise the relevant physiology and explore the existing literature on the estimation of circulating blood volume, both during pregnancy and in obesity. Building on key works and principal findings, we present a practical, nonlinear approach to the adjustment of estimated blood volume with increasing body mass. This clinical tool aims to reduce the clinical bias influencing the management of obstetric haemorrhage in a population already at increased risk of morbidity and mortality. Discussion of the limitations of this approach and the call for further research within this field completes this review. Postpartum haemorrhage continues to be a leading cause of morbidity and mortality in the obstetric population worldwide, especially in patients at extremes of body weight. Quantification of blood loss has been considered extensively in the literature. However, these volumes must be contextualised to appreciate the consequences of blood loss for individual parturients. Knowledge of a patient's peripartum circulating blood volume is essential to allow accurate interpretation of the significance of haemorrhage and appropriate resuscitation. Greater body weight in obesity can lead to overestimation of blood volume, resulting in inappropriately high thresholds for blood product transfusion and delays in treatment. The most recent Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE-UK) surveillance report demonstrated the risk to this population, with more than half of all maternal mortality recorded in parturients who were either overweight or obese. Current linear calculations used to estimate circulating blood volumes based on patients’ weights could be contributing to this phenomenon, as blood volume increases at a disproportional rate to body composition. In this review, we summarise the relevant physiology and explore the existing literature on the estimation of circulating blood volume, both during pregnancy and in obesity. Building on key works and principal findings, we present a practical, nonlinear approach to the adjustment of estimated blood volume with increasing body mass. This clinical tool aims to reduce the clinical bias influencing the management of obstetric haemorrhage in a population already at increased risk of morbidity and mortality. Discussion of the limitations of this approach and the call for further research within this field completes this review. Editor's key points•Guidelines for the management of peripartum haemorrhage often cite a linear formula for calculating maternal blood volume. However, this can generate spurious estimates in patients at extremes of body mass.•It is essential to appreciate the relationship between the nominator (blood loss) and the denominator (circulating blood volume) to quantify the significance of any bleeding.•Building on calculations of maternal blood volume with increasing body mass, this paper proposes a nonlinear approach to blood volume estimation.•Future guidance on the management of peripartum haemorrhage should incorporate a nonlinear model for maternal blood volume, which might be of particular benefit to parturients living with obesity. •Guidelines for the management of peripartum haemorrhage often cite a linear formula for calculating maternal blood volume. However, this can generate spurious estimates in patients at extremes of body mass.•It is essential to appreciate the relationship between the nominator (blood loss) and the denominator (circulating blood volume) to quantify the significance of any bleeding.•Building on calculations of maternal blood volume with increasing body mass, this paper proposes a nonlinear approach to blood volume estimation.•Future guidance on the management of peripartum haemorrhage should incorporate a nonlinear model for maternal blood volume, which might be of particular benefit to parturients living with obesity. Postpartum haemorrhage is a substantial and ongoing issue, affecting 24% of pregnancies in England in 2020–1.1NHS DigitalNHS maternity statistics, England 2020-21. The Department of Health and Social Care, 2021https://digital.nhs.uk/data-and-information/publications/statistical/nhs-maternity-statistics/2020-21Google Scholar As noted in the 2020 Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE-UK) report, there continue to be serious untoward outcomes relating to the underestimation of blood loss, especially in parturients of lower weights.2Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2016-18.2020https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2020/MBRRACE-UK_Maternal_Report_Dec_2020_v10_ONLINE_VERSION_1404.pdfGoogle Scholar Guidance arising from this report includes the recommendation to ‘ensure that the response to obstetric haemorrhage is tailored to the proportionate blood loss as a percentage of circulating volume based on the woman's body weight’.2Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2016-18.2020https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2020/MBRRACE-UK_Maternal_Report_Dec_2020_v10_ONLINE_VERSION_1404.pdfGoogle Scholar A key aspect in the management of obstetric haemorrhage is an accurate assessment of the volume of blood loss,3Hernandez J.S. Alexander J.M. Sarode R. McIntire D.D. Leveno K.J. Calculated blood loss in severe obstetric hemorrhage and its relation to body mass index.Am J Perinatol. 2012; 29: 557-560PubMed Google Scholar but this information is only half of the picture; knowledge of the patient's original circulating blood volume (CBV) is just as important. However, this value cannot be directly measured in clinical practice. Drawing upon values given in the Royal College of Obstetricians and Gynaecologists (RCOG) ‘green-top’ guideline on postpartum haemorrhage published in 2009,4Arulkumaran S. Mavrides E. Penney G.C. et al.Prevention and management of postpartum haemorrhage.2009http://www.rcog.org.uk/womens-health/clinical-guidance/prevention-and-management-postpartum-haemorrhage-green-top-52Google Scholar the recent MBRRACE-UK report advocates using a weight-based calculation volume of 100 ml kg−1 of body weight to approximate maternal CBV.2Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2016-18.2020https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2020/MBRRACE-UK_Maternal_Report_Dec_2020_v10_ONLINE_VERSION_1404.pdfGoogle Scholar Of note, these values were subsequently omitted in the 2016 update of the RCOG guideline, which instead recommends that the ‘clinical picture should be the main determinant of the need for blood transfusion’.5Mavrides E. Allard S. Chandraharan E. et al.Prevention and management of postpartum haemorrhage.2016Google Scholar One of the difficulties that may arise from arbitrarily using 100 ml kg−1 as the basis for calculating CBV and guiding therapeutic measures is the potential to create ‘cut-off values’ that may disproportionately influence patient management. For example, guidelines often use this approach to derive a value of 1.5 L blood loss (i.e. >20% of a ‘typical’ 70 kg patient's blood volume) to activate major haemorrhage pathways, as this will be sufficient for many patients. However, recent guidance acknowledges that arbitrary volumes for activation of protocols may be detrimental for women with a lower body mass.2Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2016-18.2020https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2020/MBRRACE-UK_Maternal_Report_Dec_2020_v10_ONLINE_VERSION_1404.pdfGoogle Scholar,5Mavrides E. Allard S. Chandraharan E. et al.Prevention and management of postpartum haemorrhage.2016Google Scholar Although the adoption of an individualised ‘per kilogram’ approach to blood volume estimation in parturients goes some way to addressing the risk of underestimating the significance of haemorrhage in those with lower body weights,2Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2016-18.2020https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2020/MBRRACE-UK_Maternal_Report_Dec_2020_v10_ONLINE_VERSION_1404.pdfGoogle Scholar,5Mavrides E. Allard S. Chandraharan E. et al.Prevention and management of postpartum haemorrhage.2016Google Scholar little attention is paid to how this method may affect those of a higher body mass, for whom predictions may yield unrealistic results (e.g. 12 L for a 120 kg patient). Overestimation of CBV leads to a risk of inadequate resuscitation and false reassurance. This discrepancy is likely to become more clinically significant in shorter patients with a high BMI, as blood volume does not increase proportionally to excess adipose tissue, resulting in potentially harmful underestimation of the significance of blood volume loss in patients with obesity. More than a decade ago, approximately one per 1000 parturients in the UK was classified as with extreme obesity,6Knight M. Kurinczuk J.J. Spark P. Brocklehurst P. Extreme obesity in pregnancy in the United Kingdom.Obstet Gynecol. 2010; 115: 989-997Crossref PubMed Scopus (134) Google Scholar a number that has increased in line with the prevalence of obesity in the general population; in 2018, more than 60% of women in England were classed as being overweight or having obesity,7NHS Digital. Statistics on obesity, physical activity and diet, England 2021. Available from: https://digital.nhs.uk/data-and-information/publications/statistical/statistics-on-obesity-physical-activity-and-diet/england-2020/part-3-adult-obesity-copy#overweight-and-obesity-prevalence. [Accessed 13 September 2022].Google Scholar alongside an annual rate of obesity in early pregnancy of 22.1%.8Office for Health Improvement & Disparities. Public health profiles: obesity in early pregnancy, 2018. Available from: https://fingertips.phe.org.uk/child-health-profiles#gid/1938133222/ati/165. [Accessed 13 September 2022].Google Scholar In the latest MBRRACE-UK report, more than half of maternal mortality was amongst women who were overweight or having obesity,9Knight M. Bunch K. Tuffnell D. et al.Saving lives, improving mothers’ care: lessons learned to inform maternity care from the UK and Ireland Confidential Enquiries into Maternal Deaths and Morbidity 2017-19.2021https://www.npeu.ox.ac.uk/assets/downloads/mbrrace-uk/reports/maternal-report-2021/MBRRACE-UK_Maternal_Report_2021_-_FINAL_-_WEB_VERSION.pdfGoogle Scholar and raised BMI is independently associated with an increased risk of significant postpartum haemorrhage.10Blomberg M. Maternal obesity and risk of postpartum hemorrhage.Obstet Gynecol. 2011; 118: 561-568Crossref PubMed Scopus (133) Google Scholar Therefore, pregnant women with obesity are at least as likely as women without obesity to suffer adverse outcomes, further hindered by healthcare biases. This is likely to become yet more prevalent as obesity rates increase. In this narrative review, we outline the physiological changes pertinent to blood volume in pregnancy; review the surrounding literature; and, based on the available data, suggest a methodology for adapting the estimation of CBV with increasing body mass. The physiological changes associated with pregnancy are well documented. These include increased cardiac output secondary to increased CBV and baseline HR. A concomitant decrease in systemic vascular resistance (SVR) is mediated by a combination of factors (including the action of relaxin and progesterone) and the presence of the placental circulation means that MAP decreases during the second trimester of pregnancy, returning to near baseline at term.11Fabry I. Richart T. Cheng X. Van Bortel L.M. Staessen J.A. Diagnosis and treatment of hypertensive disorders during pregnancy.Acta Clin Belg. 2010; 65: 229-236Crossref PubMed Scopus (18) Google Scholar,12Morris E.A. Hale S.A. Badger G.J. Magness R.R. Bernstein I.M. Pregnancy induces persistent changes in vascular compliance in primiparous women.Am J Obstet Gynecol. 2015; 212: 633.e1-633.e6Abstract Full Text Full Text PDF Scopus (56) Google Scholar Plasma volume and red cell mass increase during pregnancy, although the disproportionate increase in plasma volume results in a physiological anaemia attributable to haemodilution. CBV expansion, mediated by the renin–angiotensin and aldosterone system, begins at 6–8 weeks’ gestation and increases progressively to 28–30 weeks’ gestation before plateauing.13Sanghavi M. Rutherford J.D. Cardiovascular physiology of pregnancy.Circulation. 2014; 130: 1003-1008Crossref PubMed Scopus (473) Google Scholar,14Vricella L.K. Emerging understanding and measurement of plasma volume expansion in pregnancy.Am J Clin Nutr. 2017; 106: 1620S-1625SCrossref PubMed Scopus (47) Google Scholar The effects of pregnancy on cardiovascular remodelling have been studied in the context of obesity, and multiple studies have analysed the impact of obesity on maternal haemodynamics.15Vinayagam D. Gutierrez J. Binder J. Mantovani E. Thilaganathan B. Khalil A. Impaired maternal hemodynamics in morbidly obese women: a case-control study.Ultrasound Obstet Gynecol. 2017; 50: 761-765Crossref PubMed Scopus (24) Google Scholar,16Dennis A. Castro J. Ong M. Carr C. Haemodynamics in obese pregnant women.Int J Obstet Anesth. 2012; 21: 129-134Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar The majority of these studies focused on whether there is a correlation between obesity and the development of hypertensive diseases of pregnancy, such as pre-eclampsia.17Pisani I. Tiralongo G. Presti D.L. et al.Correlation between maternal body composition and haemodynamic changes in pregnancy: different profiles for different hypertensive disorders.Pregnancy Hypertens. 2017; 10: 131-134Crossref PubMed Scopus (16) Google Scholar,18Bicocca M.J. Mendez-Figueroa H. Chauhan S.P. Sibai B.M. Maternal obesity and the risk of early-onset and late-onset hypertensive disorders of pregnancy.Obstet Gynecol. 2020; 136: 118-127Crossref PubMed Scopus (25) Google Scholar However, the few studies that have examined the cardiovascular response to pregnancy in the context of obesity demonstrate conflicting results, with some reporting no difference in the observed parameters between high BMI and control groups.16Dennis A. Castro J. Ong M. Carr C. Haemodynamics in obese pregnant women.Int J Obstet Anesth. 2012; 21: 129-134Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar Other studies suggest that obesity is associated with altered haemodynamics in pregnancy, including increased sympathetic activation, producing an increase in HR and myocardial contractility, and increased SVR and MAP attributable to vasoconstriction.15Vinayagam D. Gutierrez J. Binder J. Mantovani E. Thilaganathan B. Khalil A. Impaired maternal hemodynamics in morbidly obese women: a case-control study.Ultrasound Obstet Gynecol. 2017; 50: 761-765Crossref PubMed Scopus (24) Google Scholar Parturients with obesity have an increased risk of cardiovascular diseases, such as ischaemic heart disease, increased left ventricular mass, and peripartum cardiomyopathy, and therefore may poorly tolerate a reduction in oxygen-carrying capacity and volume depletion.16Dennis A. Castro J. Ong M. Carr C. Haemodynamics in obese pregnant women.Int J Obstet Anesth. 2012; 21: 129-134Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar,19Patel S.D. Habib A.S. Anaesthesia for the parturient with obesity.BJA Educ. 2021; 21: 180-186Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar In many treatment protocols, blood product transfusion is triggered by specific quantities of blood loss, but the optimal methods to assess blood loss are yet to be determined. Visual estimations have recurrently been found to be inaccurate, especially as blood loss may be hidden, with accuracy degrading further with increasing volumes of haemorrhage.20Natrella M. Di Naro E. Loverro M. et al.The more you lose the more you miss: accuracy of postpartum blood loss visual estimation. A systematic review of the literature.J Matern Fetal Neonatal Med. 2018; 31: 106-115Crossref PubMed Scopus (34) Google Scholar Although quantification of blood loss lends itself to more suitable methodologies than historic studies of CBV, a recent Cochrane review identified only two trials suitable for comparison and was unable to advocate any one method of blood loss estimation.21Diaz V. Abalos E. Carroli G. Methods for blood loss estimation after vaginal birth.Cochrane Database Syst Rev. 2018; 9: CD010980PubMed Google Scholar Furthermore, there is little research to support the role of quantitative blood loss measurement in improving overall maternal morbidity or mortality.22Chau A. Farber M.K. Do quantitative blood loss measurements and postpartum hemorrhage protocols actually make a difference? Yes, no, and maybe.Int J Obstet Anesth. 2020; 42: 1-3Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar,23Katz D. Farber M.K. Can measuring blood loss at delivery reduce hemorrhage-related morbidity?.Int J Obstet Anesth. 2021; 46102968Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar This is likely, at least in part, to be because of the ethical and practical problems of attempting to isolate the influence of blood loss measurement from other clinical interventions. Studies examining blood loss often extrapolate from haemoglobin-based analysis to determine the accuracy of blood loss estimates.24Kahr M.K. Brun R. Zimmermann R. Franke D. Haslinger C. Validation of a quantitative system for real-time measurement of postpartum blood loss.Arch Gynecol Obstet. 2018; 298: 1071-1077Crossref PubMed Scopus (19) Google Scholar However, it has been suggested that this technique may be inherently flawed because of the poor correlation between these estimates and postpartum maternal haemoglobin values.23Katz D. Farber M.K. Can measuring blood loss at delivery reduce hemorrhage-related morbidity?.Int J Obstet Anesth. 2021; 46102968Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar Although this discrepancy has been explained previously by the influence of fluid shifts, starvation periods, and crystalloid administration, it may reflect the variability of CBV at the population level. Gravimetry (weight-based evaluation, e.g. of swabs), volumetry (based on fluid volumes, e.g. of suction canisters), and colorimetry (based on colour density analysis) can all be used individually to help quantify blood loss.25Gerdessen L. Meybohm P. Choorapoikayil S. et al.Comparison of common perioperative blood loss estimation techniques: a systematic review and meta-analysis.J Clin Monit Comput. 2021; 35: 245-258Crossref PubMed Scopus (29) Google Scholar However, it is likely that a multimodal approach, as recommended by the RCOG,5Mavrides E. Allard S. Chandraharan E. et al.Prevention and management of postpartum haemorrhage.2016Google Scholar is superior. Contemporary methods for quantifying blood loss have been shown to be effective when integrated into care systems and used in conjunction with treatment bundles, as demonstrated by the quality improvement project ‘Obstetric Bleeding Strategy for Wales (OBS Cymru)’.26Bell S.F. Watkins A. John M. et al.Incidence of postpartum haemorrhage defined by quantitative blood loss measurement: a national cohort.BMC Pregnancy Childbirth. 2020; 20: 271Crossref PubMed Scopus (27) Google Scholar To date, there are no studies specifically addressing the relationship between the numerator (estimated blood loss [EBL]) and the denominator (estimated blood volume [EBV]). This may be because of a lack of a ‘gold standard’ technique for quantification of blood loss, as existing research debates the superiority of any one method.21Diaz V. Abalos E. Carroli G. Methods for blood loss estimation after vaginal birth.Cochrane Database Syst Rev. 2018; 9: CD010980PubMed Google Scholar,22Chau A. Farber M.K. Do quantitative blood loss measurements and postpartum hemorrhage protocols actually make a difference? Yes, no, and maybe.Int J Obstet Anesth. 2020; 42: 1-3Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar,27Hancock A. Weeks A.D. Lavender D.T. Is accurate and reliable blood loss estimation the ‘crucial step’ in early detection of postpartum haemorrhage: an integrative review of the literature.BMC Pregnancy Childbirth. 2015; 15: 230Crossref PubMed Scopus (93) Google Scholar Although quantitative measurement techniques, such as gravimetry, are now recommended by multiple international bodies,23Katz D. Farber M.K. Can measuring blood loss at delivery reduce hemorrhage-related morbidity?.Int J Obstet Anesth. 2021; 46102968Abstract Full Text Full Text PDF PubMed Scopus (6) Google Scholar,28Muñoz M. Stensballe J. Ducloy-Bouthors A.-S. et al.Patient blood management in obstetrics: prevention and treatment of postpartum haemorrhage. A NATA consensus statement.Blood Transfus. 2019; 17: 112-136PubMed Google Scholar it is not only knowledge of blood loss that is important but the significance of that volume for the individual parturient that is vital for clinicians to appreciate. In an extensive literature search, we identified a paucity of research addressing the estimation of maternal CBV in the population with obesity. As such, it is useful to analyse the methodologies used by studies exploring haemodynamic adaptation to pregnancy and review how estimates of CBV were derived. A prospective, case-controlled study by Vinayagam and colleagues15Vinayagam D. Gutierrez J. Binder J. Mantovani E. Thilaganathan B. Khalil A. Impaired maternal hemodynamics in morbidly obese women: a case-control study.Ultrasound Obstet Gynecol. 2017; 50: 761-765Crossref PubMed Scopus (24) Google Scholar compared pregnant women with morbid obesity (BMI ≥40 kg m−2) with a control population (BMI 20–29.9 kg m−2), using a Doppler ultrasound technique to measure HR, cardiac output, stroke volume, and SVR, which they corrected for body surface area to produce indexed variables. They found that the population with obesity had a lower cardiac index and higher SVR index when compared with the population without obesity.15Vinayagam D. Gutierrez J. Binder J. Mantovani E. Thilaganathan B. Khalil A. Impaired maternal hemodynamics in morbidly obese women: a case-control study.Ultrasound Obstet Gynecol. 2017; 50: 761-765Crossref PubMed Scopus (24) Google Scholar They attributed this phenomenon to reduced cardiac function and an ‘impaired cardiac adaptation’ to pregnancy, although they were unable to elucidate any structural or functional anomalies during the study to explain this hypothesis.15Vinayagam D. Gutierrez J. Binder J. Mantovani E. Thilaganathan B. Khalil A. Impaired maternal hemodynamics in morbidly obese women: a case-control study.Ultrasound Obstet Gynecol. 2017; 50: 761-765Crossref PubMed Scopus (24) Google Scholar These findings were later echoed by both Sarno and colleagues,29Sarno L. Morlando M. Giudicepietro A. et al.The impact of obesity on haemodynamic profiles of pregnant women beyond 34 weeks’ gestation.Pregnancy Hypertens. 2020; 22: 191-195Crossref PubMed Scopus (2) Google Scholar who also used continuous wave Doppler ultrasound to measure cardiac output, and a haemodynamic study assessing a subset of the Combined Multimarker Screening and Randomized Patient Treatment with Aspirin for Evidence-Based Preeclampsia Prevention (ASPRE) trial that utilised a noninvasive bioreactance method to assess cardiac function.30Ling H.Z. Jara P.G. Bisquera A. Poon L.C. Nicolaides K.H. Kametas N.A. Maternal cardiac function in women at high risk for pre-eclampsia treated with 150 mg aspirin or placebo: an observational study.BJOG. 2020; 127: 1018-1025Crossref PubMed Scopus (9) Google Scholar Transthoracic echocardiography has been used to demonstrate left ventricular hypertrophy16Dennis A. Castro J. Ong M. Carr C. Haemodynamics in obese pregnant women.Int J Obstet Anesth. 2012; 21: 129-134Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar,31Buddeberg B.S. Sharma R. O’Driscoll J.M. et al.Cardiac maladaptation in obese pregnant women at term.Ultrasound Obstet Gynecol. 2019; 54: 344-349Crossref PubMed Scopus (22) Google Scholar and diastolic dysfunction31Buddeberg B.S. Sharma R. O’Driscoll J.M. et al.Cardiac maladaptation in obese pregnant women at term.Ultrasound Obstet Gynecol. 2019; 54: 344-349Crossref PubMed Scopus (22) Google Scholar in pregnant women with obesity, attributed to maladaptive response to volume overload. However, none of the aforementioned studies quantified maternal blood volume or specifically addressed its impact on maternal cardiac output. Vonck and colleagues32Vonck S. Lanssens D. Staelens A.S. et al.Obesity in pregnancy causes a volume overload in third trimester.Eur J Clin Invest. 2019; 49e13173Crossref PubMed Scopus (11) Google Scholar used bioimpedance to compare body composition and total body water (TBW) of parturients with and without obesity during each trimester of pregnancy. They found an increase in TBW in both populations throughout pregnancy, but controls without obesity demonstrated a more significant late gestational increase in TBW that was not observed in the population with obesity.32Vonck S. Lanssens D. Staelens A.S. et al.Obesity in pregnancy causes a volume overload in third trimester.Eur J Clin Invest. 2019; 49e13173Crossref PubMed Scopus (11) Google Scholar Their study also estimated volumes for extracellular water (ECW) and intracellular water, from analysis of different electrical frequencies,32Vonck S. Lanssens D. Staelens A.S. et al.Obesity in pregnancy causes a volume overload in third trimester.Eur J Clin Invest. 2019; 49e13173Crossref PubMed Scopus (11) Google Scholar,33Segal K.R. Burastero S. Chun A. Coronel P. Pierson Jr., R.N. Wang J. Estimation of extracellular and total body water by multiple-frequency bioelectrical-impedance measurement.Am J Clin Nutr. 1991; 54: 26-29Crossref PubMed Scopus (269) Google Scholar although their method was unable to provide estimates for individual ECW constituents, such as plasma volume. Although the methodologies used previously are validated, noninvasive techniques, which may be translatable into clinical practice, they do not provide an accurate estimation of CBV. Assumptions can be inferred from measuring stroke volume, which is directly affected by preload, but this does not take into consideration other compensatory mechanisms and cannot be presumed to truly reflect plasma volume. In their paper on postpartum blood loss, Hernandez and colleagues3Hernandez J.S. Alexander J.M. Sarode R. McIntire D.D. Leveno K.J. Calculated blood loss in severe obstetric hemorrhage and its relation to body mass index.Am J Perinatol. 2012; 29: 557-560PubMed Google Scholar developed a predictive formula to estimate CBV in relation to maternal height and weight at the time of delivery. This involves predicting the non-pregnant CBV and then adding an additional 50% to account for the average blood volume expansion in pregnancy:{[(maternalheightininches×50)+(maternalweightinpounds×25)÷2]×1.5} This formula was derived from measurements of maternal blood volumes using chromium isotope (51Cr) labelled red blood cells developed by Pritchard34Pritchard J.A. Changes in the blood volume during pregnancy and delivery.Anesthesiology. 1965; 26: 393-399Crossref PubMed Scopus (450) Google Scholar at Parkland Memorial Hospital, Dallas, TX, USA in the 1960s. This formula has been validated internationally,13Sanghavi M. Rutherford J.D. Cardiovascular physiology of pregnancy.Circulation. 2014; 130: 1003-1008Crossref PubMed Scopus (473) Google Scholar,31Buddeberg B.S. Sharma R. O’Driscoll J.M. et al.Cardiac maladaptation in obese pregnant women at term.Ultrasound Obstet Gynecol. 2019; 54: 344-349Crossref PubMed Scopus (22) Google Scholar but it can no longer be reproduced on ethical grounds. This equation was later used by S" @default.
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• W4297177527 title "Maternal body weight and estimated circulating blood volume: a review and practical nonlinear approach" @default.
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