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Maternal Hemodynamics by Thoracic Impedance Cardiography for Normal Pregnancy and the Postpartum Period

Morris, Rachael MD; Sunesara, Imran MBBS, MPH; Rush, Laura MD; Anderson, Belinda RN; Blake, Pamela G. RN, MSN; Darby, Marie MD; Sawardecker, Sandip MD; Novotny, Sarah MD; Bofill, James A. MD; Martin, James N. Jr MD

doi: 10.1097/AOG.0000000000000104
Contents: Original Research

OBJECTIVE: To establish normative impedance cardiography values for the second half of pregnancy and up to 48 hours postpartum after either vaginal or cesarean delivery.

METHODS: A single-center prospective observational institutional review board-approved study of normotensive women (n=168) using thoracic impedance cardiography performed at specific times during gestation. Antepartum testing was performed at three time periods: 20–27 weeks, 28–33 weeks, and 34–40 weeks of gestation. Postpartum testing was undertaken after the immediate puerperium at 6–23 hours and 24–48 hours after vaginal or cesarean delivery. Data analysis was performed using STATA software; data are expressed as mean±standard deviation.

RESULTS: All seven of the patient groups studied were comparable with regard to demographic features; 80% of the study participants were African American. Group means obtained between 20 and 40 weeks of gestation and postpartum after vaginal and cesarean delivery fell within the “normal range” of the hemodynamic graph that was developed to associate mean arterial pressure and systemic vascular resistance. The thoracic fluid content group means in both vaginal and cesarean delivery groups were higher than the antepartum patient groups. The thoracic fluid content mean after cesarean delivery at 48 hours is significantly higher than the mean value recorded between 20 and 27 weeks of gestation (P<.05). The systemic vascular resistance systemic vascular resistance means in each of the postpartum groups were significantly higher than the late second-trimester group means recorded at 20–27 weeks of gestation (P<.05).

CONCLUSION: The normative values reported in this investigation can be used to interpret and assess similarly tested patients with hypertensive or otherwise complicated pregnancy.


Normative impedance cardiography hemodynamic data are provided for normotensive pregnancy and the first 2 days postpartum after a vaginal or cesarean delivery.

Divisions of Maternal-Fetal Medicine and Biostatistics, Department of Obstetrics and Gynecology, University of Mississippi Medical Center, Jackson, Mississippi.

The Division of Maternal-Fetal Medicine in the Department of OBGYN at the University of Mississippi Medical Center funded this study.

Corresponding author: James N. Martin Jr, MD, Professor OBGYN, Director MFM Division, Vice Chair, Research & Academic Development, Department of OBGYN-UMMC, 2500 N State Street, Jackson, MS 39216; e-mail:

Presented as poster at the 33rd Annual Meeting of the Society for Maternal-Fetal Medicine: The Pregnancy Meeting, February 11–16, 2013, San Francisco, California.

Financial Disclosure The authors did not report any potential conflicts of interest.

Evaluation of maternal hemodynamic changes during pregnancy by safe, reliable, validated, and easy-to-use noninvasive techniques can be used by clinicians including obstetricians and maternal–fetal medicine subspecialists to aid them in the diagnosis and management of mothers with various pregnancy disorders including preeclampsia1–3 and impending pulmonary edema.4 For many years, reliable noninvasive techniques were not readily available to many clinicians. Instead, the invasive techniques of pulmonary artery catheterization using either thermodilution or the direct Fick method were used in limited critical care settings and circumstances. Invasive monitoring techniques for maternal hemodynamic assessment were also relatively cumbersome and the risks to the patient including arrhythmias, pneumothorax, infection, thrombosis, and even death outweighed any benefit except in very ill patients.5 Echocardiography was one of the first noninvasive tools used on a broader scale to help assess hemodynamic changes such as cardiac output, but it also has limitations for widespread clinician use secondary to logistical issues, cost considerations, and dependence on skilled operators.2 In recent years, bedside immediately available impedance cardiography by thoracic, whole-body or thoracic and whole-body approaches has emerged as a reliable and easy-to-use instrumentation to noninvasively evaluate a given adult patient's hemodynamic status and make therapeutic decisions for safe, better directed care.6–9

Impedance cardiography that provides hemodynamic profiling of late-gestation hypertensive pregnant patients has been shown to facilitate clinician differentiation of severe and superimposed preeclampsia from other hypertensive disorders, always a challenging endeavor that can be considerably accelerated by this approach to management.10 During 2006 and 2010, we prospectively assessed with thoracic impedance cardiography all pregnant outpatients evaluated greater than 20 weeks of gestation for suspected new-onset or progressive development of a hypertensive disorder of pregnancy. We were able to reliably distinguish those patients with severe disease apart from the others, especially when impedance cardiography values were supplemented by the biomarker of hyperuricemia.10 Missing from this comprehensive investigation of impedance cardiography findings in patients with five types of hypertensive disorders of pregnancy was a group of patients with normal pregnancy used as controls for comparison purposes. The present study addresses that deficiency and adds to the number of published studies investigating the findings of thoracic and whole-body impedance cardiography in uncomplicated, normal pregnancy devoid of preeclampsia or any other type of hypertensive pregnancy disorder.11–13 Our results constitute a reference healthy pregnant patient population that can be compared with values obtained in abnormal gestational circumstances.10,14,15

For the present investigation we sought to determine baseline (control) maternal hemodynamic values through thoracic impedance cardiography of normotensive, uncomplicated pregnant patients at different stages after 20 weeks of gestation as well as postpartum after vaginal or cesarean delivery. Knowledge of these baseline parameters for normal gestation can be used for comparison with values similarly obtained from patients developing preeclampsia and other hypertensive disorders as well as any other medical complications of pregnancy. Moreover, knowledge of the range of normal values during various stages of pregnancy and during the early puerperium would allow the clinician to assess the level of patient deviation from these numbers.

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This was a prospective cross-sectional observational study of normotensive pregnant patients (n=168) weighing within the acceptable limits of 66 and 342 pounds for accurate impedance cardiography testing with singleton gestations absent any evidence of preeclampsia or other medical complication of pregnancy diagnosed at any time during gestation or the puerperium. Two portable Cardiodynamics/Sonosite BioZ thoracic bioimpedance cardiography devices were used to perform all assessments at specific times during gestation. When purchased, they were equipped with recently upgraded software and algorithms for better computation of hemodynamic parameters than present in earlier models. Approval by the University of Mississippi Medical Center institutional review board 2 occurred on January 29, 2010 (institutional review board file number 2009-0234). The study was conducted at the University of Mississippi Medical Center's Winfred L. Wiser Hospital for Women & Infants, with patient recruitment completed in June 2012. Informed consent was obtained from all patients before participation.

The Cardiodynamics BioZ impedance cardiography system uses four sets of paired sensors with adhesive pads that are placed on opposite sides of the neck (between the ears and shoulders) and chest (midaxillary line at the level of the xiphoid process) to measure the change of an electrical signal passing through the thoracic cavity. The inner electrodes detect voltage changes and the outer electrodes relay current. Differences in voltage are used to determine impedance changes. The hemodynamic parameters of heart rate, cardiac output, systemic vascular resistance (SVR), mean arterial pressure, and thoracic fluid content (thoracic fluid content) are calculated by the impedance cardiography system using standard equations normalized for body size and surface area.

Study participants were stabilized in the supine position with some lateral tilt and head on a pillow for 5–10 minutes while pads with sensors were placed. After testing to assure a working system, a recording of the hemodynamic parameters was begun and continued for 1–2 minutes before a single representative hemodynamic profile was generated and printed for study purposes. A copy of the impedance cardiography study results was saved for future analysis and did not factor into patient management. No patient in the series received tocolytic agents, corticosteroids, antihypertensives, magnesium sulfate, oxytocin, or anesthetic agents while being evaluated with impedance cardiography. Subsequent to patient delivery, hospital discharge, or both data from the impedance cardiography studies and medical records were collected, reviewed for relevant clinical and laboratory information, and managed using REDCap electronic data capture tools16 hosted at The University of Mississippi Medical Center and later used for analysis.

Antepartum testing was performed during three gestational age periods: 1) 20–27 weeks (n=23); 2) 28–33 weeks (n=23); and 3) 34–40 weeks (n=21). Postpartum testing was performed in four groups of normotensive uncomplicated patients: 1) 6–24 hours after vaginal delivery (n=27); 2) 25–48 hours after vaginal delivery (n=25); 3) 6–24 hours after cesarean delivery (n=25); and 4) 25–48 hours after cesarean delivery (n=24). No testing was undertaken during labor, delivery, or the first 6 hours immediately postpartum. Hemodynamic as well as demographic and obstetric and neonatal data were recorded for all patients. Any patient sampled earlier in gestation and later found to develop a hypertensive complication of pregnancy was removed from the final data set.

Cardiac parameters obtained from impedance cardiography for three antepartum gestational age periods and four postpartum normotensive patient groups were compared using analysis of variance with Bonferroni correction. Descriptive statistics were reported in terms of mean, standard deviation, median, fifth, and 95th percentiles. Sensitivity analysis found similar qualitative results. Plots were produced using estimated mean and 95% confidence interval (95% CI). Analysis was done using STATA 12.1. No comparative statistics are reported unless statistically significant (Table 1). P<.05 was deemed statistically significant.

Table 1-a

Table 1-a

Table 1-b

Table 1-b

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Demographic characteristics for the overall cohort included a mean maternal age of 25.15 years (24.19–26.11, 95% CI), a mean body mass index (calculated as weight (kg)/[height (m)]2) of 31.81 (95% CI 30.80–32.82%), mean gravidity of 2.90 (95% CI 2.66–3.15), and mean parity of 1.84 (95% CI 1.62–2.07); 80.36% of the study participants were African American.

The means of group values obtained before and after delivery fell within the “normal” area of the hemodynamic graph developed to associate mean arterial pressure and SVR (Fig. 1). As illustrated in Table 1, evaluation of group means indicated significant differences between early third-trimester maternal heart rate and postoperative cesarean delivery at 24 hours (P<.05). The only significant difference between postpartum and postoperative patient groups was the finding of a significantly higher maternal heart rate 24 hours after cesarean delivery compared with 48 hours after vaginal delivery (P<.05). The thoracic fluid content mean measured 48 hours after cesarean delivery is significantly higher than the mean value recorded between 20 and 27 weeks of gestation (P<.05) Indeed, all thoracic fluid content group means from postpartum and postoperative groups were higher than the means of antepartum groups, particularly after cesarean delivery (Fig. 2). Systemic vascular resistance means for each of the four postpartum groups were significantly higher than the late second-trimester findings recorded for 20–27 weeks of gestation (P<.05). Figure 3 further illustrates the findings for mean arterial pressure and SVR index for the seven patient groups.

Fig. 1

Fig. 1

Fig. 2

Fig. 2

Fig. 3

Fig. 3

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The current study expands on reports from other investigators describing hemodynamic changes associated with normal pregnancy when evaluated using noninvasive thoracic (electrodes on the neck and thorax) or whole-body (electrodes on one wrist and a contralateral ankle or wrist) impedance cardiography.6–9,11–13 All of the group means fell within the normal, optimal hemodynamic profile as demonstrated by Easterling17 in obstetric patients (Fig. 1). Similarly to van Oppen's findings using thoracic impedance cardiography, we observed a consistent decrease in cardiac output and mean stroke volume in late pregnancy.12 We did not compare findings between nulliparous and multiparous women in our series.

Many innovations and enhancements regarding impedance cardiography have happened since Nyboer's first observations in 1944.18 In its current iteration, impedance cardiography is a noninvasive hemorrheology technique to detect the properties of blood flow while measuring the baseline impedance (resistance) to electrical current transmitted between receiving and transmitting sensors. Computerization enables the device to detect changes in impedance from baseline on a beat-to-beat basis to measure and calculate cardiovascular hemodynamic parameters. A new generation of very portable, easy-to-use bedside impedance cardiography devices has opened up the possibility of more widespread use of this technology to noninvasively collect hemodynamic data for assessing patients with heart failure, dyspnea, and hypertension. Application to pregnancy has progressed, particularly in patients with stress states such as preeclampsia.

Several reports have validated impedance cardiography-derived hemodynamic values in comparison to thermodilution and direct Fick methods in nonpregnant patients.19,20 The information obtained by impedance cardiography has been validated in pregnant and nonpregnant patients with high correlation and accuracy compared with invasive thermodilution techniques and M-mode echocardiography.21–23 A 1997 meta-analysis revealed that both whole-body and thoracic impedance cardiography accurately assess cardiac output in healthy patients inclusive of pregnant patients with a pooled correlation coefficient of 0.80 with 95% CIs.24 Results from Austria were less affirming,25 whereas newer algorithms in recent impedance cardiography devices have shown very good correlation with invasive cardiac output measurements.26–28 Using equipment very similar to ours, investigators in Belgium and elsewhere very recently demonstrated that impedance cardiography measurements of healthy pregnant women are reliable when undertaken in a standardized fashion, especially measures of cardiac contractility, and thoracic resistance parameters.23 Pearson correlation coefficients were consistently 0.80 or greater both in normal pregnant patients and those with preeclampsia.23 Moreover, our results are consistent with those reported from Norway using a German-built Philips impedance cardiography device.11 Paredes and colleagues29 have shown agreement between both methods using the Non-Invasive Cardiac System that is within the boundaries of the U.S. Food and Drug Administration guidelines for bioequivalence.

Surprisingly, little attention has been placed on impedance cardiography-derived hemodynamic changes incurred by the pregnant patient in the first 48 hours after either cesarean or vaginal delivery. During cesarean delivery, Finnish investigators have shown that the cardiac index increases rapidly in association with delivery, loading the heart and circulation, necessitating intact physiologic systems to adequately respond to these challenges from a combination of anesthesia, stress, placental removal, and reallocation of residual blood volume.30 The present findings for both vaginal and cesarean-delivered postpartum patients add much needed information to this area of investigation.

Some of the limitations in this study include a focus on mostly African American patients. A cross-sectional study may not be generalized to all populations. Although efforts were made to obtain a steady state before impedance cardiography testing was recorded, multiple studies over 30–60 minutes might have rendered more accurate readings. We did not control for anesthesia type because it has not been shown to significantly affect hemodynamic testing that is undertaken 6 or more hours after delivery.31,32 However, for consistency, most of the impedance cardiography testing was performed for our study by a single study nurse. Of major importance is that all of our means for the seven test groups fit within the “optimal hemodynamic profile,” suggestive of accuracy.

In conclusion, with this study, we present normative values for maternal hemodynamics in nonhypertensive or otherwise noncompromised pregnancy in seven patient groups spanning from midgestation to 48 hours after either vaginal or cesarean delivery. These data can be used for comparison with patients in abnormal pregnancy settings such as preeclampsia. Observed increases in SVR and thoracic fluid content emphasize the need for physicians to be attentive to fluid intake in the postpartum patient given the potential risks of fluid overload and decreased colloid osmotic pressure, even in normal postpartum patients absent evidence of preeclampsia.33,34

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© 2014 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.