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.
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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