TABLE 3 : Maternal Hemodynamics Calculated Mean ± Standard Deviation ( 95 % Confidence Interval ) for continuous variables
All Categories |
BMI < 30kg / m 2
|
BMI 30kg / m 2 – |
|
|
34.9kg / m2 |
BMI 35kg / m 2 – 39.9kg / m2
BMI ≥ 40kg / m 2
N 796 335 171 154 136 --- HR 91.33 ± 11.91 90.29 ± 0.65 92.10 ± 0.91 91.75 ± 0.95 92.40 ± 1.02 0.21 SBP 125.20 ± 12.93 121.58 ± 12.48 127.23 ± 12.26 127.53 ± 12.96 128.92 ± 12.68 <. 01 DBP 73.54 ± 9.54 73.10 ± 9.72 75.08 ± 8.47 73.41 ± 9.48 72.81 ± 10.31 0.11 MAP 91.01 ± 9.97 89.75 ± 10.33 92.70 ± 9.43 91.35 ± 9.31 91.60 ± 10.14 0.01 CO ( L / min ) 6.88 ± 1.63 6.42 ± 1.50 6.67 ± 1.38 7.28 ± 1.59 7.85 ± 1.74 <. 01 SVR 1076.84 ± 340.59 1151.97 ± 413.49 1104.95 ± 245.72 1004.00 ± 258.32 938.93 ± 261.24 <. 01
Anova F test p value
BMI = Body Mass Index ; HR = Heart Rate ; SBP = Systolic Blood Pressure in mmHg ; DBP = Diastolic Blood Pressure in mmHg ; MAP = Mean Arterial Pressure in mmHg ; CO = Cardiac Output in L / min ; SVR = Systemic Vascular Resistance in dynes · sec / cm -5
compared to pre-pregnancy levels . Stroke volume and heart rate also respond to maternal gravid changes , showing an approximate 30 % and 20 % increase , respectively . Gestation contributes to an overall increase in cardiac output that is observed late in the first trimester until it reaches a value of 6 to 7 L / min , an approximate 50 % increase compared to the nonpregnant state . In the absence of pathological alteration maternal arterial blood pressure decreases in early gestation secondary to the dilatory effects of progesterone . This decrease in maternal arterial blood pressure reaches its physiologic nadir during the second trimester . Arterial blood pressure should then begin to increase towards pre-pregnancy levels for the remainder of pregnancy . 7
Each cardiac cycle induces changes in systemic electrical resistance due volume changes in the arterial system . Utilizing two sensors arranged on a patient ’ s wrists , impedance cardiography devices initiate a small electrical signal ( 1.4 mA @ 30 kHz ) that is conducted through the body allowing for measurement of hemodynamic parameters . Impedance cardiography has proven to be a reliable non-invasive method for studying hemodynamic changes in both pregnant and non-pregnant individuals . 8-11
Prior studies have validated impedance cardiography for the measurement of cardiac output in obese patients . In a study of 285 patients , bioimpedance correlated well with thermodilution for the entire population ( r = . 84 , P < . 0001 ) and was reliable equally in patients with obesity ( r = . 85 , P < . 0001 ) and those without ( r = . 82 , P < . 0001 ). 12 This study by Brown et al concluded that bioimpedance technology may be used reliably as a noninvasive alternative for the assessment of cardiac performance in obese patients . 12
Historical data indicate that nonpregnant individuals with obesity typically have an elevated cardiac output secondary to the effects of increased aldosterone and central sympathetic activity . Prior studies demonstrate that both pregnant and non-pregnant individuals with obesity have an increase in total and central blood volume , as well as a low peripheral vascular resistance , and a normal or mildly elevated
13 , 14 heart rate .
Recent studies specific to the effects of obesity on gravid women have suggested hemodynamic alterations . A study by Vonck et al examined the hemodynamic profiles of 232 gravidas with obesity . 15 They demonstrated that while gestational adaptations are comparable between patients with and without obesity in the first two trimesters of pregnancy , in the third trimester there is a shifting to volume overload with decreased cardiac output and increase in systemic vascular resistance . Importantly , in their study they stratified results by two groups : BMI of 20-25 kg / m 2 and BMI ≥ 30 kg / m 2 . In 2017 , Vinayagam et al performed a case-control study using ultrasound cardiac output monitoring to investigate the cardiovascular parameters of 350 gravid women at the median gestational age of 31 weeks . 16 After accounting for body surface area , results showed that women with BMI ’ s ≥ 40kg / m 2 had an increase in their mean arterial pressure , heart rate , and systemic vascular resistance index , with a decrease in their cardiac index and stroke volume index when compared to women with BMI ’ s < 30kg / m 2 .
A strength of our study is it is the largest reported population of gravidas with and without obesity that underwent hemodynamic testing in early gestation . Given the regionalized nature of healthcare in our tri-state area , Marshall University remains the primary referral center for high-risk pregnancies , allowing for a large sampling of our patient population . Our robust analysis was conducted with stratification of our patient population into four BMI categories . Limitations of the study include the retrospective nature of the work , not having a control group with increasing obesity without co-morbid conditions ( history of preeclampsia , diabetes , chronic renal failure ), and not being able to definitively determine a referral reason for many patients . Additionally , while studies have demonstrated the reliability of bioimpedance technology of cardiac performance in patients with obesity , this method has not been validated during the first trimester of pregnancy .
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