The effect of supine positioning on maternal hemodynamics during late pregnancy

Abstract

Introduction: Recent studies have demonstrated an association between maternal supine sleep position and an increased risk of late stillbirth. During late pregnancy, the gravid uterus compresses the inferior vena cava (IVC) when a woman lies in the supine position. The azygos system of veins is the dominant pathway of collateral venous return back to the heart in the event of acute obstruction of the IVC. It is suggested that this pathway provides adequate collateral venous circulation in the event of IVC compression in the supine position during late pregnancy.

Objective: Investigate the effect of supine positioning on maternal hemodynamics during late pregnancy and the role of collateral venous circulation.

Methods: Ethics approval was obtained and 12 women with singleton pregnancies between 35- and 38-week gestation underwent magnetic resonance imaging in the supine and left lateral decubitus positions using a Skyra 3T system (Siemens). Phase-contrast images were evaluated to measure cardiac output, blood flow through the azygos vein, and blood flow through the abdominal aorta (AA) and IVC at two levels: at the level of aortic bifurcation and immediately above the renal veins.

Results: The supine position was associated with a 16.4% reduction in cardiac output when compared to the left lateral position. In addition, blood flow through the IVC decreased at its origin by 85.3% and by 44.4% at the level of the renal veins. Blood flow through the azygos vein increased in the supine position by 220%. Blood flow through the AA at the level of the renal veins did not differ significantly; however, it is reduced by 32.3% at the level of the aortic bifurcation.

Conclusions: Women in late pregnancy experience an increase in collateral venous blood flow when lying supine, likely as a response to marked compression of the IVC in this position. However, cardiac output and aortic blood flow were found to decrease while in the supine position.     

Acute effects of dihydralazine mesylate,furosemide, and metoprolol on maternal hemodynamics in pregnancy-induced hypertension

The intervillous blood flow and other maternal hemodynamic parameters were assessed in 40 mothers with pregnancy-induced hypertension in late pregnancy before and after hypotensive treatment. Furosemide caused a significant decrease in the intervillous blood flow, whereas the effects of dihydralazine and metoprolol were both negative and positive, that is, more individual and diverse. These changes in the intervillous blood flow did not have any correlation with the changes in other maternal hemodynamic parameters. The importance of placental hemodynamic studies in the evaluation of hypotensive drugs is stressed.     

Effect of exercise on maternal hemodynamics and placental blood flow in healthy women

Intervillous placental blood flow responses to standardized exercise during late pregnancy were studied using a Xenon technique in 25 healthy women. Thirteen of them were studied twice between the 32nd and 38th weeks of pregnancy, with mean 32 (range 22 to 40) days between the studies. At the end of a 6-min exercise, mean maternal heart rate had risen from 77 +/- 10 (SD) to 154 +/- 11 beats/min, amounting to 63% of maximal oxygen uptake. Stroke volume rose by 9%, cardiac output by 65% and cardiac index by 71% as a consequence of exercise, but peripheral vascular resistance declined by 41%. The placental blood flow was at a similar level after the exercise as before the exercise, being 95 +/- 19 (mean +/- SD) ml/min/100 ml of intervillous space before, 98 +/- 24 one min after, and 93 +/- 16 30 min after the cessation of exercise. No change was found in the level of placental blood flow between the 32-34th and 37-38th weeks of pregnancy. The placental blood flow had a positive correlation with maternal weight, mean arterial blood pressure and with diastolic blood pressure. Maternal heart rate, cardiac output, cardiac index, placental weight and the birth weight of the infant was not correlated with placental blood flow. It is concluded that in normal pregnancy a short submaximal exercise has little effect on placental blood flow measured after exercise.     

Studies on maternal hemodynamics during normal pregnancy: correlation between maternal hemodynamics and fetal growth

The cardiac output (CO) and cardiac index (CI) were examined by echocardiography throughout pregnancy and 1 month of puerperium and the results were compared with fetal growth as determined by birth weight. The results are as follows. The changes in CO and CI during the pregnancy and puerperium of normal pregnant women (n = 48) were similar in all subjects, and reached the maximum at the 24th-31st week's of gestation. On the 5th day of puerperium, the CO and CI values were almost the same as those obtained at the 32nd-40th weeks of gestation, and after 1 month of puerperium the values were same as those obtained in the non-pregnant period. The CO and CI of HFD child-bearing group (n = 8), AFD child-bearing group (n = 30) and LFD child-bearing group (n = 10) were compared. The CO and CI values for the LFD group were lower than those for AFD group; particularly, significant differences (p less than 0.05) were observed in 24th-31st weeks and 32nd-40th weeks of gestation. The CO and CI values of HFD group tend to be higher than those for AFD group, but there is no statistical significance between the two groups. The birth weight correlated to the rate of increase in CO and CI during pregnancy with coefficients of correlation of 0.56 (p less than 0.05) and 0.54 (p less than 0.05), respectively. The rate indicates the ratio of maximum values of CO and CI obtained during pregnancy to the values after 1 month of puerperium, which have been shown to be consistent with those during the non-pregnant period. A significant correlation (r = 0.54, p less than 0.05) was found between placenta weight and birth weight. The correlations between placental weight and the rate of increase in CO and CI were also significant with coefficients of correlation of 0.40 (p less than 0.05), and 0.38 (p less than 0.05), respectively.     

Effects of maternal postural change from supine to left-lateral on tocolysis, maternal cardiac function and pulsatility index of uterine artery.

In order to prove the usefulness of the left-lateral position (l-lat.) in preventing the cardiovascular complications associated with ritodrine, twenty-nine patients having preterm labor were employed as the subjects of this study. Before and during the administration of ritodrine, the cardiac function was recorded by impedance cardiography. (1) When ritodrine was administered in the supine position, SV and CO first increased but eventually tended to decrease. When the position was changed to the l-lat. after SV and CO had almost returned to the preadministration level, HR decreased and SV and CO increased in 70% (10/14) of the patients. However, in patients showing no change in HR in the supine position, the cardiac function did not change either. (2) The magnitude of the changes in cardiac function accompanying the postural change during ritodrine administration was compared with that observed before administration. The decrease in HR and the increase in SV due to the postural change to the l-lat. were larger during ritodrine administration than before administration. The l-lat. position is recommended as a means for preventing the side effects accompanying ritodrine administration. This study also indicated the possibility that the inferior vena cava may be more strongly compressed in the supine position by the relaxed gravid uterus.     

Uterine resting tonus and maternal hypertension during late pregnancy--existence of intrauterine capacity

The uterine resting tonus was measured with an open-end catheter beginning from the 24th gestational week through the onset of labor in order to investigate changes in the intrauterine environment from the physical aspect. The measurements were performed in 183 primiparas for a total of 272 times and 102 multiparas for a total of 120 times. The uterine resting tonus was almost constant from the 2nd trimester through the 3rd trimester, but it tended to increase gradually beginning 2-3 weeks prior to the onset of delivery. This tendency was more pronounced in primiparas, and their mean tonus was about 5mmHg higher than that of multiparas throughout the pregnancy. In 55 subjects in whom both the resting tonus and blood pressure were simultaneously measured at least twice during pregnancy, the mean maternal blood pressure increased as the resting tonus increased. The above findings revealed a positive correlation between the resting tonus and the maternal blood pressure and suggested the presence of a critical point in terms of the intrauterine capacity, which derives from the properties of the uterine muscle and the intrauterine contents, consisting mainly of the fetus. The increase in the resting tonus is surmised to be related to this critical point.     

Effect of maternal heme degradation on fetal carboxyhemoglobin concentration in pregnancy-induced hypertension

Increased red cell destruction with consequent heme catabolism results in accumulation in carboxyhemoglobin in women with pregnancy-induced hypertension. Temporally associated with this is a significant shift in the oxyhemoglobin dissociation curve to the left. In this study we assessed the relationship between carboxyhemoglobin concentration and oxyhemoglobin dissociation curve in simultaneously obtained maternal and fetal venous blood in which patients with pregnancy-induced hypertension were compared with normotensive patients. Maternal and fetal carboxyhemoglobins were significantly higher in patients with pregnancy-induced hypertension than in control subjects. Fetal carboxyhemoglobin was significantly higher than maternal carboxyhemoglobin in both clinical groups. Furthermore, maternal and fetal oxyhemoglobin dissociation curves were significantly shifted to the left in patients with pregnancy-induced hypertension compared with controls. The fetal-maternal gradient of carboxyhemoglobin was similar in both groups. The maternal-fetal gradient of oxyhemoglobin dissociation curve position was significantly reduced in pregnancy-induced hypertension. The concentrations of carboxyhemoglobin measured do not fully explain the magnitude of the shift in oxyhemoglobin dissociation curve, but the data suggest a potential impact on fetal oxygenation in pregnancy-induced hypertension.     

Effect of left-lateral position on maternal hemodynamics during ritodrine treatment in comparison with supine position.

Pregnant women are generally recommended to rest in the left-lateral position (l-lat.) to avoid caval compression. We studied the influence of postural change from the supine position (sup.) to the l-lat. on maternal hemodynamics and tocolysis. The cardiac output (CO) was measured by impedance cardiography, while the pulsatility index of the uterine artery (PIUtA) was determined by the pulsed Doppler method. When the maternal posture was changed from sup. to l-lat, the following results were obtained. (1) The frequency of uterine contractions (UC) (preterm labor = 92) was significantly reduced (p less than 0.01). (2) The resting uterine tonus (n = 5) was also significantly decreased (p less than 0.05). (3) In the cases showing a decrease in the frequency of UC, the CO value was significantly increased (p less than 0.01), whereas PIUtA was significantly decreased (p less than 0.01). It is concluded that the postural change from sup. to l-lat. resulted in a decrease in the frequency of UC and the resting uterine tonus and at the same time an increase in the CO and decrease in the vascular resistance of the uterine artery.     

Non-invasive assessment of maternal hemodynamics in early pregnancy

ObjectivesNon-invasive assessment of maternal hemodynamics in early pregnancy may be promising in evaluating maternal hemodynamic (mal)adaptation to pregnancy. We explored usage of applanation tonometry and Doppler ultrasound for assessment of cardiac output (CO), systemic vascular resistance (SVR) and arterial stiffness in early pregnancy.MethodsPregnant healthy nulliparous women were studied during first trimester. Radial artery pressure waveform (augmentation index(AIx)), carotid-femoral pulse wave velocity (PWV) and cardiac output (CO) were measured by applanation tonometry (SphygmoCor), electrocardiogram and Doppler ultrasound (USCOM) and related to maternal demographic characteristics and literature concerning advanced pregnancy and non-pregnant subjects.Results116 women were studied during gestational age range of 7⁺²–14 weeks. Systolic and diastolic central blood pressure were correlated with systolic and diastolic brachial blood pressure respectively. Both measures of arterial stiffness (heart rate corrected AIx(AIx@75) and PWV) were correlated. AIx@75, PWV and SVR were correlated with central mean arterial pressure. CO was negatively correlated with AIx and associated with BMI. PWV was associated with age and BMI, whereas SVR was associated with age.ConclusionsApplanation tonometry and Doppler Ultrasound for assessment of maternal hemodynamics in early pregnancy revealed similar associations between different hemodynamic parameters and maternal characteristics as have previously been reported in advanced pregnancy and non-pregnant subjects. The SphygmoCor and the USCOM appear to be reliable methods for the assessment of maternal hemodynamics in early pregnancy. Obtaining a comprehensive hemodynamic profile using these modalities may offer insight in maternal (mal)adaptation to pregnancy. Future work needs to be done relating such measures to pregnancy outcome.     

The possible relation between late pregnancy hypertension and 5-hydroxytryptamine levels in maternal blood.

The concentrations of the vasoactive substance, 5-hydroxytryptamine (5-HT), were estimated in whole blood samples from 12 pregnant women with hypertension in the third trimester. These were compared with a control group of 15 normotensive pregnant women of the same gestation. The hypertensive patients were admitted for observation and the whole blood 5-HT estimated again after four days bed rest: in all patients the blood pressure had, by this time, returned to within the normal range. No significant difference was found between the mean blood 5-HT concentrations of the hypertensive and the normotensive groups. A small, but not statistically significant, rise occurred in the hypertensive group after four days bed rest. Thus this study gave no evidence that 5-HT is implicated in hypertension in pregnancy.     

Acute effects of maternal smoking on fetal hemodynamics.

OBJECTIVE: To investigate acute effects of cigarette smoking on fetal hemodynamics. METHOD: Sixty seven women between 32nd to 40th weeks of gestation were evaluated. Maternal blood pressure and heart rate, fetal heart rate (FHR) tracing, umbilical and fetal middle cerebral arterial (MCA) color Doppler measurements were evaluated. Pre- and postsmoking results were compared with paired t-test. RESULTS: Maternal heart rate significantly increased after smoking. Baseline FHR and FHR variability remained unchanged. The number of participants who had a reactive NST was 60 in 67 before smoking (89.5%) and decreased to 47 after smoking (70.1%) (p=0.009). There were no significant changes between maximum and minimum flow velocities, pulsatility index (PI), resistance index (RI) and systolic/diastolic flow ratio (S/D) of umbilical and middle cerebral arteries. CONCLUSION: The nicotine load of a single cigarette may be inadequate to cause a detectable decrease in utero-placental blood flow; however, smoking prior to the FHR recording may alter the FHR reactivity.     

Influence of the increased uterine tonus on maternal hemodynamics

In an experiment using 103 late pregnant rabbits, the resting uterine tonus was increased by inserting a balloon into the amniotic cavity on the cervical side (n = 71) or the ovarian side (n = 52) and the effects of this increase on the maternal blood pressure, renal cortical blood flow (measured by a thermocouple method), renal arterial and carotid arterial blood flow (measured with an electromagnetic flowmeter) were studied. The findings are briefly described, as follows: The maternal blood pressure was slightly increased on both sides (by 12.0 +/- 3.5% on the cervical and by 12.0 +/- 3.8% on the ovarian side). However, there was a hemodynamic difference between the renal cortical and renal arterial blood flow on the cervical side and those on the ovarian side: They were slightly increased on the cervical side and slightly decreased on the ovarian side. On the basis of these findings, the following may be proposed: An increase in resting uterine tonus causes a rise in maternal blood pressure associated with blood redistribution from the uteroplacental vascular bed. However, the fact that the renal blood flow was decreased consequently upon the increase in resting uterine tonus raised a possibility of neurotransmission via the ovarian plexus as well as utero-renal reflex.     

Effect of maternal oral hydration on amniotic fluid index in women with pregnancy-induced hypertension

OBJECTIVE: To evaluate the effect of maternal oral hydration on amniotic fluid index (AFI) in women with pregnancy-induced hypertension. METHODS: Five women with pregnancy-induced hypertension and five normotensive gravidae at 32-37 weeks gestation with normal AFI (8-18cm) were hydrated by oral intake of 2L of water over 1 h. Both groups were similar in age, parity and gestation. Repeat AFI was measured in all women after 3 h. The differences between pre- and post-hydration AFI in the two groups were analyzed with Student's t-test for statistical significance. RESULTS: Maternal oral hydration resulted in a significant increase in AFI in both hypertensive and normotensive gravidae (1.7cm and 3.48cm, respectively). However, the extent of increase was significantly lower in the hypertensive group (P = 0.03). CONCLUSION: Pregnancy-induced hypertension adversely affects the increase in amniotic fluid volume with maternal hydration. This observation is of clinical relevance in the management of oligohydramnios associated with pregnancy-induced hypertension.     

Effects of maternal central hemodynamics on fetal heart rate patterns

BACKGROUND: This study was undertaken to evaluate the effects of maternal central hemodynamics on fetal heart rate patterns near term, with special regard to the maternal body position. METHODS: Brief non-stress test and bioimpedance cardiography were carried out in the supine position, then repeated in a full left lateral decubitus position of mothers with singular, 36-39 week-old normal pregnancies in 106 cases. Computer-aided data were processed by SPSS statistic program. RESULTS: Due to the appearance of inferior vena cava syndrome, examinations had to be interrupted in 6 cases. Analysis of 100 complete registrations revealed a significantly increased number of accelerations, overall and short-term variations, and longer high episodes with lower basal fetal heart rates were found in the lateral decubitus than in the supine maternal position. Turning to the left resulted in a significant increase of the stroke volume; however, due to decreasing pulse rate, the cardiac output remained unchanged. Parameters of non-stress test showed correlations to hemodynamic indices. In the supine position, the short-term variation correlated with cardiac output (r=0.232, p=0.020); in the left lateral position, the number of accelerations correlated with stroke volume (r=0.221, p=0.027) and cardiac output (r=0.220, p=0.028). Changes of stroke volume due to altered body position correlated to similar changes of overall variation (r=0.264, p=0.018), and marginally to those of short-term variation (r=0.221, p=0.051). CONCLUSION: Maternal central hemodynamics influences fetal heart rate patterns in connection with different maternal body position.     

Effect of maternal nutrient restriction and melatonin supplementation from mid to late gestation on vascular reactivity of maternal and fetal placental arteries

Introduction

Maternal nutrient restriction and decreased scotophase concentrations of melatonin have been associated with severely compromised pregnancies. We hypothesized that melatonin supplementation in a compromised pregnancy enhances the bradykinin (BK)-induced relaxations of placental arteries thereby ensuring sufficient umbilical blood flow to the developing fetus.

Methods

Pregnant ewes (n = 31) were fed an adequate (ADQ) or nutrient restricted (RES) diet supplemented with 5 mg of melatonin (MEL) or without melatonin (CON) from day 50 to 130 of gestation. On day 130 of gestation, the maternal (caruncular; CAR) and fetal (cotyledonary; COT) placental arteries were suspended in organ chambers for isometric tension recording.

Results

There were no treatment or dietary effects on CAR arteries for any vasoactive agent. However, in COT arteries, MEL ewes were more sensitive (P < 0.01) to bradykinin-induced relaxation than CON ewes. There was a melatonin by nutritional level interaction (P < 0.01) with sodium nitroprusside-induced relaxation of COT arteries where CON-RES were more sensitive to sodium nitroprusside compared to CON-ADQ, which was in contrast to when ewes were fed MEL. There was a significant melatonin by nutritional interaction (P = 0.04) for responsiveness to norepinephrine. The sensitivity of the COT arteries to norepinephrine in CON-RES ewes was decreased compared to CON-ADQ. Melatonin supplementation, regardless of maternal dietary intake, resulted in COT arteries having similar responsiveness to CON-RES ewes.

Conclusion

An increase in placental vessel sensitivity to bradykinin-induced relaxation may contribute to melatonin-induced increases in umbilical artery blood flow.