Ohnmacht und Schwangerschaft

Left Lateral Table Tilt for Elective Cesarean Delivery under Spinal Anesthesia Has No Effect on Neonatal Acid–Base Status

Background

Current recommendations for women undergoing cesarean delivery include 15° left tilt for uterine displacement to prevent aortocaval compression, although this degree of tilt is practically never achieved. We hypothesized that under contemporary clinical practice, including a crystalloid coload and phenylephrine infusion targeted at maintaining baseline systolic blood pressure, there would be no effect of maternal position on neonatal acid base status in women undergoing elective cesarean delivery with spinal anesthesia.

Methods

Healthy women undergoing elective cesarean delivery were randomized (nonblinded) to supine horizontal (supine, n = 50) or 15° left tilt of the surgical table (tilt, n = 50) after spinal anesthesia (hyperbaric bupivacaine 12 mg, fentanyl 15 μg, preservative-free morphine 150 μg). Lactated Ringer’s 10 ml/kg and a phenylephrine infusion titrated to 100% baseline systolic blood pressure were initiated with intrathecal injection. The primary outcome was umbilical artery base excess.

Results

There were no differences in umbilical artery base excess or pH between groups. The mean umbilical artery base excess (± SD) was −0.5 mM (± 1.6) in the supine group (n = 50) versus −0.6 mM (± 1.5) in the tilt group (n = 47) (P = 0.64). During 15 min after spinal anesthesia, mean phenylephrine requirement was greater (P = 0.002), and mean cardiac output was lower (P = 0.014) in the supine group.

Conclusions

Maternal supine position during elective cesarean delivery with spinal anesthesia in healthy term women does not impair neonatal acid–base status compared to 15° left tilt, when maternal systolic blood pressure is maintained with a coload and phenylephrine infusion. These findings may not be generalized to emergency situations or nonreassuring fetal status.

The fetal heart response to static antenatal exercises in the supine position

Continuous fetal heart rate traces were made in twenty-six low risk patients during static maternal exercises in the supine position, as prescribed in a hospital's antenatal education programme. Prior to exercise, fetal heart rate abnormalities were present in four patients. Three of these had an abnormal fetal outcome. During the exercises a further eight cases had reductions in fetal heart rate, variability or reactivity. An abnormal fetal outcome was recorded in two of these cases. The findings indicate uncertainty about the safety of antenatal exercises in the supine position in late pregnancy and, until further studies are available, it is prudent to advocate the practice of all antenatal exercises in a tilted position, and not at all where fetal compromise is suspected.

Perioperative vasovagal syncope with focus on obstetric anesthesia

Vasovagal syncope refers to a reflex cardiovascular depression that gives rise to loss of consciousness with bradycardia and profound vasodilatation. This response commonly occurs during regional anesthesia, hemorrhage or supine inferior vena cava compression in pregnancy. The changes in circulatory response from the normal maintenance of arterial pressure to parasympathetic activation and sympathetic inhibition may cause severe hypotension. This change is triggered by reduced cardiac venous return as well as episodes of emotional stress, excitement or pain. Occasionally, these vasovagal responses may be unpredictable and may dramatically proceed to asystole with circulatory collapse, and may even result in death. In these circumstances, hypotension may be more severe than that caused by bradycardia alone, because of unappreciated vasodilatation. Regional anesthesia, decreased venous return, hemorrhage and abnormal fetal presentation cumulatively increase the risk of vasovagal syncope in cesarean section patients. When a vasovagal response occurs, ephedrine is the drug of first choice because of its combined action on the heart and peripheral blood vessels. Epinephrine must be used early in established cardiac arrest, especially after high regional anesthesia.

Perioperative bradycardia and asystole: relationship to vasovagal syncope and the Bezold-Jarisch reflex

Reflex cardiovascular depression with vasodilation and bradycardia has been variously termed vasovagal syncope, the Bezold-Jarisch reflex and neurocardiogenic syncope. The circulatory response changes from the normal maintenance of arterial pressure, to parasympathetic activation and sympathetic inhibition, causing hypotension. This change is triggered by reduced cardiac venous return as well as through affective mechanisms such as pain or fear. It is probably mediated in part via afferent nerves from the heart, but also by various non-cardiac baroreceptors which may become paradoxically active. This response may occur during regional anaesthesia, haemorrhage or supine inferior vena cava compression in pregnancy; these factors are additive when combined. In these circumstances hypotension may be more severe than that caused by bradycardia alone, because of unappreciated vasodilation. Treatment includes the restoration of venous return and correction of absolute blood volume deficits. Ephedrine is the most logical choice of single drug to correct the changes because of its combined action on the heart and peripheral blood vessels. Epinephrine must be used early in established cardiac arrest, especially after high regional anaesthesia.

Orthostatic intolerance. A historical introduction to the pathophysiological mechanisms

Several of the pathophysiological mechanisms resulting in orthostatic intolerance (ie, tachycardia) have been recognized individually over the course of the past 100 years or more. More recent definitions of the normal ranges of orthostatic blood pressure and heart rate changes have facilitated the recognition of pathogenetic disorders that are probably shared in various proportions between orthostatic intolerance and various types of orthostatic hypotension. These include autonomic dysfunction of (1) the leg veins almost invariably causing excessive gravitational blood pooling, usually associated with (2) hypovolemia of circulating erythrocytes and plasma that is probably attributable to impaired autonomic stimulation of erythropoietin production, renin release, and (less consistently) aldosterone secretion. Improved understanding of these apparent results of lower body dysautonomia should facilitate more effective therapy in the future.

Autonomic cardiovascular control in pregnancy

Pregnancy is associated with profound adaptive changes in the maternal hemodynamics. Although the autonomic nervous system plays a central role in the adaptation of the cardiovascular system to various needs, its role in the adaptation of the circulation to the demands of pregnancy is poorly understood. This paper reviews the literature of autonomic cardiovascular control in pregnancy as studied with the cardiovascular reflex tests. A Medline search and manual cross-referencing for prior publications were used. All papers found on the hemodynamic effects of the Valsalva maneuver, the orthostatic test, the deep breathing test, the isometric handgrip test and maternal heart rate variability in pregnancy were reviewed and all publications that studied short-term changes in maternal heart rate and blood pressure were included. The beginning of pregnancy is associated with sympathetic reactivity, whereas the latter half of pregnancy is characterized by increased hemodynamic stability during orthostatic stress. The heart rate response to the Valsalva maneuver is blunted in mid-pregnancy, possibly due to changes in the baroreflex and increased maternal blood volume. Heart rate variability is significantly reduced in the second trimester. Cardiovascular reflex tests can be used to study drug effects on maternal circulation non-invasively.

An orthostatic uterovascular syndrome--a prospective, longitudinal study

OBJECTIVE

The interaction between maternal hemodynamics and uterine activity in the upright position was investigated longitudinally (358 measurements) in 40 healthy pregnant women from 20 gestational weeks to term.

STUDY DESIGN

Maternal-fetal hemodynamic parameters and uterine contractions were measured noninvasively in four different postures.

RESULTS

Hemodynamic disturbances caused by compression of pelvic vessels by the gravid uterus in the upright position were detected in two of 40 (5%) women as early as 24 weeks' gestation; a peak was reached at 38 weeks (71%). With a decrease in the stroke volume (22%, p < 0.001) neither the cardiac output (-11%, p < 0.05) nor the systolic blood pressure (-1.4%, p < 0.05) remained constant, although there was a compensatory heart rate increase.

CONCLUSION

A significantly increased number of spontaneous uterine contractions in the upright position is associated with release of the blocked venous return flow and restoration of normal maternal hemodynamics.

The implication of upright posture on pregnancy

Pregnant women spend more than half of the day in an upright position. The physiological effects of this posture on the mother and the fetus are evaluated. Changes in vascular autoregulation and anatomy lead to maternal fainting in about 8% of women during early pregnancy. The immediate effects of such episodes on the fetus are unknown. There is a positive correlation of orthostatic dysregulations and abortions. In late pregnancy we found a significant increase in functional residual capacity in the upright posture. Minute volume and oxygen consumption were also significantly increase (p less than 0.001). Regarding the cardiovascular changes we detected a rhythmic change of the maternal heart rate with the change to upright position, which had not been published before. Change from the left lateral position to unsupported standing increased maternal heart rate by a mean of 27 beats per minute and a mean duration of 105 seconds in two thirds of the women. This was accompanied by a decreased cardiac output, systolic blood pressure and an increased oxygen consumption. The gravid uterus is responsible for these changes. During the upright position, the venous flow to the right ventricle is inhibited by the relaxed uterus. Contractions, leaning forward and the muscle pump improve the venous return. The phenomenon reached its maximum during the 38th week, where 71% of pregnant women displayed a cyclic change in heart rate. The fetal heart rate baseline is significantly increased in the upright position with a significantly reduced acceleration frequency (p less than 0.001). Combined with the data from epidemiologic studies, prolonged standing during late pregnancy may signal potential risks for the fetus such as low birth weight, prematurity and stillbirths because of an 'uterovascular syndrome'. Maternal standing possibly may be used as a physiological fetal stress test.

Premature contractions: are they caused by maternal standing?

In 33 out of 51 women studied in late gestation, the uterus was found to phasically compress the pelvic vessels and impede the venous blood flow during quiet standing. This caused a reduction of the cardiac stroke volume with resultant reduction of systemic blood pressure and a compensatory increased heart rate (range of increases 9-51 beats/min). In all cases uterine contractions (mostly subclinical) coincided with the phase of circulatory readjustment. Apparently, the contracting uterus, by changing its position and/or shape, relieves the venous obstruction and prevents decompensation. In the women displaying the uterine compression syndrome (UCS), uterine activity was markedly increased in standing compared to the left recumbent position. It was also investigated whether the UCS appeared more often and earlier in gestation in women with twins. In all 9 women with twin pregnancies (mean gestational age 28 5/7 weeks) the UCS associated with uterine contractions was apparent in the standing posture. Although at present no definite conclusions can be reached on the effect on the cervix of these contractions, quiet standing especially in twin pregnancies seems to provoke an increased uterine activity and should therefore be avoided.

The oscillating 'vena cava syndrome' during quiet standing--an unexpected observation in late pregnancy

While studying the lung function of pregnant women at term in four different postures, we were surprised to note marked cyclic accelerations in the heart rate in two-thirds of the women when in a standing position. The mean cycle length was 105 s (range: 1-4 min) and the amplitude had a mean of 27 beats/min (range: 9-51). Blood flow velocity measurements with ultrasound Doppler over the femoral vein showed that there was an intermittent reduction of flow during quiet standing. When the venous return ceased, maternal heart rate increased, cardiac output decreased and blood pressure fell. After the venous blood flow was restored, maternal heart rate, cardiac output and blood pressure returned to normal until the cycle started again. Concomitant with these maternal heart rate changes, different patterns of fetal heart rate were observed. About 70% of the fetuses showed reduction in the long-term variability, increase in fetal heart rate or periodic accelerations. Although no woman fainted during quiet standing, the maternal circulatory changes were consistent with those seen in the classical vena cava syndrome.