Bamber, J. H. Mmed Sci, FRCA; Dresner, M. MB, BS, FRCA
Department of Anaesthesia, Leeds General Infirmary, Leeds, United Kingdom
Accepted for publication February 28, 2003.
Address correspondence and reprint requests to Dr. J. H. Bamber, Department of Anaesthesia, Box 93, Addenbrooke’s NHS Trust, Hills Rd., Cambridge CB2 2QQ. Address e-mail to firstname.lastname@example.org.
Presented in part at the Obstetric Anaesthetists’ Association annual scientific meeting, Winchester, UK, May 2000.
When women in late pregnancy lie flat on their backs, the gravid uterus completely occludes the inferior vena cava (1) and laterally displaces the subrenal aorta (2). This aortocaval compression reduces maternal cardiac output (3–6), an event that is often concealed, because only 10% of pregnant women will exhibit supine hypotension syndrome (7,8).
Although some pregnant women will naturally avoid lying supine (9), they are often put into this position during their medical care. To alleviate the consequent aortocaval compression, gravity is often used to laterally displace the uterus. This is done either by placing a wedge under the woman’s hip or by tilting the operating table. Despite this common practice, there has been very little study as to what constitutes an adequate amount of lateral tilt and whether more tilt is better than less. We could not find any study that investigated the effect on maternal cardiac output of varying the amount of lateral tilt.
We therefore designed a prospective randomized study to test the hypothesis that increasing the amount and direction of lateral tilt has a significant effect on maternal cardiac output.
With local Ethics Research Committee approval and written informed consent, volunteers with normal, healthy, singleton pregnancies in the third trimester were recruited from antenatal wards and clinics.
All volunteers had their height and weight measured. Each volunteer was asked to lie in 7 positions: lying horizontal on their left or right side (left lateral and right lateral), lying horizontal on their back (supine), lying on their back but tilted laterally to the left or right (left 12.5°, left 5°, right 5°, and right 12.5°). Women with a history of supine hypotension were not placed in an untilted supine position. The order in which the positions were adopted was randomized. The tilt positions were achieved by tilting an operating table. One complete revolution of a screw handle tilted the table laterally by precisely 2.5°.
Each position was adopted for a total of 5 min: 2 min were allowed to let the volunteer settle and then measurements were made over a 3-min time period. Cardiac output, stroke volume, and heart rate were measured continuously using bioimpedance cardiography with the BoMed® NCCOM3-R7 monitor (BoMed Manufacturing, Irvine, CA.) Blood pressure was measured automatically over the left brachial artery.
The sample size of 32 women was calculated to be sufficient to detect a 20% difference in cardiac output between positions for a Type 1 error probability of 0.05 and a Type 2 error probability of 0.2. Thirty-seven women were approached to enter the study, and 34 volunteered. One woman did not complete the study because of feeling faint even in the lateral tilt position.
Statistical analysis was by a one-way analysis of variance with the multiple comparison test of Bonferroni (SPSS 7.0 for Windows; SPSS, Chicago, IL)
The demographics of the volunteers are shown in Table 1. The mean measurements for cardiac output, stroke volume, and systolic blood pressure were tabulated against each position. The positions were ordered as if the subjects had rotated from the left lateral position through to the right lateral position (Table 2).
Stroke volume and cardiac output were greatest when the women lay on their left side and least when they lay on their back with the table tilted laterally to the right (Fig. 1, Table 2). There was a mean 17% reduction in cardiac output when women lying on their back were tilted laterally to the right compared with the cardiac output measured when they were lying on their left side (Table 3). When the women were supine, increasing the amount of lateral table tilt to the left or right seemed to have no effect on cardiac output.
We were unable to demonstrate that increasing the amount and direction of lateral table tilt has a significant effect on maternal cardiac output in healthy pregnant women. Maternal cardiac output was significantly reduced when maternal position was changed from lying on the left side to lying supine with lateral table tilt to the right. This observed reduction in cardiac output may not have been clinically important in our study population of healthy volunteers but may have greater importance in women with compromised cardiac output or uteroplacental circulation.
Previous studies examining the benefits of a lateral tilt position compared with the supine position have found that umbilical venous oxygenation is improved (10–12). Investigations of the effect of increasing the amount of lateral tilt have been limited to measuring maternal blood pressure, fetal heart rate, and maternal toe pulse pressure (13,14). No significant differences for variable tilt positions were found for any of these outcome measures except maternal toe pulse pressure, which was found to increase with lateral tilt up to 35°(14).
We used an operating table to tilt our subjects because it allowed accurate measurement and replication of each amount of tilt and 12.5° lateral tilt was the maximum possible. A survey of the various operating tables used in our institution found the maximal lateral tilt to be 15°. Twenty-seven degrees is the maximal angle to which nonpregnant volunteers can be laterally tilted without sliding off (15). Most pregnant women in our study found even 12.5° to be intimidating and we would suspect that in clinical practice the tilt is often much <12.5°. Alternate methods of uterine displacement, such as use of a wedge, may be better tolerated or more effective than table tilt and would warrant further investigation.
Cardiac output was chosen as our outcome measure because it is directly affected by aortocaval compression and is the key determinant of placental blood flow in healthy pregnancies (16,17). Cardiac measurement by thoracic bioimpedance remains controversial. The technique depends on measuring a complex dynamic process and using simplistic assumptions to indirectly calculate cardiac output (18). However, thoracic bioimpedance has been shown to be as reliable as thermodilution techniques in measuring cardiac output trends (19–21). In our study, each volunteer acted as her own control for each position change and trends were measured.
There may have been a bias in this study toward women who could compensate for aortocaval compression. Our findings might not be replicated in women who exhibit supine hypotension or who have a regional anesthetic.
In conclusion, to avoid any detrimental effect on maternal cardiac output, the pregnant woman would ideally be kept in the full lateral tilt position, but this is often impractical. When it is necessary to lay the woman on her back, then tilting her to the left is preferable. We were unable to demonstrate any advantage by increasing the amount of lateral table tilt up to 12.5°.
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