Lateral tilt for pregnant women: why 15 degrees?

Maternal cardiovascular compromise and fetal stress in the supine position and their relief in the full lateral position are well recognised [1-3]. The systematic use of lateral table or pelvic tilt to reduce supine aortocaval compression during Caesarean section (CS) developed in the 1970s, and is now ingrained in obstetric anaesthetic practice. Crawford designed a wedge-shaped cushion that was placed under the hip to tilt the pelvis away from the horizontal and therefore displace the uterus. The upper surface of this was, possibly arbitrarily, angled at 15°[4]. Other potentially less precise means used to achieve pelvic tilt include air-filled 3-litre fluid bags or rolled sheets, and commonly the whole operating table is tilted laterally. However, as in George Orwell's ‘Animal Farm’, the original message has gradually changed. Lateral tilt was introduced to reduce the effects of aortocaval compression in the supine position, when the full lateral position was not practical [4]. It has come to be seen as a manoeuvre that may completely prevent or avoid aortocaval compression [5, 6]. But there is strong suggestive evidence that lateral tilt, as used in routine practice, does not relieve aortocaval compression completely. Cardiac output increased when manual displacement of the uterus was performed in women who were already in the tilted position for CS [7]. There are reports of severe maternal cardiovascular collapse during CS in women who were apparently adequately tilted [8, 9]. It is also well established that, during maternal cardiopulmonary resuscitation, inferior vena cava compression may prevent the re-establishment of cardiac output until the fetus is delivered [10]. A possible explanation for these findings is that tilt is not being employed correctly. Crawford specified both a labour and an operating wedge [4], but the latter is narrower than the pelvis, and a sponge rubber wedge will be compressed to a variable extent by body weight. Lateral tilt of an operating table should be more consistent, but may be inadequately applied. Morgan et al. performed two studies on thiopental pharmacokinetics during CS. Maternal and umbilical venous blood levels of thiopental were significantly more variable in the first study, when tilt was estimated conventionally, than in the second study where it was directly measured [11]. They noted that ‘estimating the angle [of lateral table tilt] by eye is grossly inaccurate, the true angle being much smaller than the estimated angle’, and suggested that significant aortocaval compression in inadequately tilted subjects during the first study led to the variation in thiopental venous concentrations [11]. Jones et al. came to similar conclusions: tilt was overestimated by 12 out of 16 anaesthetists, and all but one applied less than 15°[12]. A simple protractor and plumb line attached to the operating table can provide a surprising insight into this! If tilt has been applied inadequately in previous research, then conclusions relating to its lack of effectiveness may be suspect. However, a small number of studies of maternal haemodynamics or fetal effects have explicitly stated that there were direct measurements of the angle of the pelvis or the surface on which the woman was lying. What are the conclusions here? Firstly, there is individual variability in the susceptibility to aortocaval compression. Morgan et al. found evidence of aortocaval compression, assessed by perfusion time to the leg, in two out of six women tilted to between 8° and 10° but none in three tilted to 15°–18°[11]. Another study found that seven out of 27 women had an increase in heart rate of greater than 10 beat.min−1 during a 5-min test period in the supine position prior to CS. These women were more likely to develop severe hypotension during surgery in the supine position with 15° pelvic tilt than those who did not [13]. An increase in heart rate indicates reduced venous return from supine inferior vena cava compression, and this effect persists in spite of ‘standard’ pelvic tilt. If women do exhibit problems, increasing the tilt may be effective. A recent paper in this journal by Mendonca et al. [14] compared the 12° supine tilted position to the full left lateral position for management of women after a combined spinal-epidural and before CS. Women were allocated to one or other position two minutes after block insertion until just before the start of surgery. The lateral position was associated with less hypotensive episodes and less use of ephedrine than the 12° tilt position. In five out of 38 women in the tilt position, hypotension was corrected merely by increasing the amount of tilt from 12° to 20°. It was noted that, at the point that women managed lateral after the spinal are repositioned supine with tilt prior to CS, further doses of ephedrine may be required [14-16]. This tends to cancel out the lower use of ephedrine just after the spinal, although it may be easier to manage hypotension from a single cause rather than that resulting from a combination of evolving spinal as well as inferior vena cava compression. Other studies have investigated the intra-individual effect of altering the amount of tilt. Cardiac output changes with position are commonly taken to reflect the effects of inferior vena cava compression. Bamber and Dresner studied 33 women during the third trimester in seven positions including supine, both lateral, as well as left and right 5° and 12.5° tilt [17]. Mean cardiac output was significantly different when comparing the left lateral position to both right tilted positions. However, there was no effect of increasing left tilt from 5° to 12.5°, and possibly a decrease in cardiac output when increasing the rightward tilt. A further study using the same methodology found that varying left tilt angles between 2.5° and 12.5° did not affect cardiac output, whether before or after establishment of spinal anaesthesia for CS [18]. Although most assessment methods cannot distinguish the separate components of aortocaval compression, aortic compression may be assessed in isolation using arterial pressure measurements in the leg. During labour, aortic compression is present in about 30% of women at the 10° range of pelvic tilt [19], a similar incidence to that found in the unmodified supine position [20]. Although steeper amounts of tilt reduce the frequency of aortic compression, it may still be identified at over 30° of tilt [19]. After establishing spinal anaesthesia for elective CS, the lowest systolic pressure in the leg in supine women with 15° table tilt was on average 10 mmHg less than women in the full lateral position [21]. Fetal effects of maternal position follow the trend found in maternal cardiovascular effects, such that lesser degrees of tilt are comparable to supine whereas large tilt angles are similar to the lateral position. Umbilical artery Doppler flow velocity waveforms were similar with supine, 5° or 10° left or 5° or 10° right tilt [22]. On the other hand, there was no difference in fetal heart rate patterns after regional analgesia in women managed supine with measured 30° lateral tilt compared to those who laboured in the lateral position [23]. However, significant fetal problems may develop in a small minority of women who have persisting severe aortic compression with this amount of tilt, that will be relieved on turning to the full lateral position [19]. The need for more than the routine 15° lateral tilt in order to minimise aortocaval compression was acknowledged during the construction of the Cardiff resuscitation wedge. This has a surface angled at 27°, the maximum tilt found to be compatible with effective external cardiac massage [10]. Although the lateral position has been recommended during establishment of elective spinal anaesthesia [15, 21], it has been argued that staff may risk back injury while bodily moving heavy women with fully developed regional anaesthesia from the lateral position back to supine. In such cases, the supine tilted position immediately after spinal insertion may be preferable [24]. It would seem prudent that if the supine tilted position is used during establishment of regional block, the largest feasible amount of tilt is used. It is then reduced to the maximum at which the surgeon is able to operate just before surgery commences. Women feel insecure when table tilt is applied in effective amounts. As an operating table was tilted laterally, normal volunteers first expressed concern at a mean angle of 9° (range 4° to 14°) [Jones S & Donald FA. Personal communication]. The use of a combination of pelvic tilt and lateral table tilt may be ideal [17]. To summarise these findings, inferior vena cava compression is demonstrable up to 12.5°–15° of lateral tilt, and aortic compression up to 30°. The use of 15° of tilt at CS and 30° during labour reduces the effects considerably, but a minority of women may have particular susceptibility even at these levels. However these amounts of tilt are probably infrequently used, even if intended. Crawford may have arrived at a figure of 15° by chance, but it seems that this amount of tilt, applied correctly, is needed to reliably reduce inferior vena cava compression. I would like to thank Dr Jim Bamber for his helpful advice.