A comparison of the pelvic angle applied using lateral table tilt or a pelvic wedge at elective caesarean section

Authors


Correspondence to: S. M. Kinsella
Email: stephen.kinsella@uhbristol.nhs.uk

Summary

Lateral table tilt or a pelvic wedge are commonly used to reduce inferior vena cava compression during obstetric anaesthesia in the supine position. Direct measurement of pelvic angle allows individual assessment of the effectiveness of these manoeuvres in achieving a tilted position. We observed routine practice during caesarean section after random allocation to one or other of these methods. The anaesthetist managing the case was asked to position the women after induction of spinal anaesthesia using either left table tilt or a wedge under the right hip. We then measured pelvic angle in all women, and the table angle in women who had table tilt. The mean (SD [range]) pelvic angle was 20.2° (8.1° [9°–37°]) in 18 women with table tilt and 21.0° (7.5° [10°–36°]) in 17 women with a wedge. The mean (SD [range]) table angle was 12.4° (3.1° [8°–21°]) in the women with table tilt. There was a significant difference between table angle and pelvic angle in the women with table tilt (p = 0.0003), but no significant difference in pelvic angle between the table tilt and wedge groups. Measurement of table angle does not represent pelvic position adequately in the majority of women. However, this study showed that lateral table tilt and a pelvic wedge were equally effective in producing tilt of the pelvis.

The term gravid uterus compresses the aorta and inferior vena cava in the supine position, which may result in maternal and fetal compromise. Various methods have been used to reduce this effect. Lateral displacement of the uterus by raising one side of the maternal pelvis with a wedge-shaped cushion [1], or the whole body by tilting the operating table surface laterally [2], has been shown to reduce compression in a progressive way. Other methods used to tilt the pelvis include an air- or liquid-filled bag or rolled towels. Lateral uterine displacement while keeping the body completely supine can also reduce inferior vena cava compression, although unlike the preceding methods, it does not also reduce aortic compression [3].

It is suggested that the tipping point for inferior vena cava compression lies at 15°, with an acceptable reduction if angles above this are achieved [4]. The assumption is that the woman herself is tilted to the same extent as the surface she is lying on, whether that is the operating table or the upper inclined surface of a wedge. Although provision of 15° of tilt by one or another method is recommended during anaesthesia, direct measurements have shown that not all women are tilted to this degree using lateral table tilt [5–8] or a wedge [1, 9]. In our unit, we use both these methods, but they have not been formally compared. We wished to establish through observing routine clinical practice whether one method is superior in providing a sufficient and consistent lateral angle of the pelvis. By direct measurement of pelvic angle, we were able to estimate the effectiveness of the wedge as well as any difference between the woman’s position and the tilt of the table.

Methods

Local Research Ethics Committee approval was gained for this research. We recruited women of ASA physical status 1–2 with a singleton fetus having elective caesarean section and verbal consent was obtained. Women were not studied if they had back problems or body mass index (BMI) > 40 kg.m−2, or if there were any fetal problems. Participants were informed that their clinical care would only be influenced by the option of using table or pelvic tilt after spinal insertion. Allocation to either lateral table tilt towards the woman’s left hand side (Table group) or pelvic tilt with a wedge under the right buttock (Wedge group) was performed with a random number list. The wedge dimensions were 56 cm in length, 38 cm in width and 9 cm in height.

As the aim of the study was to observe routine clinical practice, it was important that the anaesthetist managing the woman for her operation was not aware of the study before the angle measurements were made. Hence, each anaesthetist only participated once. The study was performed over 18 months, as trainee anaesthetists rotated through the maternity unit. In our unit, spinal anaesthesia is normally performed with the woman in a flexed right lateral position, following which she turns to the left lateral position. When appropriate to position the woman supine for surgery, the anaesthetist was asked by the study supervisor to provide suitable lateral tilt using one or other method under study. The supervisor then measured the pelvic angle in all patients, and table angle in the Table group using a protractor device (Fig. 1) [10]. For measurement of pelvic angle, the legs of the device were placed on the anterior superior iliac spines. The protractor was then zeroed against the plumb line on a horizontal surface and replaced on the spines to read the angle on the protractor. Once these measurements were obtained, the operation was managed according to the clinical circumstances. When the study was completed, the anaesthetist was asked to keep the study concealed from his or her colleagues.

Figure 1.

 The instrument used to measure pelvic or table angle. To measure pelvic angle, the distance between the legs of the instrument is adjusted by placing the ends on the anterior superior iliac spines (a). The instrument is next placed on a flat surface and the protractor is zeroed (b). Finally, it is replaced on the iliac spines and the pelvic angle is read from the protractor.

The null hypothesis was that the two methods produced the same degree of lateral angle. A power calculation was performed using the results of an observational study [5]. A study including 34 participants was predicted to detect a difference of 2.5° in angle between groups, with a power of 0.9 and p = 0.05. The groups were compared statistically using paired and unpaired Student’s t-tests as appropriate.

Results

There were 18 women in the Table group and 17 in the Wedge group. Table 1 shows maternal baseline characteristics. The lateral angle applied by the anaesthetist is shown in Table 2. Four (22%) women in the Table group had a table angle of 15° or greater. Fourteen (78%) women in the Table group and 13 (76%) in the Wedge group had a pelvic angle of 15° or greater.

Table 1. Baseline characteristics of women tilted using lateral table tilt or a pelvic wedge. Values are mean (SD) or number (proportion).
 Table tilt (n = 18)Wedge (n = 17)
  1. BMI: body mass index; CS; caesarean section. *p < 0.05.

Age; years31.8 (4.3)31.3 (5.8)
Height; m1.63 (0.06)1.65 (0.06)
Weight at booking; kg64.6 (10.0)73.9 (11.6)*
Current weight; kg77.5 (9.9)85.3 (11.4)*
Current BMI; kg.m−228.9 (4.1)31.2 (4.0)
Parity
 03 (17%)5 (29%)
 112 (67%)10 (59%)
 22 (11%)1 (6%)
 31 (6%)1 (6%)
Indication for CS
 Previous CS9 (50%)6 (35%)
 Breech3 (17%)7 (41%)
 To avoid vaginal delivery5 (28%)3 (18%)
 Others1 (6%)1 (6%)
Fetal back on maternal side
 Left14 (78%)12 (70%)
 Right4 (22%)5 (30%)
Anaesthetic experience; years3.8 (2.1)5 (3.0)
Table 2. Angle of deviation from horizontal in women tilted using lateral table tilt or a pelvic wedge. Values are mean (SD [range]).
Table tilt (n = 18)Wedge (n = 17)
  1. *p = 0.0003; p = 0.0001.

Table angle; ºPelvic angle; ºPelvic angle; º
12.4 (3.1 [8.0–21.0])*†20.2 (8.1 [ 9.0–37.0])*21.0 (7.5 [10.0–36.0])†

The mean (SD [range]) difference between table angle and pelvic angle in the Table group was 8.0° (7.4° [−4° to 22°]) (Fig. 2). There was no correlation between the amount of table angle and pelvic angle in the Table group.

Figure 2.

 Individual table (diamond) and pelvic (bar) angles in women tilted using lateral table tilt.

Discussion

It is often assumed that a patient’s supine body takes up the same degree of tilt as the surface on which it is lying. This is the rationale behind using a wedge with a 15° incline on the upper surface during obstetric anaesthesia, as well as the measurement of the angle of the table surface to estimate the effectiveness of lateral table tilt in the relief of aortocaval compression.

If we had performed measurements of table angle only, in the Table group, and compared these with pelvic angle in the Wedge group, we would have concluded that the wedge was superior to lateral table tilt. However, the pelvic angle achieved with lateral tilt of the operating table differs from the degree of table tilt. Pelvic angle was greater than table angle in 15 out of 18 patients, with a mean difference of 8°. The explanation is that when the operating table is tilted laterally in a term pregnant woman, the weight of the uterus produces further axial rotation of the abdomen and bony pelvis. This must occur by compression of the soft tissues and table mattress on the dependent side (Fig. 3).

Figure 3.

 Diagram showing the relationship of pelvic and table angle in women during lateral table tilt: (a) the conventional assumption is that table angle is equal to pelvic angle; (b) axial rotation of the pelvis with soft tissue compression produces a discrepancy between table and pelvic angles. NB for diagrammatic purposes the protractor is shown to the side of the operating table.

Although it might be expected that the discrepancy between pelvic angle and table angle would increase as the lateral table angle is increased, we did not find such a relationship. There was also no relationship with other variables that we recorded, such as parity or on which side of the maternal abdomen the fetal spine was lying. Pending further clarification, we conclude that the relationship between pelvic and table angle cannot be predicted in the individual.

Due to this rotational effect, mean pelvic angle was similar between the two groups. Kundra et al. have shown that haemodynamic variables are different in women who are tilted using a Crawford wedge depending on whether the preceding position was supine or lateral [11], although whether this holds true for lateral table tilt has not been investigated. We did not record the preceding position of the women before the tilt measurements were taken, although our routine is to manage women in the full left lateral position during the time between spinal insertion and moving supine for surgery. The similarity of pelvic angle between the two groups suggests that the anaesthetists may have been visually estimating the woman’s abdominal lateral rotation rather than the underlying table tilt, and were not necessarily aligning the woman’s body straight and parallel to the table surface.

The range of pelvic angle achieved when using a wedge indicates that the 15° angle of the upper surface of an operating theatre wedge has little bearing on its effectiveness. The wedge as described by Crawford is narrower than the table width as well as being compressible [12]. The wide variation in the pelvic angle resulting from clinical use of lateral table tilt and wedge is disappointing. It has been suggested that an inflatable wedge provides greater consistency in the pelvic angle and this may be worthy of further study [13].

A high BMI has been shown to reduce the reliability of pelvic angle achieved using a wedge, but not the median value achieved [9]. There was no significant difference in BMI between the groups in this study, but there was a significant difference in weight. However, there was no correlation between pelvic angle in the Wedge group and either booking or current weight. We therefore conclude that the weight difference between groups did not affect our results.

This finding of a difference between pelvic and table angle should lead to a re-evaluation of the literature concerning the effectiveness of lateral tilt where measurements of table angle were taken rather than pelvic angle [4]. Table angles of 12.5° or less may show little difference from supine in measured variables (De Guibert J M, Mercier F J, Roger-Christoph S et al., unpublished results. OAA/CARO Conference, Jersey 2009) [2, 14, 15]. On the other hand, an increase from below 15° tilt to 20° or full lateral can increase cardiac output and reduce hypotension [2, 16]. If the discrepancy between pelvic and table angles is a general finding, the critical angle for lateral tilt of the pelvis to relieve inferior vena cava compression is greater than the 15° table angle suggested by these publications (Fig. 3) [2, 14–16]. Measurement of pelvic angle is unfortunately not as convenient as measurement of lateral table tilt [17, 18].

The ideal solution to the problem of maternal positioning during obstetric anaesthesia would be real-time estimation of aortic and inferior vena cava compression [1, 19]. In the absence of such an individualised approach, current advice is to tilt all women by a standard amount. The method used should result in consistent positioning and improved haemodynamic effects, considering both aortic and inferior vena cava compression [13, 20]. We have shown that pelvic angle applied in our practice is similar when using either lateral table tilt or a pelvic wedge, with over three quarters of cases achieving 15° or above. However, there was a wide variability in the actual angle achieved.

Acknowledgements

No external funding and no competing interests declared. We thank Russ Davidson, Medical Illustration Department, for Fig. 3.

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