This Invited Commentary accompanies the following article:
Tsai S-E, Yeh PH, Hsu PK, et al. Continuous haemodynamic effects of left tilting and supine positions during Caesarean section under spinal anaesthesia with a noninvasive cardiac output monitor system. Eur J Anaesthesiol 2019; 36:72–74.
In this issue of the European Journal of Anaesthesiology, Tsai et al. 1 publish their comparison of cardiac output (CO) with and without tilt at caesarean section. Their finding that there was no significant difference between the two positions is not new and is perhaps unsurprising, but their work provides us with an opportunity to reflect on the practice of tilt during caesarean section.
Supine hypotension is caused by compression of the inferior vena cava in the supine position by the pregnant uterus, resulting in a fall in CO that is generally, but not always accompanied by a fall in blood pressure. It is the latter that will alert the anaesthesiologist to the fact that something is amiss. Scott's 2 venograms provided visual evidence of impaired venous return that was irrefutable, paving the way for adoption of the left tilt into routine clinical practice.
A heavily pregnant uterus may totally obliterate the inferior vena cava. Dense sympathetic blockade, transverse lie, 3 nonengagement of the foetal head and multiple pregnancy all add to this risk. Although partial loss of venous return might result in some maternal hypotension and neonatal acidosis, inferior vena caval obstruction, without a collateral circulation, will gradually sequestrate the circulating volume in the lower half of the body. The stroke volume falls progressively as the heart empties, and cardiac arrest ensues. Sudden deaths during the early days of spinal anaesthesia, before preload, tilt and vasopressor, were attributed to this cause. 4 Once flow through the vena cava has ceased, fluids and vasopressor may not be sufficient to remedy the problem. The full lateral position should permit restoration of CO provided that it is not too late and significant acidosis has not occurred. This can be a life saving measure.
Scott 2 wrote ‘It has become clear that inferior vena caval compression is the rule rather than the exception in the supine position in advanced pregnancy. Only occasionally does it result in a gross reduction of arterial pressure’. He goes on to warn that hypotension does not always accompany a reduction in CO. It is reasonable to believe that preload, tilt and vasopressor will reduce both the incidence and severity of supine hypotension and help preserve CO. Methods for vasopressor administration are varied and are beyond the scope of this article. 5 However, we cannot predict for any one individual whether or not she is liable to get supine hypotension, though women with the risk factors given above and those who report that they cannot lie flat in comfort must be regarded as higher risk. This creates a problem for the researcher who does not know how many women in any particular cohort might experience supine hypotension. A comparison of two groups of largely low risk women might be expected to show that the intervention is largely ineffective. Results must be interpreted with caution.
The first devices to prevent supine hypotension were intended to displace the uterus to the left either directly or by a pelvic wedge. Crawford et al. 6 were the first to suggest the use of a wedge to tilt the pelvis and its dimensions gave it an angle of 15° from the horizontal. These dimensions were more the result of practical application than lengthy research. Devices to push the uterus to the left complicated surgery and removing them involved disturbing surgical drapes. If the wedge was placed too high it tended to splint the lumbar spine and make positioning for intubation more difficult. Abandoning the wedge and tilting the table allowed the anaesthesiologist control from the top.
When a woman in late pregnancy is tilted to the left, the leftward movement of the uterus is augmented by gravity. Angles of pelvic tilt can exceed 20° with a pelvic wedge or less than 15° of table tilt, and it is left uterine displacement that is the object of the exercise. 7 There is no evidence to indicate that there is a magic number for the degree of left uterine displacement, it merely needs to be sufficient from the point of view of maternal well being. The table tilt angle is used as a surrogate for left uterine displacement and the relationship between the two is not constant.
Nobody measures the tilt in clinical practice anyway.
Personal experience and anecdotal reports indicate that the degree of table tilt is estimated rather than measured in routine clinical practice. Anaesthesiologists are reluctant to tilt at angles that give them concerns regarding safety and make women feel insecure. Consequently tables are rarely tilted to 15°. There is no evidence to indicate that current practice in this regard gives rise to a clinical problem.
Achieving 15° table tilt is irrelevant given that our goal is actually left uterine displacement. If we cannot predict what any individual woman before us needs we must be alert to the possibility of supine hypotension and react accordingly rather than blindly follow dogma. In the event of clinical evidence of falling CO, the recommendation is to turn the woman into the full left lateral position.
Tsai et al., through their results, are inviting us to consider whether tilting the table is necessary. Given that supine hypotension is a genuine clinical entity and that tilting is a low-risk procedure, then there is logic in applying tilt as a default procedure. Because there is no evidence to suggest a best angle of tilt it is not possible to make a recommendation as to how much tilt should be applied. For the same reason, it makes little sense to recommend that tilt is routinely measured. Maternal security and the practical need for surgical access limit the amount of tilt that can be applied.
The welfare of the neonate was not considered by Tsai et al. except in terms of Apgar scores. The lack of biochemical analysis may be forgiven as Lee et al. have shown that table tilt does not affect neonatal acidosis. 8
Until new evidence becomes available the practice of tilt and uterine displacement should still be observed, but the angle remains a matter of clinical judgement.
Those who require an in depth dissection of this topic are directed to Lee and Landau. 9
Acknowledgements relating to this article
Assistance with the commentary: none.
Financial support and sponsorship: none.
Conflicts of interest: PK has received speaker fees from TEVA ratiopharm and Fresenius Kabi.
Comment from the Editor: GL is a language and technical editor and PK is an associate editor of the European Journal of Anaesthesiology. This Invited Commentary was checked and accepted by the Editors, but was not sent for external peer review.
1. Tsai S-E, Yeh PH, Hsu PK, et al. Continuous haemodynamic effects of left tilting and supine positions during Caesarean section under spinal anaesthesia with a noninvasive cardiac output monitor system. Eur J Anaesthesiol
2. Scott DB. Inferior vena caval occlusion in late pregnancy and its importance in anaesthesia. Br J Anaesth
3. Kinsella SM, Spencer JAD. Transverse lie is a risk factor for supine inferior vena cava compression. J Obstet Gynaecol
4. Holmes F. Spinal analgesia and caesarean section. J Obstet Gynaecol Br Emp
5. Sng L, Du W, Lee MX, et al. Comparison of double intravenous vasopressor automated system using nexfin versus manual vasopressor bolus administration for maintenance of haemodynamic stability during spinal anaesthesia for caesarean delivery. A randomised double-blind controlled trial. Eur J Anaesthesiol
6. Crawford JS, Burton M, Davies P. Time and lateral tilt at caesarean section. Br J Anaesth
7. Kinsella SM, Harvey NL. A comparison of the pelvic angle applied using lateral table tilt or a pelvic wedge at elective caesarean section. Anaesthesia
8. Lee AJ, Landau R, Mattingley J, et al. Left lateral table tilt for elective cesarean delivery under spinal anesthesia has no effect on neonatal acid–base status. Anesthesiology
9. Lee AJ, Landau R. Aortocaval compression syndrome: time to revisit certain dogmas. Anesth Analg