The World Health Organization (WHO) has classified obesity according to body mass index (BMI) (Table 148.1). In the UK, around one in five pregnant women is obese by the WHO definitions. Obesity in pregnancy has been defined as weight > 91 kg, or > 110-120% of ideal body weight, or BMI > 30 kg/m2 at the first antenatal consultation. The obese mother presents significant challenges to the obstetric anaesthetist, and obesity has been highlighted as an important factor contributing to maternal mortality.
Problems and special considerations
The physiological changes that occur in pregnancy already put the parturient at risk, and obesity puts further stress on the limited physiological reserve of the pregnant mother.
The risks of diabetes, hypertension, coronary artery disease and obesity-related cardiomyopathy are all increased. Airway closure may occur within tidal volume, especially in supine and semi-supine positions. A small number of morbidly obese women may develop secondary pulmonary hypertension and chronic right ventricular failure.
Antenatal assessment, including accurate estimation of gestation, may be difficult.
Airway-related morbidity and mortality is increased in obese parturients, who have a high incidence of symptomatic reflux and are at greater risk of gastric aspiration, and of difficult or failed intubation.
Aortocaval compression will occur in all but the full upright and full lateral positions, owing to the large pannus.
Fetal growth restriction is possible, as well as the more commonly occurring macro- somic fetus. Malpositions are common, and there is a higher incidence of birth defects and stillbirth. Furthermore, monitoring the fetus ante- and intrapartum is more difficult in the obese patient.
Table 148.1 World Health Organization classification of obesity
|
Body mass index |
|
|
Underweight |
< 18.5 kg/m2 |
|
Normal |
18.5-24.9 kg/m2 |
|
Overweight |
25.0-29.9 kg/m2 |
|
Obese class 1 class 2 class 3 |
30.0-34.9 kg/m2 35.0-39.9 kg/m2 > 40.0 kg/m2 |
The obese mother has an increased likelihood of developing pre-eclampsia, of requiring operative delivery (a risk of caesarean section of 30-50% has been reported) and of postpartum haemorrhage. Thromboembolic disease and infective postoperative complications are also more common.
The massively obese woman may not fit on a standard operating table, and she may exceed the weight limit of a standard hospital lift.
Intravenous access and non-invasive monitoring of the mother may be difficult. The use of invasive arterial monitoring has been suggested where it is thought that the blood pressure cuff may not give accurate blood pressure measurements.
Regional anaesthesia may be difficult, and there is an increased risk of epidural failure and dislodgement of the epidural catheter in obese patients.
Management options
Thromboprophylaxis should be used, preferably with low-dose heparin (in increased doses; Table 148.2), and graduated compression stockings should be worn for the entire hospital admission - this alone needs special consideration, since it may be difficult or even impossible to find stockings that fit effectively. Specialised equipment such as a bariatric operating table or hoist must be made available as required. H2-antagonists and antacids should be used throughout labour.
Difficulty in securing intravenous access should be anticipated, as should difficult tracheal intubation. For labour, the benefits of regional analgesia usually outweigh the risks of epidural haematoma resulting from heparin prophylaxis. Early use of epidural analgesia should be recommended, since the risk of obstetric intervention is greater. Although identification of landmarks is difficult, standard-length needles can be used for the majority of women. The use of ultrasound may help to identify the midline and the need for a longer needle. The lowest effective concentrations of local anaesthetic combined with an opioid should be used; combined spinal-epidural (CSE) analgesia offers a suitable alternative. The aim should be to minimise any motor blockade while providing effective analgesia. There is some anecdotal evidence suggesting that the incidence and severity of postdural puncture headache is reduced in obesity, perhaps because of increased intra-abdominal pressure. Once an epidural has been sited, these patients should be reviewed regularly during labour, and there should be a low threshold to re-site a non-functioning epidural early.
Table 148.2 CMACE/RCOG recommended daily doses of low-molecular-weight heparin in obese parturients
|
Weight |
Enoxaparin |
Dalteparin |
Tinzaparin |
|
91-130 kg |
60 mg a |
7500 units |
7000 units a |
|
131-170 kg |
80 mg a |
10,000 units |
9000 units a |
|
> 170 kg |
0.6 mg/kg a |
75 units/kg |
75 units/kg a |
a May be given in two divided doses.
Reproduced with permission from RoyalCollege of Obstetricians and Gynaecologists. Reducing the Risk of Thromboembolism During Pregnancy and the Puerperium. Green-top Guideline 37a. London: RCOG, 2015. www .rcog.org.uk/en/guidelines-research-services/guidelines/gtg37a.
For caesarean section, regional anaesthesia is usually recommended in preference to general, and an existing epidural can usually be extended for emergency delivery. Where a de novo block is required, CSE anaesthesia may be preferable to single-shot spinal anaesthesia, because it allows better control over the final height of the block and surgical difficulty may lead to prolonged operating time. It is important to pay meticulous attention to avoidance of aortocaval compression.
If general anaesthesia is necessary, the risks of hypoxia and regurgitation of gastric contents should be assumed to be higher than in the non-obese pregnant woman. Adequate preoxygenation is essential, and tipping the operating table head-up may help to increase the functional residual capacity (FRC) and improve the efficiency of preoxygenation.
Difficulty with tracheal intubation should be anticipated and suitable aids should be readily available (see Chapter 38, Failed and difficult intubation). Often, extra pillows are required under the patient’s shoulders and neck to position the mother optimally. The so- called ‘ramped’ position (external auditory meatus and sternal notch in the same horizontal plane) has been advocated in obese patients as this is thought to be associated with the optimal position for intubation as well as improving the FRC; it may be achieved using ordinary pillows and/or blankets or specific wedge-shaped pillows, possibly also altering the configuration of the operating table.
Trained and experienced anaesthetic assistance is essential, and the presence of a second anaesthetist is desirable.
In order to ensure good oxygenation, higher tidal volumes, the use of positive end- expiratory pressure (PEEP) and a higher inspired oxygen concentration have been recommended. Residual neuromuscular blockade has been implicated in maternal death and is a particular hazard in the obese woman. A peripheral nerve stimulator should be used to confirm reversal of neuromuscular blockade, and the trachea should be extubated with the patient in a slightly head-up position. Recovery from general anaesthesia should take place in a well-lit recovery area under the supervision of trained recovery staff.
Good postoperative analgesia, for example with epidural or spinal opioids, is important to allow early mobilisation. Abdominal wall blocks performed intraopera- tively and intravenous patient-controlled opioid analgesia in the postoperative period are recommended for women in whom the central neuraxial route is unavailable. Postoperative physiotherapy should be provided, and high-dependency midwifery care should also be available.
Key points
• The obese mother has an increased risk of obstetric and anaesthetic complications.
• Early regional analgesia for labour should be encouraged.
• Difficulty with tracheal intubation should be anticipated.
• Thromboprophylaxis should be used in appropriate dosage.
Further reading
Centre for Maternal and Child Enquiries;
Royal College of Obstetricians and Gynaecologists. Management of Women with Obesity in Pregnancy. London: CMACE/RCOG, 2010. www.rcog.org.uk/womens-health/clinical-guidance/ management-women-obesity-pregnancy (accessed December 2018).
Ellinas EH. Labor analgesia for the obese parturient. Anesth Analg 2012; 115: 899-903.
Lim CC, Mahmood T. Obesity in pregnancy. Best Pract Res Clin Obstet Gynaecol 2015; 29: 309-19.
Mace HS, Paech MJ, McDonnell NJ. Obesity and obstetric anaesthesia. Anaesth Intensive Care 2011; 39: 559-70.
Uzoma A, Keriakos R. Pregnancy management following bariatric surgery. J Obstet Gynaecol 2013; 33: 109-14.