Anaesthesia and non-obstetric surgery in pregnancy

Non-obstetric surgery may be required at any trimester during pregnancy, which carries the unique challenge of caring simultaneously for two patients. Understanding the physiological changes of pregnancy enables safe delivery of anaesthesia. Anaesthetists may need to adjust their plans accordingly to account for the specific surgical context and maternal comorbidities. Non-elective surgery should never be denied to pregnant patients, although postponing elective cases until 6 weeks postpartum is recommended.¹ Obstetric and neonatal consultations are critical to the patient’s care, regardless of type of surgery. This article aims to guide anaesthetic management with clinical advice for each trimester.

Fetal considerations

First trimester (weeks 1–12)

The first trimester heralds the start of fetal organogenesis. Researchers have looked for links between anaesthetic agents used and fetal teratogenesis, but nothing is conclusive.^1,2 In the context of an anaesthetic, the following are harmful to the fetus and should be avoided: prolonged maternal hypo-/hyperthermia, hypoxaemia, hypercarbia and hypotension.^2,3 Commonly used preoperative medications, including benzodiazepines or opioids, may be safely offered without concern for fetal cleft palate.² The National Library of Medicine hosts resources on medication teratogenicity: https://www.nlm.nih.gov/toxnet/index.html.

Second trimester (weeks 13–26)

The second trimester is preferred for non-elective surgery. Patients are considered to be at the lowest risk for preterm delivery, surgical exposure is enhanced as the uterus is lower in the abdomen, and major embryonic development is complete by the eighth week.2, 3, 4

Third trimester (weeks 27–40)

Many of the topics relevant to the first and second trimesters also apply to the third trimester, as long as labour has not yet begun. Giving corticosteroids to facilitate fetal lung maturity should be discussed with the consulting obstetrician before surgery.¹ NSAIDs should be avoided after 32 weeks of gestation because of concerns for premature patent ductus arteriosus closure.³

Fetal monitoring

The American College of Obstetricians and Gynecologists recommends fetal heart rate (FHR) measurement using Doppler ultrasound before and after any given surgery regardless of gestational age, with the addition of contraction monitoring in the viable fetus.¹ This recommendation differs from that of the Royal College of Obstetricians and Gynaecologists and is most likely a result of the increased risk of malpractice litigation. The decision to monitor FHR during surgery is specific to the patient and often based on institutional guidelines. Most anaesthetic agents cross the placenta and may affect the FHR tracing. It is not uncommon to see reduced variability after initiating anaesthesia or analgesia. Although opinions differ regarding how and when fetal monitoring should be done, the following are helpful: fetal monitoring can alert you if optimisation of maternal positioning, oxygenation or blood pressure is needed.

Maternal considerations

Several physiological changes occur during pregnancy and have significant implications for anaesthesia.

Airway

Mucosal friability and oedema occur early in the first trimester and are most prominent in the third trimester.⁵ Nasal intubation, regardless of trimester, is not recommended, as increased vascularity can cause epistaxis. The risk of difficult intubation and mask ventilation increases in the pregnant patient. In 2015, the Obstetric Anaesthetists’ Association and the Difficult Airway Society published an obstetric-specific difficult airway algorithm.⁶ The authors recommend having a downsized tracheal tube and videolaryngoscope available if possible.

Respiratory

Respiratory changes begin in the first trimester and continue to have an effect throughout pregnancy. Maternal oxygen consumption increases steadily to meet the demands of the growing fetus. Increased minute ventilation, driven mostly by increased tidal volumes, leads to compensated respiratory alkalosis with a pH nearing 7.44.⁵ This should guide ventilatory settings to a target Pe′co₂ of 3.7–4.3 kPa during general anaesthesia or blood gas analysis.⁵ A decrease in functional residual capacity of 20% can cause rapid desaturation during periods of apnoea, often within 2–3 min.⁵ It is important to allow ample time for preoxygenation before induction of general anaesthesia.

Cardiovascular

Cardiac output increases by 50% above baseline by the end of the second trimester, which may predispose patients with congenital heart anomalies, heart failure or valve disease to dysrhythmia or worsening heart failure.⁵ A transthoracic echocardiography during pregnancy may be helpful to assess cardiac function.

Renal

Renal changes impact fluid and electrolyte balance, and drug metabolism and elimination. Renal blood flow and glomerular filtration rate increase by 75% and 65% above baseline, respectively, both of which are maintained at these levels throughout the remainder of pregnancy.⁵ Serum creatinine decreases by the end of the first trimester; therefore, elevated creatinine levels should be investigated and can alter drug dosing strategies.

Gastrointestinal

Gastric emptying and acid secretion do not change before labour regardless of the patient’s BMI.⁵ Standard guidelines for fasting should be used before surgery. Lower oesophageal sphincter tone decreases with progesterone concentrations and reaches a nadir at 36 weeks.⁵ Combined with the accompanying mechanical displacement of the stomach by the gravid uterus, the patient is at increased risk for regurgitation and aspiration. In addition, 80% of patients can experience nausea and vomiting as early as 4 weeks.⁵ There is no consensus for a specific gestational age at which tracheal intubation is required. However, rapid sequence induction with aspiration prophylaxis should be considered in any trimester if the patient is experiencing nausea, vomiting, pain, infection, and gastro-oesophageal reflux; has a history of hiatus hernia; or has a full stomach.

Type of anaesthesia

Regional or neuraxial anaesthesia, monitored anaesthesia care and general anaesthesia can be safe during any trimester, depending on the individual patient’s needs and medical history. Maintaining normotension and avoiding hypoxia are critical for both fetal and maternal well-being. The sedative effects of all anaesthetics may be observed at lower doses than in non-pregnant patients, thus decreasing minimum alveolar concentration requirements.

Positioning

Inferior vena cava compression as early as 13–16 weeks has been reported.⁵ Applying 15° of left uterine displacement (LUD) can prevent aortocaval compression by the gravid uterus, which can impede venous return to the heart. Published data are inconsistent as to when to initiate LUD. However, we recommend using LUD starting at 18 weeks. Multiple gestation and obesity may require using LUD earlier in the pregnancy. Fetal monitoring can assist with positioning, as fetal bradycardia can be a sign of aortocaval compression. Increased weight gain and enlarged breast tissue often make positioning of the patient challenging. A ramp or reverse Trendelenburg position helps to optimise respiratory mechanics.

Laparoscopy

Although laparoscopy is considered safe during pregnancy and has reduced morbidity compared with open procedures, CO₂ insufflation confers a higher risk of hypercarbia.⁷ Hypercarbia, hypotension and hypoxaemia may lead to vasoconstriction and subsequent reduction in uteroplacental perfusion, which may lead to fetal distress.² Consequences of CO₂ pneumoperitoneum can be reduced by keeping insufflation pressures between 12 and 15 mmHg, and maintaining baseline maternal Paco₂ and arterial blood pressure.⁷ Hypotension may be treated with either phenylephrine or ephedrine, as both are safe for use in pregnancy.

Reversal of neuromuscular block

Unless confronted with a ‘cannot intubate, cannot ventilate’ scenario, the 2019 Society for Obstetric Anesthesia and Perinatology consensus statement on sugammadex recommends against its use during pregnancy because of concerns about progesterone binding.⁸ Neuromuscular block may be reversed using neostigmine and atropine. In the UK, sugammadex is becoming widely used, as increasing numbers of practitioners are replacing suxamethonium with rocuronium for rapid sequence induction.

Newly postpartum

Neonatal (breastfeeding) considerations

Many mothers will ask when they can resume breastfeeding after surgery. The old recommendation of discarding breast milk while in the PACU is not supported by the ASA or the Association of Anaesthetists.^9,10 Although all anaesthetic medications transfer to breast milk to some degree, the vast majority are in very low concentrations, which are considered safe for the newborn.^9,10 Some exceptions to this are opioids that have genetic variants for metabolism (e.g. codeine and pethidine [meperidine]), certain antibiotics (e.g. tetracycline) and cardiovascular agents (e.g. amiodarone and statins).¹⁰

Maternal considerations

The return to maternal physiological baseline is gradual and often takes place over weeks. Cardiac output may reach values 150% above pregnancy baseline immediately postpartum and may take up to 24 weeks to return to normal.⁵ Maternal heart rate stabilises within 2 weeks. The dilutional anaemia seen throughout the pregnancy resolves by 3 weeks postpartum. Gastric emptying, volume and pH return to pre-pregnancy levels at 18 h postpartum.⁵

Conclusions

The anaesthetist caring for an obstetric patient must be prepared for distinct challenges in each trimester to maintain maternal and fetal well-being. Pregnancy induces complex changes in respiratory, cardiac, gastrointestinal, haematological and renal physiology. With a good understanding of maternal physiology, the anaesthetist can provide best care for these vulnerable patients.

Acknowledgements

The authors are deeply grateful to Dr Rebecca Minehart for her dedication and help with this article.

Biographies

Erin Haggerty is an instructor of anaesthesia at the Massachusetts General Hospital, where she completed a fellowship in obstetric anaesthesia.

Jaime Daly is an assistant professor at the University of Colorado and former associate director for the Obstetric Anesthesia Fellowship program at Massachusetts General Hospital. She completed a fellowship in obstetric anaesthesia at the University of Chicago, Chicago, IL, USA.

Matrix codes: 1A01, 2A09, 3B00

Declaration of interests

The authors declare that they have no conflicts of interest.