Obstetric failed intubation algorithm.
Premedication
Premedication aims to reduce the risk of aspiration and should be given to all parturients planned for, or at risk of, operative delivery.
Ranitidine (H2 receptor antagonist) blocks the action of histamine on gastric parietal cells, decreasing gastric acid production. The oral preparation reaches peak plasma concentrations within 1–3 hours. There is a relationship between plasma concentrations of ranitidine and suppression of gastric acid production, but wide interindividual variability exists. Aim for oral administration 60–90 minutes prior to anaesthesia (or 30 minutes for IV administration). Its effects are sustained for up to 8 hours. It is recommended to be administered 6 hourly when in high risk labour.
Metoclopramide (dopamine antagonist) inhibits gastric smooth muscle relaxation. It accelerates intestinal transit, increases the lower oesophageal sphincter resting pressure, increases amplitude of oesophageal peristaltic contractions and raises the threshold of activity in the chemoreceptor trigger zone. Given orally it can take between 30 and 75 minutes to act, but 1–3 minutes when given IV. Its effects are sustained for 1–2 hours. Use of prokinetic agents in labour is not as widespread as it once was.
Sodium citrate 0.3 M 30 mL has the benefit of instantaneous efficacy as an acid-neutralizing agent; however, its effects are short lived. It is best used within 20 minutes of induction.
Anaesthesia
Induction
Successful performance of a Category 1 caesarean section under general anaesthetic requires the anaesthetist to maintain clear communication, with good leadership and team working skills.
Of UK obstetric units, 70% perform anaesthesia for caesarean section in the operating theatre rather than an anaesthetic room; this reduces the time from induction to delivery.
To minimize delay, intravenous induction is performed once the patient is catheterized, the abdomen draped and surgeons scrubbed. The patient is positioned with left lateral tilt to avoid aortocaval compression. Some practitioners also advocate a 30° head-up tilt, arguing improvement in maternal wellbeing through an increased functional residual capacity (FRC), reduced breast interference with intubation and reduced gastro-oesophageal reflux.
The ramped position, with the parturient sat up approximately 15°, with the tragus of the ear lying in the same horizontal plane as the sternal notch and the head extended, is particularly useful for the obese parturient; reducing the risk of acid reflux, pulmonary shunting and improving the view at laryngoscopy. Preoxygenation in this position is improved as FRC is increased compared to the supine position, prolonging the safe apnoeic time after induction.
Aside from the importance of a tight-fitting oxygen mask, there is little consensus regarding the conduct of preoxygenation. Described techniques include tidal volume breathing for 3 min or performing 4, 5, or 8 vital capacity breaths. Regarding adequacy of preoxygenation, there is debate that use of a set time period for preoxygenation should be abandoned in favour of a target FE02 (>0.8).
Induction agent
Properties of an ideal induction agent for obstetric anaesthesia include:
Fast onset
Clear end point
Cardiovascular stability
Minimal adverse maternal and neonatal effects
Ample user experience
Appropriate licensing
Low cost
No premixing.
In a survey of UK consultant OAA members 2011, 93% of obstetric anaesthetists favoured thiopentone as the induction agent of choice for an obstetric RSI with only 7% of respondents using propofol. Of those surveyed in 2013, 55% ‘hardly or never’ used thiopentone outside of obstetric practice.
See Table 12.2.
| Thiopentone | Propofol | |
|---|---|---|
| Availability | Sole UK manufacturer Long-term production of drug not guaranteed | No manufacturing or availability issues |
| Association with obstetric anaesthesia | Long history of association | Introduced into clinical practice in 1980s Comparatively short history of association. Most commonly used induction agent for general anaesthesia in obstetrics outside of UK (2013) |
| Awareness | No difference (although studies small with poor standardization and power). To minimize maternal awareness dose suggested not less than 5 mg/kg | Lack of evidence of increased awareness To minimize maternal awareness dose suggested 2.5 mg/kg |
| Neonatal outcome | Small number of studies showing better Apgar scores Longer-term studies of neurodevelopment post maternal GA lacking | No evidence that propofol advantageous Some studies show no difference in Apgar scores |
| Maternal haemodynamics | Higher maximal maternal noradrenaline concentrations Less hypotension | Less hypertension at laryngoscopy and intubation More hypotension In combination with suxamethonium can cause severe maternal bradycardia |
| User experience | Still extensively used for obstetric emergency anaesthesia. Largely replaced by other induction agents for non-obstetric emergencies owing to lack of familiarity | Extensively used for elective anaesthesia Good experience profile for all grades of anaesthetist |
| Drug error | Highlighted by the UK patient safety report as a risk in obstetric practice Reconstitution error Syringe-swap with antibiotics | Minimal risk Nil reconstitution Distinctive appearance |
| Summary of product characteristics | Reconstituted drug should be used within 7 hours Pre-filled syringes 90 day shelf life ‘Can be used without adverse effects in pregnancy’ (in doses not exceeding 250 mg) | ‘Should not be used for obstetric anaesthesia unless clearly necessary’ However, used worldwide by countries where thiopentone not available e.g. USA |
Related posts:
Non-regional analgesia techniques for labour
Anaesthesia for other obstetric indications: cervical suture, external cephalic version, controlled ARM, manual removal of placenta and perineal repair
Regional analgesia techniques for labour
Endocrine disease in pregnancy, including diabetes mellitus, thyroid and adrenal disease
Maternal collapse, including massive obstetric haemorrhage, amniotic fluid embolism and cardiac arrest
Haematologic disease in pregnancy
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