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Maternal Airway and General Anaesthesia

Introduction

• General anaesthesia (GA) for caesarean delivery carries higher maternal morbidity and mortality, more pronounced haemodynamic fluctuations at induction and greater postoperative analgesic requirements.
• In South Africa, airway-related events (failed intubation, failed ventilation, aspiration) account for over 50 % of anaesthesia-related maternal deaths
• Fetal depression is minimised if delivery occurs within 10 minutes of induction; surgical practice aims for decision-to-delivery intervals < 30 minutes and incision-to-delivery < 3 minutes to optimise neonatal Apgar scores and acid–base status.
• Incidence of difficult or failed tracheal intubation in obstetric GA:
  • Difficult intubation reported in up to 8.6 % (1 in 12) of parturients in the UK DREAMY cohort
  • Failed intubation occurs in approximately 0.3 % (1 in 309) in the UK and as low as 0.13 % (1 in 750) in South African high-volume centres
• Contributing factors to improved obstetric airway safety include early use of second-generation supraglottic airway devices (SADs), simulation-based training, routine availability of video laryngoscopy (VL) and increased regional anaesthesia utilisation.

The Obstetric Airway

Anatomical and Physiological Changes

• Anatomical: Increased body mass index, breast enlargement, elevation of the diaphragm and progesterone-mediated mucosal oedema narrow the pharyngeal lumen.
• Physiological: Oxygen consumption rises by ~20 %, functional residual capacity falls by ~20 %, shortening safe apnoea time to 2–3 minutes
• Clinical implication: Reduced margin for error mandates meticulous preoxygenation and airway planning.

Rapid Sequence Induction (RSI) and Cricoid Pressure

• Traditional RSI: No mask ventilation after induction, manual cricoid pressure throughout until airway secured.
• Modified approach (Difficult Airway Society-adapted):
  • Gentle mask ventilation with ≤ 12 cmH₂O to prevent desaturation.
  • Selective cricoid pressure only if aspiration risk high; release immediately if it hinders mask ventilation or intubation.
  • Use of rocuronium (1.2 mg kg⁻¹) with plan for sugammadex reversal as an alternative to succinylcholine (1–1.5 mg kg⁻¹).

Airway Devices

• Second-generation SADs (e.g., LMA ProSeal®, LMA Supreme®):
  • First-pass success > 98 % in elective, fasted, low-risk parturients; incorporate a gastric drain channel
  • Recommended as rescue devices and, in selected low-risk cases, as primary airway conduits.
• Video Laryngoscopy:
  • Improves view of the glottis and reduces failed intubation in predicted difficult airways; mixed-methods meta-analysis shows no difference in first-pass success for routine cases but clear benefit in difficult airway scenarios
  • Should be immediately available in all obstetric theatres.

Maternal GA Technique

Premedication and Fasting

• Fasting guidelines: Solids ≥ 6 h, clear fluids ≥ 2 h.
• Emergency or high-risk cases:
  • Non-particulate antacid (sodium citrate 0.3 M, 30 mL).
  • H₂-receptor antagonist (famotidine 20 mg IV) and metoclopramide 10 mg IV 1–2 h pre-induction.
• Anti-sialagogue: Glycopyrrolate 0.2 mg IV if indicated.

Positioning

• Ramped position: Align external auditory meatus with sternal notch.
• Left uterine displacement: Wedge under right hip to minimise aortocaval compression.

Induction and Maintenance

1. Preoxygenation: 3–5 minutes tidal breathing (or 8 vital‐capacity breaths in 60 s) targeting end-tidal O₂ > 90 %.
2. Modified RSI:
   • Optional opioid (fentanyl 1–2 µg kg⁻¹).
   • Propofol 2–2.5 mg kg⁻¹ or ketamine 1–2 mg kg⁻¹.
   • Neuromuscular blockade with rocuronium 1.2 mg kg⁻¹ (or succinylcholine 1–1.5 mg kg⁻¹).
   • Gentle mask ventilation ≤ 12 cmH₂O if required.
   • Cricoid pressure applied selectively.
3. Intubation: Prefer VL; confirm placement with waveform capnography.
4. Maintenance: Oxygen/air or O₂/N₂O with volatile agent ≤ 1 MAC; avoid hyperventilation (keep PaCO₂ ≥ 4.5 kPa).
5. Uterotonics: Oxytocin 5 IU IV over 1 minute, then infusion of 10–20 IU over 4 h.

Emergence

• Reverse blockade with neostigmine/glycopyrrolate or sugammadex; extubate awake with intact protective reflexes.

Predictors of Difficult Airway

Tracheal Intubation	Mask Ventilation	SAD Insertion
BMI > 35 kg m⁻²	BMI > 35 kg m⁻²	Thyromental distance < 6 cm
Thyromental distance < 6 cm	Neck circumference > 50 cm	Mouth opening < 4 cm
Mallampati grade 3–4	Cricoid pressure
Reduced jaw protrusion	Fixed cervical flexion deformity
Significant airway oedema

Accidental Awareness During GA

• Represents ~10 % of reported cases of accidental awareness under GA
• Risk factors: Young age, obesity, difficult airway, trainee provider, out-of-hours or urgent surgery, rapid sequence induction, neuromuscular block without adequate anaesthetic depth.
• Prevention:
  • Achieve end-tidal volatile ≥ 0.7 MAC as rapidly as possible.
  • Use BIS or other depth monitoring if available.
  • Administer an opioid at induction to reduce the IV–inhalation gap.

THRIVE for Maternal Airway

• Physiological rationale: THRIVE (Transnasal Humidified Rapid‐Insufflation Ventilatory Exchange) may extend safe apnoea time by providing continuous apnoeic oxygenation in parturients.
• Healthy term parturients: Small RCTs show THRIVE (30–70 L min⁻¹) achieves lower end-tidal O₂ than a tight-fitting facemask and often fails to reach EtO₂ ≥ 90 %.
• Obese parturients: THRIVE can produce higher PaO₂ and EtO₂ compared with facemask pre-oxygenation, though the clinical significance remains uncertain.
• RSI safety: Limited trials suggest THRIVE maintains SpO₂ ≥ 90 % during rapid sequence induction for caesarean section with comparable neonatal outcomes.
• Observational series: Majority of term women do not attain target EtO₂ ≥ 90 % on first expiration after THRIVE alone, indicating it may be inadequate as sole pre-oxygenation.
• Case reports: Successful use in complex or pathological airways (e.g., subglottic stenosis) demonstrates feasibility in select high-risk scenarios.
• Guideline stance: Major obstetric anaesthesia guidelines do not yet recommend routine THRIVE for caesarean delivery; standard pre-oxygenation and modified RSI remain standard of care.
• Bottom line: THRIVE is physiologically appealing and may benefit obese or high-risk parturients, but routine adoption awaits larger, definitive trials.

Maternal Difficult Airway Algorithm

Master Algorithm – Obstetric General Anaesthesia and Failed Tracheal Intubation

A structured, stepwise approach to anticipate and manage unanticipated difficult airways in the parturient:

1. Algorithm 1: Safe Obstetric General Anaesthesia
   1. Pre-induction planning and preparation
      • Multidisciplinary team briefing: airway plan, roles, equipment check.
   2. Modified rapid sequence induction (RSI)
      • Gentle facemask ventilation (peak pressure ≤ 20 cmH₂O) if required.
   3. Laryngoscopy
      • Maximum of two intubation attempts by the primary operator.
      • Third attempt only by a more experienced anaesthetist.
   4. Success → Confirm tube placement (capnography), proceed to surgery, plan extubation.
   5. Failure → Declare failed intubation and activate Algorithm 2.
2. Algorithm 2: Obstetric Failed Tracheal Intubation
   1. Declare “failed intubation” and call for senior help.
   2. Maintain oxygenation: insert second-generation supraglottic airway device (SAD) or revert to facemask ventilation.
      • Maximum two SAD insertion attempts.
      • Remove or adjust cricoid pressure to facilitate placement.
   3. Success →
      • Proceed if surgery is essential and airway is secure.
      • Wake the patient if surgery can be deferred safely.
   4. Failure → Activate Algorithm 3.
3. Algorithm 3: Can’t Intubate, Can’t Oxygenate (CICO)
   1. Declare “CICO”, call for help, administer 100 % oxygen.
   2. Exclude laryngospasm—ensure full neuromuscular blockade.
   3. Perform immediate front-of-neck access (emergency surgical airway).
   4. If maternal oxygenation cannot be re-established promptly, consider perimortem caesarean section.

Key Points

• Intubation attempts: Limited to two by the initial operator; third attempt only by an expert.
• SAD use: Second-generation devices preferred; limited to two insertion attempts.
• Pre-induction briefing: Essential to clarify roles, backup plans and equipment.
• Front-of-neck access: Should be anticipated and prepared for as part of the primary plan in high-risk cases
• Modified RSI: Gentle mask ventilation permitted to prevent desaturation; cricoid pressure applied selectively and released if it impedes ventilation or intubation.

Proceed with Surgery?

Factors to Consider	Wake	Proceed
Maternal condition	Stable, no ongoing compromise	Hypovolaemia requiring correction, deteriorating physiology
Fetal condition	Reassuring traces, no distress	Sustained bradycardia, suspected uterine rupture, fetal haemorrhage
Anaesthetist experience	Junior or trainee	Consultant or senior specialist
Obesity	Super-morbid (BMI > 50 kg m⁻²)	BMI < 35 kg m⁻²
Surgical complexity	Major haemorrhage expected, complex repair	Routine procedure, minimal anticipated risk
Aspiration risk	Recent oral intake	Adequately fasted
Alternative anaesthesia	Neuraxial contraindicated	Neuraxial feasible but declined
After failed intubation—ventilation status	Poor facemask ventilation	Effective SAD ventilation
After failed intubation—airway hazards	Airway oedema, stridor	No additional airway path

Links

• Airway guidelines
• Airway equipment and techniques
• Maternal conditions

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References:

1. Butterworth J, Mackey D, Wasnick J. Morgan and Mikhail’s Clinical Anesthesiology, 7th Edition. 7th edition. New York: McGraw Hill Medical; 2022.
2. Bishop D. Obstetric airway management. Southern African Journal of Anaesthesia and Analgesia. 2015;21(1):40–42. Available from: [http://creativecommons.org/licenses/by-nc-nd/4.0
3. Preston R, Jee R. Obstetric airway management. Int Anesthesiol Clin. 2014;52(2):1-28.
4. Boutonnet M, Faitot V, Katz A, et al. Mallampati class changes during pregnancy, labour, and after delivery: can these be predicted? Br J Anaesth. 2010;104(1):67-70.
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6. Difficult Airway Society. (2015). DAS Algorithms. Retrieved from https://das.uk.com/files/01-15%20DAS-algorithms-web-PRINT20092015.pdf
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12. Howle R, Onwochei D, Harrison S-L, Desai N. Comparison of videolaryngoscopy and direct laryngoscopy for tracheal intubation in obstetrics: a mixed-methods systematic review and meta-analysis. Anaesthesia. 2021;76(4):847–854. researchgate.net
13. Practice Guidelines for Obstetric Anesthesia: An Updated Report by the American Society of Anesthesiologists Task Force on Obstetric Anesthesia and the Society for Obstetric Anesthesia and Perinatology. Anesthesiology. 2016;124(2):270–300.
14. Mushambi M, Kinsella SM, Popat M, et al. Obstetric Anaesthetists’ Association and Difficult Airway Society guidelines for the management of difficult and failed tracheal intubation in obstetrics. Anaesthesia. 2015;70(11):1286–306.
15. rerk C, Mitchell VS, McNarry AF, et al. Difficult Airway Society 2015 guidelines for management of unanticipated difficult intubation in adults. Br J Anaesth. 2015;115(6):827–848.
16. ASA Task Force on Obstetric Anesthesia; Society for Obstetric Anesthesia and Perinatology. Practice Guidelines for Obstetric Anesthesia: An Updated Report. Anesthesiology. 2016;124(2):270–300.

Summaries:

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