Phenylephrine in Obstetric (MD) Anaesthesia

Why Spinal Anaesthesia Causes Hypotension

• Reduction in SVR and venous return
• Maternal hypotension (SBP fall >20% from baseline or <100 mmHg)
• Reduced uteroplacental perfusion, risking fetal acidosis and neonatal depression

Phenylephrine: The Vasopressor of Choice

• Crosses the placenta readily
• Stimulates fetal beta receptors, increasing fetal metabolic rate
• Causes higher rates of fetal acidosis (lower umbilical artery pH)

• Has less placental transfer
• Results in less fetal acidosis - favourable fetal acid-base profile
• Is now the vasopressor of choice for treating post-spinal hypotension in obstetrics

Dosing Regimens

1. Prophylactic Infusion (Preferred)

• Starting rate: typically 25-100 mcg/min (0.25-1 mcg/kg/min), titrated to blood pressure
• A systematic review found prophylactic infusion vs placebo significantly reduced:
  • Risk of hypotension (RR 0.36; 95% CI, 0.18-0.73)
  • Nausea and vomiting (RR 0.39; 95% CI, 0.17-0.91)

2. Rescue Bolus Dosing

• 50-100 mcg IV bolus for acute hypotension episodes
• Can be repeated as needed
• Variable-rate infusion + rescue boluses strategy is widely used

3. Combined Strategy

Key Side Effect: Reflex Bradycardia

• Heart rate decrease and decreased cardiac output are common
• This is usually well tolerated, but monitor HR

• Norepinephrine maintains similar BP control
• Associated with greater heart rate and cardiac output compared to phenylephrine
• Emerging evidence supports its use, though phenylephrine remains the current standard

Practical Protocol Summary

Special Considerations

• Cardiac disease in pregnancy: Phenylephrine's increase in SVR and decrease in CO may not be well tolerated in some cardiac lesions (e.g., regurgitant lesions may benefit; stenotic lesions with fixed output require careful titration)
• Pre-eclampsia: Recent meta-analyses (PMID 39437476, 2024) suggest norepinephrine may be preferable to phenylephrine in pre-eclamptic patients - evidence is still evolving
• Bradycardia management: If maternal HR falls below 55-60 bpm, reduce infusion rate; atropine 0.6 mg IV or ephedrine can be used
• Fetal outcome: Despite causing reflex bradycardia and lower CO, umbilical artery pH and neonatal Apgar scores are better with phenylephrine than ephedrine

Recent Evidence Updates

• PMID 39437476 - Norepinephrine vs phenylephrine in pre-eclampsia: norepinephrine shows comparable or better outcomes in pre-eclamptic patients
• PMID 39715512 - Phenylephrine bolus vs infusion: infusion regimens result in better maternal haemodynamic stability and fetal outcomes
• PMID 40522505 - Network meta-analysis 2025: norepinephrine and phenylephrine are comparable; norepinephrine has higher HR/CO

Summary: Phenylephrine infusion (25-100 mcg/min, titrated) + rescue boluses (50-100 mcg) is the current standard of care for prevention and treatment of spinal hypotension during Caesarean section. It is preferred over ephedrine for better fetal acid-base outcomes. Its main limitation is reflex bradycardia with reduced cardiac output, which makes norepinephrine an increasingly attractive alternative.

Case Presentation: 16-Year-Old Girl with Chronic Tonsillitis for Bilateral Tonsillectomy

1. Patient Profile

2. Pre-operative Assessment

History

• 7+ documented episodes of sore throat in the preceding year, or
• 5+ episodes/year for 2 consecutive years, or
• 3+ episodes/year for 3 consecutive years
• Episodes must be disabling and prevent normal functioning (school, work)

• Frequency, severity, and documentation of sore throat episodes
• Prior antibiotic treatments and response
• Any symptoms of obstructive sleep apnoea (OSA) - snoring, witnessed apnoeas, daytime somnolence, nocturnal restlessness
• Mouth breathing - important to exclude nasal obstruction as the primary cause
• Dysphagia, voice change, or drooling (suggests significant tonsillar enlargement)
• Recent acute infection - surgery must be postponed if active acute tonsillitis is present
• Previous anaesthetics and any family history of anaesthetic complications

• Any personal or family history of abnormal bleeding
• Easy bruising, prolonged bleeding after minor cuts, menorrhagia
• Prior dental/surgical procedures and haemostasis

• Current medications (especially NSAIDs, aspirin - withhold pre-op)
• Allergies

• Cardiovascular: in severe, longstanding OSA/airway obstruction, check for signs of pulmonary hypertension or cor pulmonale
• Respiratory: any asthma or reactive airway disease (relevant to airway management)
• ENT: prior ear or nasal surgery

Examination

• Grade I: Tonsils within the pillars
• Grade II: Tonsils at the pillars
• Grade III: Tonsils extend beyond the pillars
• Grade IV: "Kissing" tonsils - touching at midline; risk of difficult airway

Investigations

3. Pre-operative Preparation

• Postpone surgery if any evidence of active acute tonsillitis or clotting abnormality
• Nil by mouth: 6 hours for solids, 2 hours for clear fluids
• Anticholinergic (e.g., glycopyrrolate or atropine): decreases pharyngeal secretions, reduces secretion-related airway complications
• IV access established
• Consent: Include risks of haemorrhage (primary and secondary), pain, nausea/vomiting, dental injury, rare risk of velopharyngeal insufficiency

4. Anaesthetic Management

Induction

• IV induction is standard in a 16-year-old (inhalational induction is reserved for young children or those with needle phobia)
• Propofol + fentanyl (or remifentanil) + muscle relaxant (atracurium/rocuronium)
• Total IV Anaesthesia (TIVA) with propofol + remifentanil infusion is an excellent option:
  • Rapid wake-up
  • Propofol has inherent antiemetic properties - reduces post-op nausea/vomiting (PONV)
• If history of airway obstruction/OSA: consider inhalational induction without muscle relaxant until ability to ventilate with positive pressure confirmed

Airway

• Oral RAE (Ring-Adair-Elwyn) tube or reinforced/armoured endotracheal tube:
  • Preformed curve keeps the tube and circuit away from the surgical field
  • Reduces risk of kinking by the surgeon's Boyle-Davis mouth gag
• Throat pack placement:
  • Gauze throat pack placed to absorb blood and secretions, preventing them from reaching the larynx/trachea
  • Critical: must be documented and removed before extubation
• Tube secured centrally in the midline of the lower lip, positioned for surgeon access

Positioning

• Supine with neck extended (shoulder roll) and head-ring
• Boyle-Davis mouth gag inserted by surgeon for access and retraction

Maintenance

• Volatile agent (sevoflurane/isoflurane) in O2/air or air/O2, OR TIVA with propofol/remifentanil
• Dexamethasone 0.15 mg/kg IV at induction (max 8 mg):
  • Cochrane review: single IV dose significantly reduces PONV and post-operative pain
  • Now widely accepted as standard of care
• Ondansetron 0.1 mg/kg IV (antiemetic) - given during procedure or at end
• Adequate analgesia throughout

Analgesia (Multimodal)

Blood Loss

• Usually modest (50-200 mL) but can be significant
• Maintain vigilance - occult blood may be swallowed without obvious external evidence
• Blood transfusion rare; type and screen is adequate pre-operatively
• Haemostasis by surgeon: bipolar diathermy, ligatures, haemostatic gauze

5. Extubation

1. Gentle pharyngeal suction under direct vision before extubation (inspect for bleeding points, remove clots)
2. Gastric aspiration via suction/NG tube - swallowed blood causes significant postoperative vomiting
3. Awake extubation is generally preferred - maintains airway reflexes and reduces aspiration risk
4. Deep extubation is an alternative practiced by some - reduces coughing/straining which could dislodge clots and cause laryngospasm, but carries higher aspiration risk
5. After extubation: position patient laterally (tonsil position / recovery position) - promotes drainage of secretions and blood away from the airway

6. Post-operative Care

Immediate Recovery (PACU)

• Lateral (tonsil) position maintained until fully awake
• Continuous SpO2, HR, BP monitoring
• Vigilance for post-tonsillectomy haemorrhage:
  • Signs: restlessness, pallor, tachycardia, hypotension, frequent swallowing
  • Frequently swallowing = sentinel sign of active bleeding into the pharynx
• Regular analgesia: paracetamol ± NSAIDs scheduled; minimal opioids
• Antiemetics: ondansetron

Analgesia Discharge Plan

• Scheduled paracetamol (1g four times daily for a 16-year-old)
• Ibuprofen (if no contraindications)
• Encourage normal diet - swallowing helps pain and prevents infection
• Patient warned: sore throat for at least 1 week; return to normal activities in 1-2 weeks
• Referred otalgia (ear pain) is common - due to shared glossopharyngeal nerve supply

Admission vs Day Case

• Day-case tonsillectomy in adolescents is safe and widely practiced
• Criteria for overnight admission:
  • Significant OSA
  • Age <3 years (less relevant here)
  • Distance from hospital
  • Post-operative bleeding concerns
  • Inadequate pain control

7. Complications

Haemorrhage (Most Important)

• Restore intravascular volume before GA unless airway obstruction is imminent
• NG tube to evacuate stomach contents (full of blood = aspiration risk)
• Rapid sequence induction (RSI) with cricoid pressure
• Consider IV access x2, cross-match, call senior anaesthetist
• Greatest anaesthetic risk in the ENT emergency repertoire

Other Complications

• PONV - very common (30-40% without prophylaxis)
• Pain/otalgia - referred ear pain from glossopharyngeal nerve
• Infection - halitosis, fever; antibiotics not routinely indicated post-op
• Velopharyngeal insufficiency - rare; nasal regurgitation, hypernasal speech
• Atlantoaxial subluxation (Grisel syndrome) - rare non-traumatic subluxation from inflammatory process
• TMJ injury - from over-wide mouth opening with Boyle-Davis gag
• Dental injury - document pre-existing dental work
• Airway fire - rare but catastrophic if diathermy used near ignition sources (O2-enriched environment)

8. Summary Anaesthetic Plan

Pre-op:   FBC, clotting screen, G&S | Postpone if acute infection
          Dexamethasone + anticholinergic pre-medication
          
Induction: IV propofol + fentanyl + rocuronium
           Oral RAE tube + throat pack
           
Maintenance: TIVA (propofol/remifentanil) OR volatile agent
             Dexamethasone 0.15 mg/kg IV
             Paracetamol IV + NSAID
             
Emergence: Suction oropharynx under vision + gastric emptying
           Awake extubation → lateral (tonsil) position
           
Post-op:   Multimodal analgesia (paracetamol + ibuprofen)
           Ondansetron antiemetic
           Close monitoring for haemorrhage
           Day-case if criteria met