Modified RSI short note for DNB anaesthesia as millers and barash as reference
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I now have comprehensive material from Miller's and Barash's. Let me compose the short note.
Modified Rapid Sequence Induction (Modified RSI)
Definition
Modified RSI (mRSI) is a variant of the classical RSI technique in which gentle positive-pressure mask ventilation is permitted during the interval between drug administration and tracheal intubation, in contrast to the classical technique where any ventilation is strictly avoided. The term is frequently used but no single universally accepted definition exists; a US survey found it most commonly refers to mask ventilation combined with cricoid pressure.
(Miller's Anesthesia 10e, Ch. 29)
Classical RSI — Basis for Comparison
| Feature | Classical RSI |
|---|---|
| Indication | Full stomach / high aspiration risk |
| Preoxygenation | 3–5 min; target EtO₂ >90% |
| Induction agent | IV hypnotic (propofol / etomidate / ketamine) |
| Muscle relaxant | Succinylcholine 1–1.5 mg/kg |
| Cricoid pressure | Applied at 10 N (awake) → 30 N (after LOC) |
| Ventilation | None until ETT confirmed |
| Goal | Minimise time from LOC to cuffed ETT |
(Miller's Anesthesia 10e)
Why Modify? — Rationale for mRSI
Classical RSI carries the risk of rapid oxygen desaturation in patients with limited oxygen reserve before intubating conditions are established. Populations at risk include:
- Morbid obesity
- Pregnancy
- Critically ill / septic patients
- Paediatric patients (low FRC, high O₂ consumption)
- Emergency situations where preoxygenation is incomplete
- Patients requiring a longer time to adequate intubating conditions (e.g. when standard-dose non-depolarising NMBDs are used)
(Miller's Anesthesia 10e; Barash Clinical Anesthesia 9e)
mRSI — Key Modifications
1. Gentle Mask Ventilation
- Allowed during the drug effect interval, unlike classical RSI
- Inspiratory pressure kept < 20 cm H₂O (Miller's) / 10–20 cm H₂O (Barash) to limit gastric insufflation
- Cricoid pressure is typically maintained during ventilation to reduce regurgitation risk
- Large tidal volumes and high peak pressures are avoided (lower oesophageal sphincter may be breached above 25 cm H₂O)
2. Muscle Relaxant — Rocuronium as Alternative
- When succinylcholine is contraindicated (hyperkalaemia, myopathy, malignant hyperthermia risk, burns, prolonged immobility, denervation injury), rocuronium 1.0–1.2 mg/kg or vecuronium 0.3 mg/kg can be used
- Provides adequate intubating conditions in < 90 seconds
- The previous disadvantage of prolonged blockade is offset by sugammadex availability (reversal within 3 minutes)
- In paediatrics: rocuronium 1.2 mg/kg or succinylcholine 2 mg/kg
(Miller's Anesthesia 10e)
3. Induction Agent Flexibility
- Classical RSI used a fixed dose of thiopental; in modern practice, propofol, etomidate, or ketamine are commonly substituted based on haemodynamic status
- Titration of the induction agent to loss of consciousness (rather than a fixed predetermined dose) is also advocated by some
4. Cricoid Pressure — Selective Use
- Applied but may be released if it worsens laryngoscopic view
- Cricoid pressure does not compress the oesophagus reliably (MRI studies show lateral displacement rather than occlusion)
- It reduces LES tone and can obstruct the subglottic airway
- Current evidence has led to removal of Class 1 recommendation by EAST guidelines for trauma; AHA recommends against use in cardiac arrest
5. Ultra-Modified RSI
- Uses transnasal humidified rapid-insufflation ventilatory exchange (THRIVE/HFNO) during induction to extend apnoeic oxygenation time — described particularly in paediatrics
(Barash Clinical Anesthesia 9e)
Indications for mRSI (vs Classical RSI)
| Scenario | Reason |
|---|---|
| Obesity / pregnancy | Rapid desaturation risk |
| Critically ill / sepsis | Cannot achieve adequate preoxygenation |
| Paediatric emergency | Low FRC, high VO₂, poor mask tolerance |
| Non-depolarising NMBD use | Longer time to intubating conditions |
| Renal failure (ESRD) | Succinylcholine-related hyperkalaemia → rocuronium-based mRSI |
| Zenker's diverticulum | mRSI without cricoid, head-up 20–30° position |
| Post-pneumonectomy / thoracic | Modified RSI with bronchoscopic guidance |
Contraindications to Succinylcholine (prompting mRSI with rocuronium)
- Known or suspected myopathy / muscular dystrophy
- Burns (> 24–48 hours post injury)
- Prolonged immobilisation / denervation
- History of malignant hyperthermia
- Hyperkalaemia or ESRD with K⁺ > 5.5 mEq/L
- Personal or family history of pseudocholinesterase deficiency
Comparison Table: Classical vs Modified RSI
| Parameter | Classical RSI | Modified RSI |
|---|---|---|
| Ventilation after drugs | None | Gentle PPV (<20 cm H₂O) |
| Cricoid pressure | Mandatory | Applied; released if needed |
| Muscle relaxant | Succinylcholine | Succinylcholine or rocuronium |
| Induction dose | Fixed | Titrated or fixed |
| Preoxygenation | Mandatory (complete) | Attempted; mRSI used if incomplete |
| THRIVE/HFNO | Not used | Ultra-modified RSI |
| Primary use case | Elective full stomach | High desaturation risk / paediatric / NMBD-based |
Key Exam Points (DNB Anaesthesia)
- No universally accepted definition — most commonly defined as mask ventilation + cricoid pressure
- Classical RSI avoids PPV; mRSI allows gentle PPV <20 cm H₂O with cricoid pressure
- Sugammadex has made high-dose rocuronium a viable succinylcholine alternative
- Cricoid pressure: 10 N awake → 30 N after LOC; release if intubation is difficult
- Preoxygenation target: EtO₂ > 90% or SpO₂ > 97%; neonates/infants desaturate fastest
- Ultra-modified RSI uses THRIVE — relevant in obese and paediatric airway
- mRSI commonly employed in ESRD (rocuronium instead of succinylcholine to avoid hyperkalaemia)
References:
- Miller's Anesthesia, 10th Edition — Chapter 29: Induction of Anesthesia; Chapter 65 (Trauma); Chapter: Thoracic Anesthesia
- Barash, Cullen & Stoelting's Clinical Anesthesia, 9th Edition — Chapter: Paediatric Anaesthesia (Full Stomach & RSI); Chapter: Airway Management
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