Difference between rsi and modified rsi for My MD Anaesthesiology exam notes
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"rapid sequence induction" AND "modified" AND intubation
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RSI vs Modified RSI - MD Anaesthesiology Exam Notes
1. RAPID SEQUENCE INDUCTION (RSI) - Classic / Standard
Definition
RSI is a specialized IV induction technique used when there is an increased risk of pulmonary aspiration of gastric contents. The goal is to achieve optimal intubating conditions as rapidly as possible, minimizing the time between loss of consciousness (LOC) and securing the airway with a cuffed ETT.
- Miller's Anesthesia 10e
- Morgan & Mikhail's Clinical Anesthesiology 7e
Indications
| Category | Examples |
|---|---|
| Full stomach (absolute) | NPO guidelines not followed, emergency surgery |
| GI pathology | Bowel obstruction, pyloric stenosis |
| Gastroparesis | Diabetic, opioid-induced, post-trauma |
| Reflux disease | Poorly controlled GERD |
| Pregnancy | 2nd trimester onwards (though now questioned) |
| Predicted difficult mask ventilation | Edentulous + bearded patient with reassuring airway |
| ICU/critical care | High-risk intubations |
| Nasogastric tube in situ | |
| Morbid obesity |
Classic Steps (Seven P's Mnemonic)
- Preparation - IV access, monitors, drugs drawn up, suction, positioning (ramped/sniffing)
- Preoxygenation - 3-5 min of 100% O2 by tight-fitting mask (target EtO2 >90% or SpO2 >97%); achieves denitrogenation and maximizes apnoea safe time
- Pretreatment (optional) - Atropine (peds), fentanyl (attenuate laryngoscopy response), lidocaine (ICP protection)
- Paralysis with induction - Induction agent immediately followed by succinylcholine 1-1.5 mg/kg; administered in rapid succession without waiting for LOC
- Protection of airway - Cricoid pressure (Sellick manoeuvre) applied at 10 N while awake, increased to 30 N at LOC
- Placement with proof - Intubate once paralysis confirmed (loss of twitch on nerve stimulator); confirm with EtCO2 + auscultation
- Post-intubation management - Inflate cuff, confirm position, THEN release cricoid pressure
Key Features of Classic RSI
- NO bag-mask ventilation (BMV) between induction and intubation - to avoid gastric insufflation and increase in aspiration risk
- Induction agent + succinylcholine given as bolus, back to back
- Cricoid pressure maintained throughout
- Succinylcholine (1-1.5 mg/kg) is the NMB of choice - fastest onset (~60 sec), ultra-short duration
- Thiopentone was traditional induction agent (propofol, ketamine, etomidate now common)
- Patient remains apnoeic until intubating conditions confirmed
- Cummings Otolaryngology 6e; Miller's Anesthesia 10e
Drugs Used
| Drug | Class | Dose | Onset | Notes |
|---|---|---|---|---|
| Propofol | Induction agent | 1.5-2.5 mg/kg | 30-45 sec | Haemodynamic depression; most common |
| Thiopentone | Induction agent | 3-5 mg/kg | 30-45 sec | Classic agent; no longer available many centres |
| Ketamine | Induction agent | 1.5-2 mg/kg | 45-60 sec | Preferred in haemodynamically unstable |
| Etomidate | Induction agent | 0.3 mg/kg | 30-45 sec | Preferred in cardiovascular compromise; adrenal suppression concern |
| Succinylcholine | Depolarizing NMB | 1-1.5 mg/kg | ~60 sec | Gold standard NMB for RSI |
| Rocuronium | Non-depolarizing NMB | 1.0-1.2 mg/kg | 60-90 sec | RSI alternative; reversible with sugammadex |
| Vecuronium | Non-depolarizing NMB | 0.3 mg/kg | 90 sec | Less preferred |
2. MODIFIED RSI (mRSI)
Definition
"Modified RSI" has no single standardized definition. A US survey of anaesthesia residents and attendings showed the most common usage refers to: allowing gentle mask ventilation in conjunction with cricoid pressure during the interval between induction and intubation.
- Miller's Anesthesia 10e, pg. 5864
Key Difference from Classic RSI
The sole major difference is that BMV is permitted, provided:
- Cricoid pressure is maintained throughout
- Inflation pressure is kept gentle (< 20 cm H2O)
- This avoids gastric insufflation while preventing hypoxaemia
Morgan & Mikhail state directly: "A modification of the classic rapid-sequence induction allows gentle ventilation as long as cricoid pressure is maintained."
Indications for Modified RSI (over Classic RSI)
These are patients at risk of rapid desaturation where safe apnoea time may be insufficient:
| Indication | Rationale |
|---|---|
| Morbid obesity | Reduced FRC, rapid desaturation |
| Pregnancy (especially term) | Elevated diaphragm, reduced FRC |
| Paediatric patients (especially neonates, infants) | Low FRC + high O2 consumption = very short safe apnoea time |
| Critically ill patients | Poor baseline reserves |
| Incomplete preoxygenation (emergency) | Could not achieve target EtO2 |
| Standard doses of non-depolarizing NMBDs used | Longer time to acceptable intubating conditions |
- Miller's Anesthesia 10e; Barash Clinical Anesthesia 9e
Variants of Modified RSI
| Variant | Description |
|---|---|
| mRSI (classic modified) | Gentle PPV (< 20 cm H2O) with cricoid pressure while awaiting paralysis |
| Ultra-modified RSI | Transnasal Humidified Rapid Insufflation Ventilatory Exchange (THRIVE / High-flow nasal O2) used during apnoeic phase - extends safe apnoea time significantly |
- Barash Clinical Anesthesia 9e
3. COMPARISON TABLE - Exam Summary
| Feature | Classic RSI | Modified RSI |
|---|---|---|
| Preoxygenation | Yes (3-5 min, 100% O2) | Yes (same) |
| Induction agent | Fixed bolus (thiopentone/propofol/ketamine/etomidate) | Same; may titrate to LOC |
| NMB agent | Succinylcholine 1-1.5 mg/kg (preferred) | Succinylcholine or rocuronium 1.2 mg/kg; or standard dose rocuronium (longer wait) |
| Cricoid pressure (Sellick) | Yes, mandatory (10 N awake, 30 N at LOC) | Yes, maintained throughout ventilation |
| Bag-mask ventilation | NONE (strict no-BMV rule) | Allowed - gentle (< 20 cm H2O) |
| Timing of intubation | After confirmed paralysis only | After confirmed paralysis; may ventilate in interval |
| Primary goal | Minimize apnoea-to-intubation time | Same, but prevent hypoxaemia in high-risk groups |
| Main indication | Full stomach, aspiration risk | Full stomach + high risk of rapid desaturation |
| Risk mitigated vs classic | Aspiration | Aspiration + hypoxic harm from prolonged apnoea |
| Paediatric preference | Older children | Neonates, infants (very short safe apnoea times) |
| Failed intubation plan | No prior test of BMV - highest risk of cannot intubate-cannot oxygenate | Easier rescue - BMV already confirmed effective |
4. CRICOID PRESSURE - The Controversy (High-Yield for Exams)
Arguments FOR (Sellick 1961):
- Occludes upper oesophagus by compressing against cricoid ring (the only complete tracheal ring)
- Prevents passive regurgitation of gastric contents into pharynx
- Recommended force: 10 N (awake) → 30 N (at LOC)
Arguments AGAINST (Modern Evidence):
- MRI studies show cricoid pressure causes lateral displacement of oesophagus, not true compression
- Decreases lower oesophageal sphincter tone - may paradoxically increase regurgitation risk
- Worsens laryngoscopic view (Cormack-Lehane grade worsens)
- Can cause subglottic airway occlusion
- Excessive pressure during active vomiting → risk of oesophageal rupture
- Current consensus: use cricoid pressure, but release it if it impedes laryngoscopy
- Miller's Anesthesia 10e
5. ADDITIONAL HIGH-YIELD POINTS
Succinylcholine Contraindications (when to use Rocuronium instead):
- Hyperkalaemia or risk of (burns > 24h, crush injury, denervation, prolonged immobility)
- Known or suspected myopathy / muscular dystrophy
- Malignant hyperthermia susceptibility
- Pseudocholinesterase deficiency
- Penetrating eye injury (theoretical)
- Paediatrics: risk of undiagnosed myopathy → hyperkalemic cardiac arrest
- Barash Clinical Anesthesia 9e
Rocuronium for RSI:
- Dose: 1.0-1.2 mg/kg provides intubating conditions in < 90 seconds
- Key advantage: Fully reversible with sugammadex (16 mg/kg for immediate reversal)
- Increasingly replacing succinylcholine where sugammadex is available
Preoxygenation Targets:
- EtO2 > 90% or SpO2 > 97% before commencing
- Neonates/infants desaturate fastest (low FRC + high O2 consumption) - key reason to prefer mRSI in this group
- THRIVE (high-flow nasal O2 at 60-70 L/min) can extend apnoeic oxygenation time to 25+ minutes (ultra-modified RSI)
Pre-medication to Reduce Aspiration Risk (given before RSI):
- Sodium citrate 30 mL oral (clear antacid - immediate action)
- Metoclopramide 10 mg IV (prokinetic + ↑ lower oesophageal sphincter tone)
- H2 antagonist (ranitidine) or PPI (omeprazole) - requires time to act
- Glycopyrrolate (reduces secretions, no CNS effects)
6. ONE-LINE MEMORY AID
RSI = preoxygenate → induction agent + succinylcholine simultaneously → NO mask ventilation → cricoid pressure → intubateModified RSI = same as RSI, but gentle BMV (< 20 cm H2O) WITH cricoid pressure is ALLOWED to prevent hypoxaemia in high-risk groups
Sources:
- Miller's Anesthesia, 2-Volume Set, 10e (Sections: Rapid-Sequence Induction and Intubation, p. 5862-5864)
- Morgan & Mikhail's Clinical Anesthesiology, 7e (How does rapid-sequence induction differ from routine induction, p. 538)
- Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (Full Stomach and Rapid Sequence Induction, p. 3749)
- Cummings Otolaryngology Head and Neck Surgery (Standard vs RSI, p. section 5)
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