2021A Question 1
Describe the advantages and disadvantages of using nitrous oxide as part of a general anaesthetic.
Examiner Report
23.1% of candidates achieved a pass in this question.
Candidates who structured their answer showing advantages and disadvantages relating to the physico-chemical, pharmacokinetic and pharmacodynamic properties of N2O generally passed. To score well, candidates needed to discuss N2O in the context of modern general anaesthesia, and show understanding of the properties of this agent to make their argument.
Recurring problems in the submitted answers included:
- Writing down pharmacokinetic parameters without any attempt to explain their relevance
- Inaccurate descriptions of fundamental pharmacokinetic principles
- Vague statements (eg N2O has analgesic properties) with no other detail or comment regarding significance
Comments regarding Entonox use in the Emergency Department or Labour Ward did not receive marks.
Model Answer
Structure:
- PC: Pros and cons
- PK: Pros and cons
- PD: Pros and cons
Pharmaceutics
| Pros | Cons |
|---|---|
| Liquid form → Efficient storage | - Impurities from manufacturing: NH3, N2, NO, NO2 - e.g. NO2: Airway damage, headache, heart failure |
| Sweet smell, non irritant → Good for gas induction | Set-up and maintenance costs: Wall supply, cylinder manifolds etc. |
| Inert: No reaction with rubber, plastic, sodalime | Need to reduce filling ratio from 0.75 to 0.67 in hot climates |
| Low potency (MAC 105%): Use as carrier gas | - Entonox lamination: Occurs if below pseudocritical temperature (-6°C) - Delivery of hyperoxic then hypoxic mixture |
- Supports combustion, although not itself flammable - Risk of fire, explosion in airway laser surgery - Destructive to ozone layer |
Pharmacokinetics
| Stage | Pros | Cons |
|---|---|---|
| A | N2O 30x more soluble, blood-gas partition coefficient 0.47; equilibrates across membranes rapidly Rapid onset during gas induction: 1) Concentration effect: - Seen only with high volume carrier gases - Switch from N2/O2 to N2O/O2 - Rapid uptake N2O from alveolus, but very slow output of N2 - ↓ Alveolar volume and pressure → Rapid inflow of N2O-rich fresh gas - Accelerated ↑ FA/FI N2O 2) Second gas effect: - Rapid uptake N2O, very slow output of N2 - Concentration of remaining alveolar gas - Accelerated ↑ FA/FI volatile drug |
|
| D | - Low muscle:blood PC (1.2) - Low fat:blood PC (2.3) - Minimal accumulation in long case |
Expansion of existing closed air spaces: - ↑ Size pneumothorax - ↑ Bowel gas → Difficult laparoscopy - ↑ Ocular gas (if surgery using SF6) - ↑ Pneumocephalus (if brain surgery) |
| M | Minimal (0.004%) | |
| E | Diffusion hypoxia: - Reverse of concentration effect - N2O/O2 off, N2/O2 on - N2O diffuses rapidly out of blood into alveolus - N2 slow to be taken up from alveolus - Dilution of alveolar O2 → Hypoxia |
Pharmacodynamics
| System | Pros | Cons |
|---|---|---|
| Resp | Mild ↑ RR, ↓ TV, minimal ↑ PaCO2 | - ↑ RR, ↓ TV - ↑ SNS output → ↑ PVR → ↑ RV strain - Carrier gas → ↓ FiO2 |
| CNS | - Additive with volatile anaesthetics - Analgesia (NMDA antagonist), may reduce risk of chronic pain |
- MAC 105%, MAC-awake 67%; inadequate alone for anaesthesia - Pro-emetic - ↑ CBF - Minimal effect on EEG; BIS/entropy unhelpful - Euphoria, abuse potential |
| Other | - No vasodilation, MAC sparing → ↓ Bleeding - Does not trigger MH |
Mutagenesis: - Oxidises cobalt atom in Vitamin B12, a cofactor for methionine synthetase - Impaired DNA synthesis - Megaloblastic anaemia - Subacute combined degeneration of the cord (chronic abuse) - Teratogenesis |