2020A Question 04
Compare and contrast the effects of sevoflurane and nitrous oxide on the cardiovascular system.
Examiner Report
The major domains assessed in this question were:
- General
- Dose dependency
- Comparison of doses used, understanding of differences between agents
- Pharmacodynamics
- Mechanisms and effects of both agents on cardiovascular parameters
Credit was given for other relevant correct material including clinical utility of combining agents at reduced doses to promote cardiovascular stability. Common problems were:
- Not providing a mechanism for Nitrous Oxide’s cardiovascular effects
- Not providing a reason why Sevoflurane and Nitrous Oxide have different effects
- Not describing the dose dependent nature of effects see
- Omitting effects on pulmonary vasculature Omitting effects on regional circulations
- Inaccurate effect of Sevoflurane on the baroreceptor reflex
There was confusion about how cardiovascular parameters interacted. Many candidates incorrectly stated that cardiac output decreases due to decreased systemic vascular resistance or that decreased cardiac output was the main cause of decreased mean arterial pressure with Sevoflurane.
This material can be found in:
- Miller (8th edition) Chapter 28 Inhaled Anesthetics: Cardiovascular Pharmacology
- Katzung, Basic and Clinical Pharmacology 14e Chapter 25 General Anesthetics
- Stoelting’s Pharmacology and Physiology in Anesthetic Practice, 5e Chapter 4 Inhaled Anaesthetics
Model Answer
| Overall Effects | Sevoflurane | Nitrous oxide |
|---|---|---|
| Summary: | Dose-dependent depression | Minimal overall effect Use to spare sevoflurane (e.g. Haemorrhaging patient) |
| Direct effects | ↓ L-Ca2+ activity ↑ NO activity |
Minimal Some negative inotropy |
| Indirect effects | Baroreceptor reflex → ↑ HR | ↑ SNS outflow from brainstem Transient (≤1 hour) |
| Cardiac Effects | Sevoflurane | Nitrous oxide |
|---|---|---|
| Rate | ↑ | ↔↑ |
| Contractility | ↓ | ↔ |
| Output | ↔ | ↔↑ |
| Systemic circulation | ↓ SVR ↓ MAP |
↔↑ SVR ↔↑ MAP |
| Pulmonary circulation | ↓ PVR (including ↓ HPV) | ↑ PVR Exaggerated if already PHTN |
| CNS circulation | Dose-dependent\ | ↑ CMRO2 → CBF Avoid if ↑ ICP |
| Anaesthetic preconditioning | Yes (some disagreement) | No |
| QTc prolongation | Yes | No |
Special Mention
| Sevoflurane | Detail |
|---|---|
| CBF | - ≤0.5MAC: ↓ CBF (↓ CMRO2 predominates) - 0.5-1MAC: CBF normalises - >1MAC: ↑ CBF (vasodilatation predominates) |
| Preconditioning | - Activation of mitochondrial and sarcolemmal ATP-sensitive K+ channels → Hyperpolarization - Occurs at MAC >0.25 - Mimics ischaemic preconditioning |