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


Some negative inotropy

Indirect effects Baroreceptor reflex → ↑ HR

↑ SNS outflow from brainstem

Transient (≤1 hour)

Cardiac Effects Sevoflurane Nitrous oxide
Rate ↔↑
Output ↔↑
Systemic circulation



↔↑ SVR

↔↑ MAP

Pulmonary circulation ↓ PVR (including ↓ HPV)


Exaggerated if already PHTN

CNS circulation Dose-dependent\


Avoid if ↑ ICP

Anaesthetic preconditioning Yes (some disagreement) No
QTc prolongation Yes No

Special Mention

Sevoflurane Detail

- ≤0.5MAC: ↓ CBF (↓ CMRO2 predominates)

- 0.5-1MAC: CBF normalises

- >1MAC: ↑ CBF (vasodilatation predominates)


- Activation of mitochondrial and sarcolemmal ATP-sensitive K+ channels → Hyperpolarization

- Occurs at MAC >0.25

- Mimics ischaemic preconditioning

Last updated 2021-08-23

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