Kinetics of potent inhaled anesthetics and carrier gases at reduced fresh gas flows - A primer

Jan F.A. Hendrickx, A. A.J. Van Zundert, Andre M Dewolf*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

When using a circle system, rebreathing will ensue if total fresh gas flow (FGF) decreases below minute ventilation: exhaled gases will be reinhaled, filling the bellows of the ventilator (controlled ventilation) or the breathing bag (spontaneous ventilation) with a mixture of fresh gas and exhaled gas. Assuming a minute ventilation of 5 L.min-1, rebreathing will minimally affect the composition of the inspired mixture at FGF above 3.5 L.min-1, because the reinhaled gas will mainly be composed of dead space gas. As FGF is further reduced (especially below 1 L.min-1), the composition of the exhaled gas will progressively affect the composition of the inspired mixture. For potent inhaled anesthetic agents, this implies that the delivered (vaporizer) concentration will have to be increased to maintain the same alveolar concentration. In addition, vaporizer setting variability increases as FGF decreases, At the same FGF, these effects are least pronounced for the agents with the lower blood solubility. When using FGF well below 1 L.min-1, the carrier gas will also determine the required vaporizer setting because different carrier gases will lead to different losses via the pop-off valve at the same FGF. The use-ofair at low FGF will lead to reinhalation of O2 depleted gas, therefore progressively more O2 has to be added to the O2/air mixture to maintain appropriate inspired O2 concentrations. While the same holds true for O2/N2O mixtures, the decreasing uptake pattern of N2O has to be taken into account: the N2O FGF will have to be decreased as uptake gradually decreases. This article summarises these effects and the practical measures required to safely use low flow anaesthesia in clinical practice.

Original languageEnglish (US)
Pages (from-to)78-84
Number of pages7
JournalCPD Anaesthesia
Volume6
Issue number2
StatePublished - Sep 24 2004

Keywords

  • Air
  • Carrier gas
  • Circle system
  • Inhaled anesthetics
  • Low-flow anesthesia
  • Nitrous oxide
  • Oxygen
  • Rebreathing

ASJC Scopus subject areas

  • Critical Care and Intensive Care Medicine
  • Anesthesiology and Pain Medicine

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