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Canadian Anaesthetists’ Society Journal

, Volume 25, Issue 2, pp 117–121 | Cite as

How rebreathing anaesthetic systems control Paco 2: Studies with a mechanical and a mathematical model

  • Richard L. Keenan
  • C. Paul Boyan
Article

Summary

Results from a proposed equation for rebreathing systems,\(Pa_{CO_2 } = \frac{{\dot V_{CO_2 } (P_B - P_{H_2 O} )}}{\begin{gathered} \dot V_E (1 - V_D /V_T ) \hfill \\ + \frac{{\dot V_{CO_2 } (P_B - P_{H_2 O} )}}{{\dot V_F }} \hfill \\ \times \frac{{\dot V_E - \dot V_F }}{{\dot V_E }} \hfill \\ \end{gathered} }\) were compared with results from a mechanical model “lung” ventilated either with a Bain Breathing Circuit, or a circle system (Eger-Ethans type A) without soda lime. When values for carbon dioxide excretion (Vco2), dead space-tidal volume ratio (Vd/Vt), minute volume ventilation (Ve), and fresh gas flow (Vf) were varied over a wide range, the model and the equation yielded identical values of Paco 2.

When Vco2 = 2.25 ml/kg, Vd/Vt = 0.5, and Ve = 140 ml/kg, then fresh gas flows (Vf) with both the equation and the model produced values of Paco 2 which were very close to those found by Bain and Spoerel in anaesthetized, artificially ventilated humans. It is concluded that the equation is an accurate mathematical representation of how rebreathing anaesthetic systems control Paco 2.

It is expected that the equation will be useful in the clinical application of rebreathing anaesthetic systems, allowing the selection of minute volumes and fresh gas flows which will yield predictable Paco 2 values.

Keywords

Soda Lime Minute Volume PH2O Breathing System Bain Circuit 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

RéSUMé

Les auteurs proposent un modèle mathématique représentatif du contrôle de la Pco2 dans les circuits anesthésiques avec réinspiration. Ľusage ďun modèle mécanique de poumon, ventilé au moyen ďun circuit de Bain ou ďun circuit sans réinspiration (Eger-Ethans) a permis de vérifier la validité de ľéquation.

La formule proposée est la suivante:\(Pa_{CO_2 } = \frac{{\dot V_{CO_2 } (P_B - P_{H_2 O} )}}{\begin{gathered} \dot V_E (1 - V_D /V_T ) \hfill \\ + \frac{{\dot V_{CO_2 } (P_B - P_{H_2 O} )}}{{\dot V_F }} \hfill \\ \times \frac{{\dot V_E - \dot V_F }}{{\dot V_E }} \hfill \\ \end{gathered} }\) Paco 2 = Pression partielle de la Pco2 artérielle Vco2 = Excrétion-minute de CO2 Ve = Ventilation-minute Vd/Vt = Rapport espace mort/volume courant Vf = Apport de gaz frais-minute Pb - Ph2o = Pression barométrique - Pression de vapeur ďeau.

Cette équation apparaît ďutilité clinique car elle permet la sélection des volumes de ventilation-minute et des débits de gaz frais nécessaires pour maintenir la Pco2 à des niveaux prévisibles.

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Copyright information

© Canadian Anesthesiologists 1978

Authors and Affiliations

  • Richard L. Keenan
    • 1
  • C. Paul Boyan
    • 1
  1. 1.Department of AnesthesiologyMedical College of VirginiaRichmond

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