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Factors influencing MAC reduction after cardiopulmonary bypass in dogs

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Abstract

Background

Anaesthetic requirements may be reduced following surgery employing cardiopulmonary bypass (CPB). This study, in dogs, determined the role of a) volatile agents (enflurane [E] vs isoflurane [I]), b) oxygenator (bubble [B] vs membrane [M]), and c) presence [FL] vs absence [NoFL] of an in-line arterial filter in the bypass circuit in altenng anaesthetic requirements following CPB.

Methods

Male mongrel dogs were anaesthetized with either enflurane (n=24) or isoflurane (n=24). They were randomly assigned to one of eight groups (n=6 per group); Group I (E/B/FL), Group 2 (E/M/FL), Group 3 (E/M/NoFL). Group 4 (E/B/NoFL), Group 5 (I/M/FL), Group 6 (I/B/FL), Group 7 (I/M/NoFL) or Group 8 (I/B/NoFL). MAC was determined using the tail-clamp method at hourly intervals, twice before and three times after a one hour normothermic perfusion using aortoatrial cannulation and CPB.

Results

Prior to CPB, MAC was reproducible (enflurane: MAC 1 2.17 ± 0.29 vs MAC2 2.14 ± 0.28%; isoflurane: MAC1 1.42 ± 0.31 vs MAC2 1.41 ± 0.33%) and differed among groups only for the volatile agent employed. Following CPB, MAC was reduced in all groups (P < 0.05 vs pre-CPB measurements) except Group I (E/B/FL). The degree of MAC reduction in other groups ranged from 39–64% and was not different based on type of agent employed, use of a membrane or bubble oxygenator, or presence or absence of an in-line arterial filter.

Conclusion

In dogs, MAC reduction following CPB was variable, not related to type of volatile agent employed, use of a membrane or bubble oxygenator, or presence or absence of an in-line artenal filter. The explanation for reductions in anaesthetic requirements following CPB in this model remains speculative.

Résumé

Objectif

On peut observer une diminution du besoin d’anesthésie après une intervention sous circulation extracorporelle (CEC). Cette étude réalisée chez des chiens visait à déterminer l’influence sur l’altération du besoin d’anesthésie après la CEC: a) de l’agent volatil (enflurane [E]vs isoflurane [I]; b) de l’oxygénateur (bulles [B] vs membrane [M] et c) de la présence [FL] vs l’absence [noFL] d’un filtre artériel en ligne sur le circuit extracorporel.

Méthodes

Des chiens mâles de race commune ont été anesthesies sort avec de l’enflurane (n=24) soit avec de l’isoflurane (n=24). Us ont été répartis aléatoirement en huit groupes (n=6 par groupe): Groupe I (E/B/FL), Groupe 2(E/M/FL), Groupe 3 (E/M/noFL), Groupe 4 (E/B/noFL), Groupe 5 (I/M/FL), Groupe 6 (I/B/FL), Groupe 7 (I/M/noFL) ou Groupe 8 (I/B/noFL). La méthode du pincement de la queue aux heures a servi à déterminer le MAC, deux fois avant et trois fois après une heure de perfusion normothermique sous CEC par canulation aortoatriale.

Résultats

Avant la CEC, le MAC était reproductible (enflurane MACI 2,17 ± 0,29 vs MAC2 2,14 ± 0,28%; isoflurane: MACI 1,42 ± 0,31 vs MAC2 1,41 ± 0,33%) et ne différait entre les groupes que selon l’agent volatil utilisé. Après la CEC, le MAC était réduit dans tous les groupes (P < 0,05 vs les mesures pré-CEC) excepté dans le Groupe I (E/B/FL). Le degré de réduction du MAC dans les autres groupes s’étendait sur une gamme de 39 à 64% et ne différait pas selon l’agent, l’oxygénateur à membrane ou à bulles et la présence ou l’absence d’un filtre artériel en ligne.

Conclusion

Chez des chiens après la CEC, la réduction du MAC variait, mais sans relation avec l’agent volatil administré, l’utilisation d’un oxygénateur à membrane ou à bulle et la présence ou l’absence d’un filtre arténel en ligne. Sur ce modèle, l’explication de la réduction du besoin anesthésique demeure hypothétique.

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Author information

Correspondence to Richard I. Hall.

Additional information

Supported by the New Brunswick Heart and Stroke Foundation. Dr. Hall is a PMAC/MRC Health Research Foundation Scholar.

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Neumeister, M.W., Li, G., Williams, G. et al. Factors influencing MAC reduction after cardiopulmonary bypass in dogs. Can J Anaesth 44, 1120–1126 (1997). https://doi.org/10.1007/BF03019237

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Keywords

  • Isoflurane
  • Cardiopulmonary Bypass
  • Enflurane
  • Minimum Alveolar Concentration
  • Volatile Agent