Canadian Anaesthetists’ Society Journal

, Volume 28, Issue 5, pp 422–430 | Cite as

The effect of pco2 on hypoxic pulmonary vasoconstriction

  • William H. Noble
  • J. Colin Kay
  • Joseph A. Fisher
Article

Abstract

Lung areas with a low V/Q ratio cause hypoxaemia. The low alveolar oxygen concentration may cause local hypoxic pulmonary vasoconstriction (HPV) which reduces perfusion, raises the V/Q ratio, and hence reduces the tendency to a low Pao2. By changing Pco2, the HPV response can be altered. We examined this relationship in anaesthetized dogs by using a tracheal divider to separate hypoxic (nitrogen ventilated) from oxygenated (100 per cent oxygen ventilated) lung. Relative perfusion was assessed from total133Xe exhaled from each lung area after intravenous infusions. When PaCO2 was changed by changing ventilation, we found that an increasing PaCo2 increased HPV and also Pao2. At a Paco2 of 3.3 kPa, HPV was abolished and Pao2 fell. We also changed PaC02 by altering PiCO2 to one or both lung areas while ventilation remained constant throughout the experiment. Again as PaCO2 increased, HPV and Pao2 increased. When PaC02 fell and end tidal carbon dioxide in the hypoxic lung (Petco2) remained elevated (by maintaining Pico2 in the hypoxic lung and removing CO2 from the oxygenated lung)HPV was maintained. Thus it is the alveolar concentration of CO2 in the hypoxic lung which is important in modifying HPV.

We conclude that in this model a low PetCO2(3.3 kPa) in hypoxic lung will reduce HPV, and will result in more severe hypoxaemia. This may have relevance in both anaesthetized and intensive care unit patients when a higher Pao2 may be obtained by increasing hypoxic lung Petco2. The effect of PetC02 on Pao2 will be influenced by other variables, but when hypoventilated or hypoxic lung areas exist, increasing PetCO2 may reinforce hypoxic pulmonary vasoconstriction and thus may increase Pao2.

Key Words

Lung hypoxic vasoconstriction carbon dioxide 

RéSUMé

Les zones pulmonaires dont le rapport V/Q est bas produisent de l’hypoxémie. La baisse de concentration de l’oxygène alvéolaire par contre entraine de la vasoconstriction hypoxique locale, ce qui a pour effet de diminuer la perfusion, améliorer le rapport V/Q et ainsi diminuer la tendance à l’hypoxémie. En modifiant la Pco2, on peut altérer la vasoconstriction hypoxique pulmonaire. Nous avons examiné cette relation sur des chiens anesthésiés en utilisant une cloison trachéale qui séparait un poumon rendu hypoxique par ventilation à l’azote, d’un poumon oxygéné avec 100 pour cent d’oxygène. La perfusion relative a été évaluée par la mesure du133Xe expiré par chaque poumon après injection intraveineuse.

Lorsqu’on a modifié la PaCO2 en changeant la ventilation, nous avons trouvé qu’une augmentation de la PaCo2 augmentait la vasoconstriction pulmonaire hypoxique et ainsi la Pao2. A une Paco2 de 3.3 kPa, la vasoconstriction hypoxique pulmonaire a été abolie et la Pao2 s’est abaissée. Nous avons modifié aussi la PaC02 en changeant la PiCo2 pour un ou les deux poumons alors que la ventilation demeurait constante pendant l’expérience: encore là, lorsque la PaCo2 a augmenté, la vasoconstriction hypoxique pulmonaire et la Pao2 ont aussi augmenté. Lorsque la PaCo2 s’est abaissée et que la mesure du CO2 du poumon hypoxique en fin d’expiration (PetC02) est demeuré élevée (en maintenant la PiCo2 dans le poumon hypoxique et en retirant le CO2 du poumon oxygéné) la vasoconstriction hypoxique a été maintenue. On peut en conclure que c’est la concentration alvéolaire de CO2 dans le poumon hypoxique qui permet de modifier la vasoconstriction hypoxique.

Nous avons conclu que sur ce modèle une PetCo2 basse (3.3 kPa) va diminuer la vasoconstriction pulmonaire hypoxique sur le poumon hypoxique et va ainsi augmenter la gravité de l’hypoxémie. Ceci peut avoir de l’importance chez le malade sous anesthésie et aux soins intensifs où une augmentation de la PetCO2 du poumon hypoxique permettra d’obtenir une Pao2 plus élevée. L’influence de la PetCo2 sur la PaC02 peut être modifiée par d’autres facteurs variables mais quand il existe des zones hypoventilées ou hypoxiques, l’augmentation de la PetC02 peut augmenter la vasoconstriction hypoxique et par le fait même augmenter la Pao2.

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Copyright information

© Canadian Anesthesiologists 1981

Authors and Affiliations

  • William H. Noble
    • 1
  • J. Colin Kay
    • 1
  • Joseph A. Fisher
    • 1
  1. 1.Department of AnaesthesiaUniversity of Toronto, St. Michael’s HospitalToronto

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