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Canadian Journal of Anesthesia, Vol 28, 422-430, Copyright © 1981 by Canadian Anesthesiologists' Society

The Effect of Pco2 on Hypoxic Pulmonary Vasoconstriction

WILLIAM H. NOBLE 1, J. COLIN KAY 1, and JOSEPH A. FISHER 1

1 Department of Anaesthesia, University of Toronto, St. Michael's Hospital, 30 Bond Street, Toronto, Ontario, M5B 1W8

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 Paoo2. 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 total 133Xe exhaled from each lung area after intravenous infusions. When Pacoco2 was changed by changing ventilation, we found that an increasing Pacoco2 increased HPV and also Pacoco2. At a Pacoco2 of 3.3 kPa, HPV was abolished and Paoo2 fell. We also changed Pacoco2 by altering Picoco2 to one or both lung areas while ventilation remained constant throughout the experiment. Again as Pacoco2 increased, HPV and Paoo2 increased. When Pacoco2 fell and end tidal carbon dioxide in the hypoxic lung (PETcoco2) remained elevated (by maintaining Picoco2 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 PETcoco2 (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 Paoo2 may be obtained by increasing hypoxic lung PETcoco2. The effect of PETcoco2 on Paoo2 will be influenced by other variables, but when hypoventilated or hypoxic lung areas exist, increasing PETcoco2 may reinforce hypoxic pulmonary vasoconstriction and thus may increase Paoo2.

Key Words: LUNG • hypoxic vasoconstriction • carbon dioxide






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Copyright © 1981 by the Canadian Anesthesiologists' Society.