THE APPLICATION OF THE FICK PRINCIPLE TO THE MEASUREMENT OF PULMONARY BLOOD FLOW

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Abstract
Though fundamentally sound, the Fick method, based on the principle that the volume of O2 entering the pulmonary capillaries equals the volume of O2 gained by the flowing blood, is less accurate under experimental conditions. Potential sources of error include the necessity for measuring the O2 uptake at the mouth, the impossibility of measuring the variables instantaneously, and the uncertainty of the relationship between the arterial and mixed venous blood at the points where blood is sampled. Methods for minimizing the potential errors depend mainly on the exactness with which the O2 uptake measured at the mouth reflects the rate at which O2 enters the capillary bed, the constancy of the concentration of O2 in the arterial and mixed venous blood, and the extent to which the average outputs of the right and left ventricles determined over a finite period are the same. The effect of anastomoses between pulmonary and bronchial vessels on calculated value of flow is also discussed.