Interpretation of radioactive gas clearance rates in the lung

¹ Department of Medicine, Postgraduate Medical School, and The Medical Research Council, Radiotherapeutic Research Unit, Hammersmith Hospital, London, England

The clearance rates of short-lived radioactive gases from defined areas of the lung have been used to measure regional blood flow and gas exchange. The clearance rates are measured by external counting over the chest during a short period of breath holding following a rapid inspiration of the gas. Two processes are necessary for the clearance of any radioactive gas from the counting field, namely, transfer from alveolar gas to pulmonary capillary blood, and subsequent removal of the active gas from the field. Recently it has been shown that oxygen-labeled CO₂ is lost from alveolar gas exceedingly rapidly, and it can therefore be used to separate these two clearance rate processes. A theoretical analysis shows how that can be done, and experiments designed to test the theory are described. Although considerable approximations are necessary, the analysis clarifies the factors controlling the clearance rate of any radioactive gas. One consequence is that the large variations in the regional uptake of radioactive CO and O₂ previously reported are probably overestimates.