Regional variations in uptake of radioactive CO in the normal lung

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

Carbon monoxide labeling with O¹⁵ was produced by passing O¹⁵, made in the Medical Research Council's cyclotron, over charcoal at 1000° C. After a single breath of the radioactive gas the rate of fall of activity during apnea was measured by external counting in different regions of the lungs of normal subjects in the sitting position. The carbon monoxide ‘clearance rate’ calculated from these data is proportional to the diffusion per unit volume in the zone under study. There was a gradient of diffusion per unit volume, the highest value being found in the basal regions of the lung and much lower values toward the apex. On exercise the clearance rate in the upper zone increased to a value similar to that in the lower zone at rest; the clearance rate in the lower zone was unchanged. Measurements of the O¹⁵ clearance rate showed a similar gradient but on exercise both upper and lower zone values increased so that the gradient was maintained. The results are interpreted as showing that in the resting state all the basal capillaries are open but that the proportion of patent capillaries diminishes toward the apex. On exercise there is an increase in flow at both apex and base and in consequence the remaining apical capillaries open.

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