The oxygen clock-a dual tracer physiological timer

In vivo differential decay of the radioisotopic pair 19O/15O is used to measure the regional label 'age'. Simplified expressions are developed relating the measured age to oxygen transit times and utilization. Simultaneous production of the label pair employs a continuous flow, mixed-gas target irradiated by the University of Wisconsin tandem Van de Graff accelerator. Flow-through chemistry follows, with transport to the user site up to several hundred metres distant. Gamma energy spectra from a twelve-probe detector array are recorded in an on-line computer during prolonged normal breathing of doubly labelled air. While the dynamics of oxygen compounds are of great physiological importance, the lack of compartment specificity of the 19O/15O pair raises fundamental questions in the interpretation of the age results.