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Regional measurements of tissue isotope concentration, made using positron emission tomography (PET), allow tracer models to be used in a quantitative manner to provide topographic distributions of many structural and functional parameters, each derived for the same well-defined lung element. In this paper we describe a technique to measure regional ventilation-perfusion ratios (V/Q), in absolute units, by use of PET and the continuous intravenous infusion of an inert gas isotope, 13N, and report on measurements made in 12 normal subjects (4 smokers). Data were obtained from a single lung section (slice thickness, 1.7 cm full width at half-maximum response to a line source) at the level of the right ventricle in the supine posture during quiet breathing. For the 12 subjects, volume-weighted mean values of V/Q, averaged over individual right and left lung fields, ranged from 0.50 to 1.29. Analysis of these means showed no difference between lungs: right, 0.80 +/- 0.23 SD; left, 0.76 +/- 0.20 SD. Topographically, a systematic fall of V/Q in the ventrodorsal direction was observed in eight of the subjects (mean ventrodorsal difference 0.39, range 0.19-0.90), whereas two showed a clear increase toward dependent lung regions (range 0.16-0.26). Seven of the subjects with a falling ventrodorsal V/Q gradient also exhibited discrete regions of low V/Q at the dorsal lung border. We conclude that, in normal subjects, ventilation and perfusion are generally well matched in the supine posture, but isolated mismatching often occurs in dependent lung regions.
Rhodes et al. (1989) studied this question.