Journal of Applied Physiology, Vol 66, Issue 4 1896-1904, Copyright © 1989 by American Physiological Society

ARTICLES

Quantification of regional V/Q ratios in humans by use of PET. I. Theory

C. G. Rhodes, S. O. Valind, L. H. Brudin, P. E. Wollmer, T. Jones and J. M. Hughes
Medical Research Council Cyclotron Unit, Royal Postgraduate Medical School, Hammersmith Hospital, London, United Kingdom.

With positron emission tomography, quantitative measurements of regional alveolar and mixed venous concentrations of positron-emitting radioisotopes can be made within a transaxial section through the thorax. This allows the calculation of regional ventilation-to-perfusion (V/Q) ratios by use of established tracer dilution theory and the constant intravenous infusion of 13N. This paper considers the effect of the inspiration of dead-space gas on regional V/Q and investigates the relationship between the measured V/Q, physiological V/Q, and V/Q defined conventionally in terms of bulk gas flow (VA/Q). Ventilation has been described in terms of net gas transport, and the term effective ventilation has been introduced. A simple two-compartment model has been constructed to allow for the reinspiration of regional (or personal) and common dead-space gas. By use of this model, with parameters representative of normal lung the effective V/Q ratio for 13N [(VA/Q)eff(13N)] is shown to overestimate VA/Q by 18% when VA/Q = 0.1 but underestimate VA/Q by 68% when VA/Q = 10. For physiological gases, the model predicts that the behavior of O2 should be similar to that of 13N, so that, in terms of gas transport, V/Q ratios obtained using the infusion of 13N closely follow those for O2. Values of the effective V/Q ratio for CO2 [(VA/Q)eff(CO2)] lie approximately halfway between (VA/Q)eff(13N) and VA/Q. These results indicate that dead-space ventilation is far less a confounding issue when V/Q is considered in terms of net gas transport (VAeff), rather than bulk flow (VA).(ABSTRACT TRUNCATED AT 250 WORDS)

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