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

• Published in: Journal of applied physiology (1985) Vol. 66; no. 4; p. 1896
• Main Authors: Rhodes, C G, Valind, S O, Brudin, L H, Wollmer, P E, Jones, T, Hughes, J M
• Format: Journal Article
• Language: English
• Published: United States 01.04.1989
• Subjects: Humans; Tomography, Emission-Computed; Ventilation-Perfusion Ratio
• ISSN: 8750-7587
• DOI: 10.1152/jappl.1989.66.4.1896

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).