Determination of regional ventilation and perfusion in the lung using xenon and computed tomography.

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Abstract
We propose a model to measure both regional ventilation (V˙) and perfusion (Q˙) in which the regional radiodensity (RD) in the lung during xenon (Xe) washin is a function of regional V˙ (increasing RD) and Q˙(decreasing RD). We studied five anesthetized, paralyzed, mechanically ventilated, supine sheep. Four 2.5-mm-thick computed tomography (CT) images were simultaneously acquired immediately cephalad to the diaphragm at end inspiration for each breath during 3 min of Xe breathing. Observed changes in RD during Xe washin were used to determine regional V˙ and Q˙. For 16 mm3, Q˙ displayed more variance than V˙: the coefficient of variance of Q˙ (CVQ˙) = 1.58 ± 0.23, the CV of V˙ (CVV˙) = 0.46 ± 0.07, and the ratio of CVQ˙ to CVV˙ = 3.5 ± 1.1. CVQ˙(1.21 ± 0.37) and the ratio of CVQ˙ to CVV˙ (2.4 ± 1.2) were smaller at 1,000-mm3 scale, but CVV˙ (0.53 ± 0.09) was not. V˙/Q˙ distributions also displayed scale dependence: log SD of V˙ and log SD of Q˙ were 0.79 ± 0.05 and 0.85 ± 0.10 for 16-mm3 and 0.69 ± 0.20 and 0.67 ± 0.10 for 1,000-mm3 regions of lung, respectively. V˙ and Q˙ measurements made with CT and Xe also demonstrate vertically oriented and isogravitational heterogeneity, which are described using other methodologies. Sequential images acquired by CT during Xe breathing can be used to determine both regional V˙ and Q˙ noninvasively with high spatial resolution.