Figure 1. Comparison between measured arterial oxygen partial pressure (P^A-aO²) and that calculated from the V [circle, open over]^A/Q [circle, open over] distributions in patients with coronary artery disease (CAD; n = 12, open circles) and mitral valve disease (MVD; n = 12, solid circles). Data were obtained while the patients were awake and spontaneously breathing air. Note the similarity between the regression line (broken line) for measured and calculated P^aO2values (y =-0.508 + 1.0003x, r²= 0.90, P < 0.001) in the CAD group and the identity line (solid line). In the MVD group, calculated P^aO2was systematically higher than measured P^aO2, as indicated by a shift of the regression line (broken line; y = 10.753 + 0.923, r²= 0.77, P < 0.01). One patient with mitral regurgitation showed remarkably well-preserved oxygenation despite pulmonary hypertension.