Introduction: Decreases in oxygen saturation (SO2) and lactate concentration [Lac] from superior vena cava (SVC) to pulmonary artery have been reported. These gradients (Delta SO2 and Delta[Lac]) are probably created by diluting SVC blood with blood of lower SO2 and [Lac]. We tested the hypothesis that Delta SO2 and Delta[Lac] result from mixing SVC and inferior vena cava (IVC) blood streams.
Methods: This was a prospective, sequential, observational study of hemodynamically stable individuals with pulmonary artery hypertension (n = 9) who were about to undergo right heart catheterization. Catheters were advanced under fluoroscopic guidance into the IVC, SVC, right atrium, right ventricle, and pulmonary artery. Samples were obtained at each site and analyzed for SO2, [Lac], and glucose concentration ([Glu]). Analysis of variance with Tukey HSD test was used to compare metabolite concentrations at each site.
Results: There were no differences in SO2 or [Lac] between IVC and SVC, both being greater than their respective pulmonary artery measurements (P < 0.01 for SO2 and P < 0.05 for [Lac]). SO2 and [Lac] in right atrium, right ventricle, and pulmonary artery were similar. Delta SO2 was 4.4 +/- 1.4% (mean +/- standard deviation) and Delta[Lac] was 0.16 +/- 0.11 mmol/l (both > 0; P < 0.001). Delta[Glu] was -0.19 +/- 0.31 mmol/l, which was not significantly different from zero, with SVC [Glu] being less than IVC [Glu].
Conclusion: Mixing of SVC with IVC blood does not account for the development of Delta SO2 and Delta[Lac] in hemodynamically stable individuals with pulmonary artery hypertension. An alternate mechanism is mixing with coronary sinus blood, implying that Delta SO2 and Delta[Lac] may reflect changes in coronary sinus SO2 and [Lac] in this patient population.