Consequences of inspired oxygen fraction manipulation on myocardial oxygen pressure. Adenosine and Lactate concentrations: A combined myocardial microdialysis and sensitive oxygen electrode study in pigs

Adenosine is a potent vasodilator whose concentration has been shown to increase in cardiac tissue in response to hypoxia. However, the time-dependent relationship between the levels of myocardial interstitial adenosine and tissue oxygenation has not yet been completely established. Therefore, the purpose of this study was to investigate the complex relationship between tissue myocardial oxygen tension (PtiO₂) and interstitial myocardial adenosine and lactate concentrations by developing a new technique which combines a cardiac microdialysis probe and a Clark-type Po₂ electrode. The combined and the single microdialysis probes were implanted in the left ventricular myocardium of anesthetized pigs. The consequences of the combined use of microdialysis and Po₂ probes on myocardial PtiO₂ and microdialysis performances against glucose were evaluated. A moderate but significant reduction in the relative recovery against glucose of the combined probe was observed when compared to that of the single microdialysis probe (42±2 v 32±1%, mean±S.E.M.: n = 5: P<0.05), at 2 μl/min microdialysis probe perfusion flow. Similarly, myocardial oxygen enrichment, measured by the Po₂ electrode, was negligible when microdialysis probe perfusion flow was 2 μl/min. Systemic hypoxia (FiO₂ = 0.08) resulted in a significant decrease in PtiO₂ from 30±4 to 11±2 mmHg, limited increase in coronary blood flow (CBF), and a significant increase in myocardial adenosine and lactate concentrations from 0.34±0.05 to 0.98±0.06 μmol/l and from 0.45±0.05 to 0.97±0.06 mmol/l respectively (P<0.05). Increasing the FiO₂ to 0.3 restored the PtiO₂ and hemodynamic parameters to baseline values with no changes in interstitial adenosine and lactate concentrations. Nevertheless, myocardial interstitial adenosine remained significantly higher than baseline values. In conclusion, this study demonstrates the ability of a combined probe to measure simultaneously regional myocardial PtiO₂ and metabolite concentration during hypoxia. The hypoxia-induced increase in myocardial adenosine persists after correction of hypoxia. The physiological significance of this observation requires further studies. (C) 2000 Academic Press.