Quality control of perfusion: Monitoring venous blood oxygen tension to prevent hypoxic acidosis

Abstract

The long-held belief that venous oxygen tension mirrored tissue oxygen tension became suspect in the 1960s when new instrumentation consistently showed that tissue oxygen tension was 10 to 30 torr less than venous oxygen tension. Moreover, a countercurrent of oxygen exchange between terminal arteries and veins was shown to exist. Despite this conflict in scientific theory, however, monitoring venous oxygen tension as a means to control hypothermic cardiopulmonary bypass has been repeatedly urged, since myocardial acidosis is clearly extremely detrimental. This study of the relationship between venous oxygen tension during hypothermic bypass and a concurrent increment in lactacidemia yields strong objective evidence to support the use of on-line venous oxygen tension monitoring to guide perfusion. In a random series of 36 patients, venous blood samples were drawn at five preselected intervals during operation and were analyzed for pH, carbon dioxide tension, oxygen tension, lactic acid, hematocrit, and base excess. Analysis of the data revealed that venous pH and base excess showed no correlation to venous oxygen tension. However, lactic acid showed a strong correlation with venous oxygen tension, with a correlation coefficient of 0.4338 at a confidence level of p less than 0.0001. If the patients were divided into three clinically pertinent groups based on the lowest venous oxygen tension recorded, a strong relationship between venous oxygen tension and lactic acid emerged. If the lowest measurement of venous oxygen tension was greater than 35 mm Hg (group A), the mean rise in lactic acid was only 0.12 microns/ml. If the lowest measurement was between 30 and 34 mm Hg (group B), the mean rise was 0.64 microns/ml. Whereas, if any venous oxygen tension value fell below 30 mm Hg (group C), the mean rise in lactic acid was 2.56 microns/ml. Analysis of variance showed that group C values were significantly different from groups A and B values (p less than 0.0002). A scientific hypothesis relating venous oxygen tension to adequate tissue oxygenation is proposed. Use of venous oxygen tension monitoring with the goal to maintain the level above 35 mm Hg is strongly supported by this study.