We appreciate the interest expressed by Drs. Schroth, Reyle-Hanh, and Rossaint with regard to our recent article. First, using a Marquette Gas Analyzer (Model 1100; Marquette Gas Analysis Corp., St. Louis, MO), we verified that the oxygen and nitrogen fractions were approximately 20 and 40%, respectively, and 20 and 0%, respectively, to give estimated xenon fractions of 40 and 80%, respectively, in the gas reservoir bags. In addition, effective xenon concentrations in solutions were verified by placing the samples and xenon standards into sealed 1-ml vials and conducting a head-space analysis using the H-P 5989 MS-ENGINE Mass Spectrometer (Hewlett-Packard, Palo Alto, CA).
The second concern was that the hearts might become hypoxic with 20% oxygenated perfusate solution, despite the presence of approximately 2.8 g hemoglobin/100 ml perfusate. Most Langendorff preparations that are not perfused with crystalloid solutions are perfused with washed erythrocytes obtained from other species. As the authors pointed out, the venous oxygen tension and p H do not suggest hypoxia. Oxygen consumption of Langendorff hearts is approximately 50–70% of in vivo hearts, and lactate is not produced with carbogen equilibrated in crystalloid perfusate. This is caused in part by the lack of kinetic (stroke) work and decreased potential (isometric) work. Nevertheless, in our erythrocyte-perfused hearts, we conducted a control experiment suggested by the authors; that is, erythrocyte perfusion of hearts with 95% and 5% CO2before switching to the reservoirs containing 20% O2. The change from 95% to 20% O2produced no appreciable change in left ventricular pressure, and so we doubt that the hearts became hypoxic. However, we believe that the use of erythrocyte solution might result in a lower left ventricular pressure because of buffering of the calcium in the solution. A small increase in viscosity may also contribute to somewhat lower left ventricular pressure.