Control of the heterogeneity of O2 saturation in small myocardial veins

There is a wide variability in the measured O2 saturations (SvO2) of small myocardial veins (20-100 microns). The purpose of the present study was to examine several factors that contributed to this variability. Studies were conducted on anesthetized open-chest dogs. The role of beta-adrenoceptors and flow in the control of the heterogeneity of SvO2 was assessed. The hearts were quick-frozen in liquid N2 and the SvO2 in the subepicardial and subendocardial (ENDO) regions of the left ventricule was determined by microspectrophotometry. Under control conditions, ENDO SvO2 averaged 35% with a variance of 337. The control variance was much more than that of the methodological errors. With propranolol or practolol, variance was reduced by about half, e.g. ENDO SvO2 variance was 145 with propranolol. This reduction in SvO2 variance could be prevented by isoproterenol. When flow is reduced by partial occlusion the SvO2 values were skewed toward lower saturations. When nitroglycerin is administered during this partial occlusion, the SvO2 distribution is restored toward normal. It can be concluded that some of the variability of SvO2 in the heart is control by beta 1-adrenoceptors. There also appears to be an inverse relationship between coronary blood flow and SvO2 heterogeneity. Further work is necessary to determine what other factors control the variability of SvO2.