This study was undertaken to examine the impact of the variations of cardiac mechanical work in heart’s respiratory quotient (RQ) and venoarterial carbon dioxide difference (ΔPCO2). In twenty-seven patients undergoing coronary artery bypass grafting under extracorporeal circulation, a coronary sinus line was instituted and it’s correct position was verified by the waveform displayed in the monitor. Immediately after cannulae placement, a haemodynamic profile was obtained and simultaneous arterial and coronary sinus (a & cs) sampling for blood gas analysis was done in an ABL 720 (Radiometer Copenhagen) analyzer. A second collection of the same data was obtained five minutes later with the patients in a slight (15-20°) “head-down” position. Conditions for exclusion was intersample variation of haemoglobin’s concentration greater than 15% and sodium ion concentration difference greater than 10% of the greater value. ΔP(acs) O2, ΔP(cs-a)CO2, blood gas content differences and heart’s respiratory quotient were calculated and correlated to cardiac output (CO) and other haemodynamic parameters. All samples were drawn before any intraoperative blood transfusion. No ischemia was detected during sampling. Statistical analysis employed t-paired test and linear regression. “Head-down” position had a significant impact to all haemodynamic parameters except heart rate. In both data rows, although CO ranged widely and altered significantly, coronary sinus oxygen saturation and arteriovenous O2 content difference were stable and showed insignificant correlations to all the haemodynamic parameters that were studied. Carbon dioxide content difference (coronary sinus-arterial) did not show statistically significant decrease associated with higher systemic flow. Heart’s venoarterial (coronary sinus-arterial) CO2 difference (ΔPCO2) was found stable and independent of flow. Finally, respiratory quotient decreased significantly from 0.91 ± 0.4 to 0.86 ± 0.4 (p=0.037). Our findings are in accordance to other observations of uselessness of coronary sinus saturation monitoring as indicator of coronary flow. Heart’s little extraction reserve is faced with the coronary flow reserve. Our study documents that ΔPCO2 also is of limited value as well, fact related to alterations of respiratory quotient together with the CO2 clearance augmentation, during states of increased mechanical work. In the physiologic range and under the conditions of cardiac anesthesia, ΔPCO2 appears stable and is not suitable for clinical decisions concerning heart’s coronary flow.