Factors affecting the respiratory ratio during cardiopulmonary by-pass

Despite the wide use of hypothermic cardiopulmonary bypass (CPB) during ope n heart surgery there is little information about the patient metabolism. I n particular no complete studies addressed the assessment of the respirator y ratio (RR) during CPB at different core temperatures. Therefore a clinica l study was performed in order to evaluate the oxygen consumption (VO2) and carbon dioxide production (VCO2) in adult patients with valvular or corona ry heart disease undergoing CPB. Twenty-five patients (16 male, 9 female) a ged between 26 and 76 (54.2+/-12.4 mean+/-SD) were the subjects of this stu dy. Measurements (102) were taken at various perfusion flow rates (from 1.6 to 2.9 L/min(-1) . m(-2)) and temperatures (from 24 to 37 degrees C). Arte rial and mixed venous gas analyses were performed and O-2 and CO2 concentra tions were calculated, including the carbamate contribute. We calculated VO 2, VCO2 and then RR from artero-venous differences in O-2 and CO2 contents. Both VO2 and VCO2 showed a positive linear correlation with temperature (r =0.82 and r=0.59 respectively) and with blood flow rate (r=0.61 and r=0.29 respectively). The mean RR was 0.78+/-0.28 and more than 84% of RR Values f ell in the range 0.5-1.2. No significant correlation between RR and tempera ture and blood flow rate was observed. VCO2 and RR showed a positive linear correlation with the gas to blood flow rate ratio (r=0.37 and r=0.49 respe ctively).