Factors Affecting the Respiratory Ratio during Cardiopulmonary By-Pass

Abstract

Despite the wide use of hypothermic cardiopulmonary bypass (CPB) during open heart surgery there is little information about the patient metabolism. In particular no complete studies addressed the assessment of the respiratory ratio (RR) during CPB at different core temperatures. Therefore a clinical study was performed in order to evaluate the oxygen consumption (VO₂) and carbon dioxide production (VCO₂) in adult patients with valvular or coronary heart disease undergoing CPB. Twenty-five patients (16 male, 9 female) aged between 26 and 76 (54.2±12.4 mean±SD) were the subjects of this study. 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°C). Arterial and mixed venous gas analyses were performed and O₂ and CO₂ concentrations were calculated, including the carbamate contribute. We calculated VO₂, VCO₂ and then RR from artero-venous differences in O₂ 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 fell in the range 0.5–1.2. No significant correlation between RR and temperature 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 respectively). (Int J Artif Organs 1998; 21: 802–8