Outline of calculations further detailed in Additional file 1. (a) Starting out from input parameters, analytical calculations of intermediate parameters were performed using standard equations. (b) Input parameters, together with venous oxygen content (CvO2) and arterial oxygen content (CaO2), define unique values of venous oxygen saturation (SvO2) and arterial oxygen saturation (SaO2), which were iteratively determined. Venous carbon dioxide content (CvCO2) was calculated in accordance with the method reported by Giovannini and coworkers . (c) In an extensive system of iterations, arterial carbon dioxide tension (PaCO2) was iteratively adjusted until veno-arterial difference in carbon dioxide content (ΔC [v-a]CO2) multiplied by total cardiac output (Qt) became equal to carbon dioxide elimination (V'CO2). In a step parallel to that shown in panel c, end-capillary carbon dioxide tension (PcCO2) was iteratively determined employing the value of QA (blood flow to ventilated alveoli) instead of Qt.