Carbon dioxide output

This chapter describes how carbon dioxide is produced from metabolism and also from buffering of lactic acid. The volume of carbon dioxide exhaled (VCO2) is calculated from the concentration in exhaled gas and minute ventilation. If the lungs are less efficient than normal, with a high dead space, the amount of ventilation needed to achieve any given VCO2 is much higher. This index, known as the ventilatory equivalent for carbon dioxide, is an important prognostic marker. Early on in a cardiopulmonary exercise test (CPET), VCO2 is slightly less than the oxygen uptake (VO2). As exercise reaches its maximum, VCO2 increases more quickly when acidaemia starts to stimulate ventilation.