In the management of the hypoxic patient it should beremembered that tissue O2 delivery
depends not just onPao2 but also on the haemoglobin concentration andcardiac output. The most useful index of tissue oxygenationis the mixed venous (pulmonary arterial)/arterial O2content difference. The normal oxygen combining capacityis 1.4mL O2/g haemoglobin (approximately 0.06mmol/g).The oxygen content of blood is the oxygen combiningcapacity x haemoglobin concentration x saturation (plusa small amount of dissolved oxygen in plasma, which isnegligible at normal atmospheric pressure). Assuming thehaemoglobin concentration is 15g/100mL, and the oxygencontent of arterial blood is 1.4 x 15 x 0.95 = 20.58 mL/100 mL, and the oxygen content of mixed venous blood is1.4 x 15 x 0.75 = 15.75, the arterial/mixed venous oxygencontent difference is 4.83mL/100mL. If oxygen delivery isreduced (e.g. reduced cardiac output) the arterial/venouscontent difference widens.Type II hypercapnic ventilatory failure represents inadequacyof the respiratory muscle 'pump'. The concept of'pump' failure leading to hypercapnia is a useful one, focusingattention on the importance of CNS output, neuromuscularfunction and chest wall movement in themaintenance of adequate ventilation, althoughsevere impairment of gas exchange can contribute to anelevated CO2.