Increase in PaCO2 or alveolar ventilation versus shunt fraction. Increase in arterial carbon dioxide tension (PaCO2; %) at constant alveolar ventilation versus shunt fraction. This is equivalent to required increase in alveolar ventilation to maintain PaCO2. Examples are as follows: 'Basal': Qt = 5 l/minute, haemoglobin (Hb) = 145 g/l and base excess (BE) = 0; 'Qt = 3': Qt = 3 l/minute, Hb = 145 g/l and BE = 0; and 'Metab. acidosis': Qt = 3.5 l/minute, Hb = 97 g/l and BE = -13.