Increase in total ventilation versus shunt fraction at constant PaCO2. Required increase in total ventilation (%) at different shunt fractions (Qs/Qt) to maintain arterial carbon dioxide tension (PaCO2) constant. 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. Airway dead space (Vdaw) and the alveolar dead space caused by uneven ventilation/perfusion (VdAVQ) were assumed to be 0.2 l and 0.06 l, respectively.