Simulation of a patient with a constant shunt of 20% of the cardiac output who is breathing 100% O2. a) Arterial partial pressure of oxygen (PO2) is very sensitive to cardiac output because as the latter falls, so must pulmonary arterial PO2 (perfusing the ... Simulation of a patient with a constant shunt of 20% of the cardiac output who is breathing 100% O2. a) Arterial partial pressure of oxygen (PO2) is very sensitive to cardiac output because as the latter falls, so must pulmonary arterial PO2 (perfusing the shunt pathway). This highlights the importance of accounting for differences in cardiac output from normal (here taken as 6 L·min−1). b) Apparent shunt computed in the same simulation from equation 11 based on arterial PO2 values in panel a when cardiac output is not normal (but is assumed to be normal). The true shunt may thus be over- or under-estimated considerably. Peter D. Wagner Eur Respir J 2015;45:227-243 ©2015 by European Respiratory Society