This information was extracted from plotting FEV1 volumes against CO2 in patients with obstructive lung disease. CO2 retention can be anticipated once the FEV1 volume is less than 0.8 litres. Hence the logic of wanting to have at least a postop FEV1 value larger than 0.8 litres. 5 year survival of CO2 retainers may not be better than the benefit from cancer surgery.

This concept is important because you will encounter some old or short patients whose FEV1 predicted value may be less than 1.8 litres. 0.8 rule applies only to obstructed patients. You may be able clear such patients on clinical grounds.