In the first formula, the calculated ppo FEV1 values are always almost 150 to 250 mL less than the values calculated by the second formula upon the existence of obstructed segments. In both situations, the preoperative FEV1 values are the same. This condition is very important for the patients with borderline preoperative FEV1 values. The patients who are accepted inoperable according to the first formula may indeed be in the operable group. For example, a patient is being planned to undergo left upper lobectomy: a:2 and b:3. Preoperative FEV1 value is 1.6 L. According to the first formula, ppo FEV1 = 1.184 L; according to the second formula, epo FEV1 = 1.318 L. The difference is 134 mL. The obstructed segments to be resected do not have any contribution to the preoperative FEV1. So, only the unobstructed segments to be removed should be taken into account while calculating the epo FEV1. As a result, the first formula does not reflect the real value. In conclusion, the second formula should be used to calculate the percentage of ppo FEV1 in order to give the chance of operability to the patients with borderline respiratory functions.