The paper by Schachter et al 1 in the January 2001 issue ofThorax is interesting in that it has a number of unusual and, it is suggested, inexplicable findings that appertain to various indices of ventilatory capacity. With all due deference, we would suggest that there is an explanation for these unusual findings.

Firstly, mild, moderate and severe obesity are all associated with an incremental reduction in both the forced expiratory volume in 1 second (FEV₁) and the forced vital capacity (FVC).2-4 Secondly, in normal subjects and in those who have pure restrictive impairment, the FVC and FEV₁ are within 2–3% of each other when expressed as a percentage of predicted. The FVC cannot be significantly smaller than the FEV₁ when expressed as a percentage of predicted except in certain neurological diseases. It is noted that the criterion for acceptance of the spirometric volumes was “two measurements of the FEV₁ within 100 ml of each other”, suggesting the FVC was ignored. Table 3 in the paper by Schachter et al shows that, when expressed as a percentage of predicted, the FVC in every instance is less than the FEV₁. In most groups there is a relatively small difference except for those who are moderately or severely obese.

The reason for the disparity in the FEV_{1 …}