Total effective time of the forced expirogram in disease: sources of error and a correction factor

Effective time of the forced expirogram is a sensitive index for the detection of mild airways obstruction. However, there is evidence that this measurement is not superior to maximum flow rates in the lower half of the forced vital capacity or even FEV1 and FEV1% in some patients suffering from obstructive lung disease. Furthermore we noticed that in some patients with a decrease of the forced vital capacity caused by exacerbation of airways obstruction, the effective time was not appreciably changed. We concluded that this apparent disadvantage of the effective time is the result of the different forced vital capacities. To eliminate this error we transformed the forced expirogram to the equivalent curve that the patient would produce, if his forced vital capacity was equal to the predicted mean value for his age, sex, and height. The derived corrected total effective time seems to have increased sensitivity for detection of small changes in expirograms obtained from the same subject or from different subjects.