Many methods have been used to measure the functional residual capacity of the lung (FRC), some simple, others complicated and elaborate. Nevertheless, an accurate measurement of functional residual capacity in patients with pulmonary emphysema has been more difficult than expected and, as time has passed, it has become apparent that the functional residual capacity in patients with emphysema is larger than previously thought.
In 1940, Darling, Cournand, and Richards (1) described the open circuit nitrogen (7-minute washout) method, now widely used, for estimating functional residual capacity, but pointed out that imperfect gas mixing in the lung could impair the accuracy