The FVL permits discrete measurements to be taken right down to the last ml of the expired breath. Because these measures represent the smallest diameter airways, a new term entered the pulmonary function lexicon, small airway disease. For reasons to complex to discuss here this did not prove to be the case. Predicted Normal Values Patient results are compared to a set of values obtained from a group of normal subjects. It is important for the clinician selecting a set of normal predicted values to become familiar with the characteristics of the study group.
Most automated spirometers provide a limited choice of predicted normal values.
These include studies performed by Crapo, Knudson, Miller and Polgar, the latter a popular pediatric study. All are well-designed studies that have met ATS criteria. A better approach is to use a statistical interpretation of the data from the normal study group. For each parameter measured during a normal study, a measure of the average value and a measure of the variability around the average is calculated. Each SD represents the probability, expressed as a percentage, that a single measured value falls within a certain distance from the mean.