The vital capacity (IVC or FVC) in adults increases linearly with standing height and declines linearly with age from age 25 yr. In children and adolescents the VC is similarly determined by height and age, but the relationship is more complex. The greater complexity arises particularly from the change in the relationship between leg and trunk height (and hence between standing height and volume of the chest) from childhood to adulthood. The assessment is based on the standard deviation score (SDS) or z-score, i.e. the difference between observed and predicted value divided by the residual standard deviation of the predicted mean in a healthy population. If the IVC or FVC is below the 5th percentile for length and age in a healthy population, the VC is regarded to be too small, and the ‘abnormality’ classified as:
A moderately low VC corresponds with about 60-80 % of the predicted value, severe lowering with < 60% predicted.
In 7699 spirometric tests performed at Dijkzigt University Hospital a moderately low IVC was observed in 20,7% of patients, severely lowered in 14,4% of patients. In a normal population (Vlaardingen and Vlagtwedde) a low FVC is uncommon: moderately low FVC 5,6%, severe lowering 1,4% of spirometric tests.