Global causes of a small vital capacity
The IVC and FVC may be diminished due to:
|Expiratory airway obstruction||Premature airway closure during expiration due to
|Restrictive lung disease||Decreased maximum lung volume from
- Spirometric tests cannot be used to disclose a restrictive ventilatory defect, but they can be used to exclude it: a normal VC excludes restrictive lung disease.
- The severer the airway obstruction, the more common it is to find that the vital capacity is too small. In general practice, however, restrictive lung disease is rare, so that in the case of airway obstruction and a low VC a restrictive ventilatory defect need not be considered seriously. If the FVC is too small and the IVC has not been measured one might consider assessing both: if the FVC is appreciably smaller than the IVC this is evidence in favor of obstructive lung disease causing the low VC.
- If the VC is too small but there is no evidence of airway obstruction the first concern is to ascertain that this is not due to insufficient subject co-operation for whatever reason.
- A restrictive ventilatory defect can only be demonstrated from an abnormally low total lung capacity. Measurement of the TLC or making a chest X-ray to ascertain the diagnosis is only warranted if there are clinical signs compatible with a restrictive defect, in particular if this points to intrathoracic pathology. Because of the low prevalence of a restrictive syndrome even in a clinical population the predictive value of a positive test result is very unsatisfactory, and the cost/benefit ratio for establishing a single case of restrictive disease very poor (Aaron). A general rule of thumb is that measurement of lung volumes should only be performed if FEV1%FVC > 55% and FVC%pred < 85%. This rule has a 96% sensitivity for predicting a low TLC and an 98% negative predictive power for excluding restriction (Glady).
Prevalence of restrictive disease and cost effectiveness of diagnostic procedures
Aaron SD, Dales RE, Cardinal P. How accurate is spirometry at predicting restrictive pulmonary impairment? Chest 1999; 115: 869-873.
Glady CA, Aaron SD, Lunau M, Clinch J, Dales RE. A spirometry-based algorithm to direct lung function testing in the pulmonary function laboratory. Chest 2003; 123: 1939–1946.
Swanney MP, Beckert LE, Frampton CM, et al. Validity of the American Thoracic Society and other spirometric algorithms using FVC and forced expiratory volume at 6 s for predicting a reduced total lung capacity. Chest. 2004; 126: 1861–1866.