Confirming a restrictive ventilatory defect
A low VC is a feature of both restrictive and obstructive ventilatory defects. Expiratory airway obstruction is associated with premature closure of intrapulmonary small airways during a deep expiration; the residual volume is increased at the expense of the vital capacity. This is a common feature of obstructive lung disease, being due to:
- thickening of the airway wall by
- influx of inflammatory cells
- hypersecretion and accumulation of secretions
- hypertrophy and hyperplasia of glandular cells
- hypertrophy and hyperplasia of smooth muscle cells
- remodeling by deposition of fibrocollagenous material
- increased smooth muscle tone
- diminished lung elastic recoil pressure generated by surrounding alveolar tissue.
In restrictive lung disease a low VC is most often due to the lung being too small. The prevalence of restrictive ventilatory defects in clinical practice, and even much more so in general practice, is much lower than that of obstructive ventilatory defects. On that account the predictive value of a small VC for restrictive lung disease is low. In case of a low (F)VC a restrictive ventilatory defect should only be considered if there is no evidence of
- airway obstruction (normal FEV1/FVC ratio) and
- increased RV.
and there is clinical evidence compatible with a restrictive ventilatory disorder. Do keep in mind that in alveolar fibrosis small intrapulmonary airways are embedded in the pathological process, so that a restrictive syndrome may be associated with signs of mild airway obstruction. If the FVC is unaffected by a bronchodilator drug this increases the likelihood of an restrictive disorder.
A restrictive ventilatory defect can only be ascertained by a small TLC assessed in a lung function laboratory.