Flow-volume curves: Restrictive lung disease
This is a characteristic pattern of a low FVC and comparatively high expiratory flow, giving the flow-volume curve a pointed cap appearance. Both FEV1 and FVC are well below predicted levels, but the FEV1/FVC ratio is normal or even high. Measurement of the total lung capacity may be considered to ascertain a restrictive ventilatory defect if its presence is not already evident from extrathoracic causes. If pathological changes of lung parenchyma are suspected it may be useful to assess the transfer factor for carbon monoxide, as the TL,CO is often diminished in interstitial lung disease.
In general practice the prevalence of restrictive ventilatory defects is very low indeed. In only a moderate proportion of subjects referred to hospital with suspected restriction on the basis of the above criteria is a restrictive ventilatory defect confirmed by measurement of lung volumes (Aaron, Swanney). Therefore entertain this 'diagnosis' only if spirometric findings and clinical symptoms are compatible with a restrictive syndrome. 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, Swanney).
Prevalence of restrictive disease and low cost effectiveness of diagnostic procedures:
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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.