Interrelationship between lung volume, expiratory flow, and lung transfer factor in fibrosing alveolitis

Fifty patients with fibrosing alveolitis studied on 104 occasions exhibited significant direct correlations between vital capacity (VC), maximum mid-expiratory flow rate (MMFR), and transfer factor for carbon monoxide (TLCO). Forced expired volume in the first second (FEV1)/VC ratio bore a weak negative correlation with VC. Peak expiratory flow, MMFR, and maximum flow rates at 50% and 25% of VC were often reduced in patients with severe grades of pulmonary dysfunction. It appears that as the severity of the fibrotic process increases, the lung volumes shrink and the transfer factor for CO decreases. The total lung capacity decreases predominantly on account of reduction in VC. With a decrease in lung volume the MMFR also falls. Decrease in flow rates at low lung volumes is greater as compared to the fall in peak flow. The expiratory flow rates however were normal or even increased when related to absolute lung volume. Some patients exhibit disproportionate expiratory slowing as evidenced by a decrease in MMFR which is out of proportion to the reduction in VC. These patients also have a reduced FEV1/VC ratio. These changes are probably the consequence of associated peripheral airway narrowing.