Lung volumes in exacerbations of asthma

Abstract

Lung volumes and forced expiratory volume in one second (FEV 1) were measured serially in thirty patients during recovery from episodes of severe asthma. Abnormalities of lung volumes were present in all patients at some stage during the course of the illness. Vital capacity (VC) was significantly reduced in most (but not all) patients at the time of acute dyspnoea, and improved with therapy to reach normal levels in all but two patients. Functional residual capacity (FRC) was increased above normal levels in twenty-eight patients on admission to hospital, and in some the increases were of the order of 3 to 5 L. In most patients FRC decreased with treatment, but in three it remained unchanged and in seven it increased. Residual volume (RV) showed changes similar in direction and magnitude to those of FRC. Total lung capacity (TLC) was increased by amounts up to several liters in about half the patients initially and in all of these it decreased with treatment; in the other patients it was normal on admission and either increased or remained unchanged during recovery. Gross changes in lung volumes were demonstrated in an acute episode of asthma with recovery in less than an hour. Analysis of relative changes in TLC, FRC and inspiratory capacity (IC) during recovery from episodes of asthma, combined with a consideration of changes in the chest roentgenogram, provide a unified explanation of the lung volume changes in asthma. It is suggested that all subjects react to increased expiratory airways resistance with an increase in RV, FRC and TLC, sometimes of considerable magnitude. The measured changes of lung volume in individual subjects are influenced by the development of non-ventilated regions in the lungs which do not necessarily depend on complete airways closure or atelectasis. The considerable increases in FRC must lead to large increases in the elastic work required in inspiration during episodes of asthma, and must account for an appreciable portion of the respiratory distress. This factor probably also accounts for the complaint of inspiratory difficulty made by some patients. In the presence of the large changes in TLC demonstrated, measurements of FEV 1 alone do not provide a reliable index of change in the degree of airways obstruction. In particular, when TLC decreases appreciably, even a constant FEV 1 indicates a considerable lessening in the degree of airways obstruction. Adequate assessment of the response of a patient to therapy requires the measurement of lung volumes as well as of FEV 1. Otherwise, objectively beneficial therapy may be withheld from patients in whom it appears to produce insignificant increases in FEV 1.