EXPIRATORY SPIROGRAM

Abstract

In healthy subjects, the forced expiratory spirogram begins with a small effort-dependent portion. The second and largest part is characterized by orderly deceleration of flow at a rate of 2.7 liters per second for every liter exhaled. It ends when flow becomes 0.5 liters per second, a point at which lung tension has been found to reach zero. At any selected point in the second part, flow cannot be increased by applying greater than threshold pressure; hence, it is called critical flow. In the third terminal part, flow is slow and variable and volume is small. Constant flow at a selected point is attributed to check-valve narrowing of cartilaginous large airways. The deceler- ating rate at which flow is regulated is ascribed to bronchiolar narrowing which parallels the decrease of volume and is mediated through decreasing lung tension. These characteristics are constant for all healthy lungs served by healthy airways. The amount of participating lung determines the inspiratory extremity of the spirogram, but the configuration is constant in health. Disease produces quantitative variation in the characteristics exhibited in health. Bronchospasm makes bronchioles small and more resistant to stretch, so dV/dV is lower than the normal value of 2.7, but responds to bronchodilators. Interstitial pneumonitis renders bronchioles wide, so dV/dV is high. Secretions reduce the amount of participating lung and so do not alter dV/dV. In emphysema, the fixed low value for dV/dV is ascribed to low lung tension and the aberrant relation to volume. In this disease, flow is judged to be sequential. First, a restricted volume is exhaled from relatively intact parenchyma. Thereafter, slow leakage from emphysematous parenchyma, makes up an enlarged terminal stage of the spirogram. Value for dV/dV varies inversely with the flow-resistive state of the airway system. A method of analysis based on the foregoing, and with ancillary knowledge of volume-restricting lesions and of the effect of bronchodilators, appreciably extends the useful information to be obtained from the spirogram. In an extensive clinical experience it has proved to be a helpful diagnostic measure.