Tests of minimal obstruction in the small airways: interrelationships and physiologic significance

Several technics have become popular to detect airway obstruction in the small (less than 2 mm diameter) peripheral bronchi which appear to be the site of the early lesions in chronic obstructive pulmonary disease (COPD). These include determining maximal midexpiratory flow rate (MMFR) from the expiration spirogram, maximum expiratory flows at low lung volumes from the maximum expiratory flow-volume (MEFV) curve, a comparison of flow-volume curves during air and helium/O2 breathing, demonstration of frequency dependence of compliance, and the measurement of closing volume (CV). This discussion attempts to interpret the literature relative to these tests, examining their physiologic basis and clinical significance while relating them to the standard spirometric tests. The aim is to provide the clinical anesthesiologist with a better understanding of the information they provide, since their physiologic basis relates well to the many aspects of abnormal lung function which appear during anesthesia and the entire perioperative period.