The successful development of non-invasive assisted ventilation via anasal mask has led to the requirement for improved methods of monitoring therespiratory system. Measurements of respiratory impedance during assistedventilation using acoustic methods are showing considerable promise inproviding this clinical data.The method involves imposing very low amplitude acoustic oscillations onto thespontaneous or assisted respiratory airflow. The subsequent interaction withthe respiratory system modifies the pressure and flow relationship of theforced oscillations. The ratio of pressure to flow can be used to measure inreal time the absolute value of respiratory input impedance. As a result aversatile, non-invasive and repeatable profile of respiratory mechanics isobtained. Using a 5 Hz conventional forced oscillation frequency, data highlysensitive to airflow obstruction and capable of tracking mechanical events andresponses become available to the clinician. Recent studies with ventilatedand sleeping subjects confirm future potential for the forced oscillationtechnique in vivo.We describe an instrument named the respiratory impedance monitor (5 Hz), whichutilizes the forced oscillation technique (FOT). This fully developed deviceis designed in accordance with agreed international clinical guidelines.Comparative clinical data from normal subjects and patients with chronicobstructive pulmonary disease is presented.