An analogue instrument for the measurement of respiratory impedance using the forced oscillation technique

The successful development of non-invasive assisted ventilation via a nasal mask has led to the requirement for improved methods of monitoring the respiratory system. Measurements of respiratory impedance during assisted ventilation using acoustic methods are showing considerable promise in providing this clinical data. The method involves imposing very low amplitude acoustic oscillations onto the spontaneous or assisted respiratory airflow. The subsequent interaction with the respiratory system modifies the pressure and flow relationship of the forced oscillations. The ratio of pressure to flow can be used to measure in real time the absolute value of respiratory input impedance. As a result a versatile, non-invasive and repeatable profile of respiratory mechanics is obtained. Using a 5 Hz conventional forced oscillation frequency, data highly sensitive to airflow obstruction and capable of tracking mechanical events and responses become available to the clinician. Recent studies with ventilated and sleeping subjects confirm future potential for the forced oscillation technique in vivo. We describe an instrument named the respiratory impedance monitor (5 Hz), which utilizes the forced oscillation technique (FOT). This fully developed device is designed in accordance with agreed international clinical guidelines. Comparative clinical data from normal subjects and patients with chronic obstructive pulmonary disease is presented.