Low Frequency Forced Oscillation Technique in Clinical Practice

According to the World Health Organization, more than 300 million people are suffering from respiratory diseases like asthma or chronic obstructive pulmonary disease. Currently, diagnosis and monitoring of function tests, that cannot fully capture the underlying mechanisms of respiratory dysfunction. In particular, the deeper lung consisting of numerous smallest airways and lung tissue, where structural damage in most respiratory diseases are thought to begin, cannot be adequately represented. A test with high potential in this respect, is the forced oscillation technique (FOT). It applies pressure oscillations at the mouth of the patient and measures the resulting flows. The ratio of pressure to flow yields a measure of the lung’s mechanical behavior, named “respiratory impedance”. Furthermore, FOT performed in the low frequency range, i.e. below 5Hz, is known to be particular sensitive to the mechanical behavior of the deeper lung. However, low frequency FOT has not yet found its way into routine clinical use, because of the disturbance created by the patient’s breathing. Current solutions of low frequency FOT to circumvent the breathing disturbance are very demanding on the test subject and hence unfeasible in the clinic. This work is a major step in making low frequency FOT fit for routine clinical use. We have built a fan-based system which allows patients to freely breathe while receiving pressure oscillations allowing impedance measurements down to 0.1 Hz. Using state-of-the-art modeling techniques the experimental signals were analyzed and interpreted, to identify and eliminate breathing disturbances from the FOT measurements. In a second stage, an adaptive measurement technique was developed, tailored to the patient’s own natural breathing frequency. In this way, high quality impedance measurement could be obtained in asthma and COPS patients of various disease severities. In addition, the adaptive FOT measurement allowed us to also extract the time-varying behavior of the respiratory system to better capture all aspects of the mechanical behavior of the deep lung.