Monitoring of fluctuating airway obstruction and episodes of coughing by thoracic electrical impedance.
The most prominent feature of bronchial asthma is the fluctuating airway obstruction of the affected subjects. Cough is also one of the major symptoms of asthma, but of other conditions as well. The continuous registration of airway obstruction and coughing in the working or open-air environment or at the homes of the potential sufferers may help establish a sometimes elusive diagnosis. To this purpose we developed a method using a measurement of the changes in the chest basal electrical impedance (Z0). A portable device for long-term recording of the Z0 signal was built using a microprocessor system. In order to assess both gradual Z0 changes, suggestive of altering airway patency and coughing episodes (characterized by abrupt changes), a continuous analogue-to-digital conversion (ADC) loop with a sampling frequency of 16 Hz was programmed. After completion of the recording, the memory of the portable instrument can be downloaded to a PC system for assessment and analysis. Appropriate averaging and filtering procedures have been worked out. This device may be further upgraded to give out a signal when Z0 starts to increase, so as to prevent a full-blown asthmatic crisis.