Human Airflow Measurement for Asthma Monitoring
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Asthma is a chronic health condition that restricts pulmonary airflow by reduction of airway diameter either through a combination of inflammation, excess mucus secretion and smooth muscle constriction or independently. This disease cannot be cured, but can be held at bay with a well-designed asthma management plan. A crucial part of the management plan is home asthma care through peak expiratory flow, a measure of flow rate of forced exhalation of the subject. Forced expiratory volume at 1 second measures the volume exhaled at 1 second from the same maneuver and is considered as an important supporting data. Literature reveals devices that measure peak expiratory flow and forced expired volume at 1 second and devices that measure peak expiratory flow alone. Existing peak expiratory flow devices in the market can be broadly classified into two categories: Mechanical and electronic devices. The mechanical devices cost fewer than thirty US dollars. Their disadvantages are manual data logging and lack of key digital features related to data communication. Research reveals data fabrication and reduction of compliance with the use of mechanical devices. The electronic devices improve measurement compliance but cost more than eighty US dollars. There does not exist a device that costs less than thirty US dollars and yet provides the benefits of the electronic devices. This thesis work approaches the above-mentioned problem by starting with variable orifice flow-sensing mechanism proposed in a previous work and developing a low-cost smartphone interface for the sensor data processing, recording and transmission. The smartphone interface is a combination of a voltage-controlled oscillator based circuit with discrete components that connects to the audio jack of a smartphone. A variation of variable orifice flow-sensing mechanism named pneumotachograph is then adopted due to better resistance to flow offered and custom modifications are made to this mechanism to lower development cost. This is achieved through creating 3D designs of peak flow tubes that can be printed with low-cost 3D printers. A smartphone application that processes the incoming signal in the audio jack and returns peak expiratory flow reading is developed. The concept is demonstrated in android and can be extended to all operating systems. The system is designed for adult peak expiratory flow devices of 300-800 LPM initially and is later extended to include child range of 90-500 LPM by modifying the peak flow tube. Following this, a new low-cost flow sensing mechanism is developed and demonstrated for the entire peak flow range. Based on this, a key tag like form factor is also proposed. This thesis work demonstrates the development of an electronic peak expiratory flow and forced expiratory volume at 1-second device that costs less than fifty US dollars by using smartphone for data processing, data recording and data communication, therein eliminating cost related to data processing such as microcontrollers and data recording such as memory cards. This thesis work demonstrates a novel flow sensing mechanism applied to peak expiratory flow monitoring that reduces the device cost to twenty US dollars. This thesis work also opens possibilities to smartphone-based advantages like alerts, reminders, e-mail communication of data to physicians and so on by developing and demonstrating a low-cost cross-operating system compatible interface to smartphones via the audio jack.