What Is Asthma?

Asthma is a chronic inflammatory disorder of the lower airways characterized by variable, largely reversible expiratory airflow limitation and by bronchial hyperresponsiveness to stimuli that leave healthy airways unaffected. Symptoms include wheeze, cough, chest tightness, and breathlessness, and their defining feature is variability: severity fluctuates over hours, days, and seasons rather than following the steady decline typical of other obstructive pulmonary diseases. That variability is what makes asthma a measurement problem as much as a clinical one, since a single office visit may catch a patient with entirely normal lung function.

The condition is common at every level of development. A systematic analysis of the global burden of asthma from the Global Burden of Disease program placed prevalence in the range of a quarter of a billion people worldwide, with mortality and disability concentrated in low- and middle-income countries where access to inhaled controller therapy is limited.

Pathophysiology and Phenotypes

Three mechanisms interact to produce the obstruction: contraction of airway smooth muscle, inflammatory swelling of the mucosa, and excess mucus secretion. Chronic inflammation drives structural remodeling of the airway wall, including thickening of the basement membrane and increased smooth muscle mass, which over years can convert reversible obstruction into a fixed component.

Asthma is best understood as a set of overlapping phenotypes rather than one disease. Type 2 high asthma, marked by eosinophilic airway inflammation and often by allergic sensitization, responds well to corticosteroids and is the target of monoclonal antibody therapies directed at immunoglobulin E, interleukin-5, and the interleukin-4 receptor. Type 2 low, neutrophilic, and paucigranulocytic phenotypes respond poorly to steroids and remain a research problem. Occupational asthma, exercise-induced bronchoconstriction, and aspirin-exacerbated respiratory disease are further distinct presentations.

Diagnosis and Pulmonary Function Measurement

Diagnosis rests on a compatible history plus objective demonstration of variable airflow limitation. Spirometry is the primary instrument: the patient performs a forced expiratory maneuver, and the ratio of forced expiratory volume in one second to forced vital capacity establishes obstruction, while the change in that volume after an inhaled bronchodilator establishes reversibility. Guidance from the NHLBI on asthma diagnosis sets out this sequence and the role of bronchoprovocation testing when baseline spirometry is normal.

Fractional exhaled nitric oxide provides a non-invasive marker of eosinophilic airway inflammation, measured by chemiluminescence or electrochemical sensing of a breath sample at a controlled flow rate. Peak expiratory flow monitoring, oscillometry, and, in research settings, hyperpolarized gas magnetic resonance imaging and exhaled breath condensate analysis add further dimensions. The 2020 focused updates to the asthma management guidelines formalized the place of nitric oxide testing alongside spirometry in ambiguous cases.

Management and Monitoring Technology

Treatment combines inhaled corticosteroids as controller therapy with short- or long-acting bronchodilators for relief, delivered by pressurized metered-dose inhalers, dry powder inhalers, or nebulizers. Device engineering matters directly to outcomes: particle size distribution determines regional deposition in the lung, and incorrect actuation timing wastes a large fraction of the delivered dose. Electronic monitoring adaptors and sensor-instrumented inhalers record actuation time and location, which has made adherence and trigger exposure measurable rather than self-reported. Ambient particulate, pollen, ozone, and humidity sensing feeds exposure models, and machine learning applied to symptom diaries, spirometry traces, and acoustic cough analysis is being evaluated for exacerbation prediction.

Applications

Asthma diagnosis, monitoring, and treatment draw on a range of engineering disciplines, including:

• Biomedical instrumentation for spirometry, oscillometry, and gas analysis
• Aerosol science and drug delivery device design
• Wearable and connected health sensing for adherence and symptom tracking
• Environmental sensor networks for air quality and allergen exposure
• Signal processing and machine learning for exacerbation prediction
• Medical imaging, including computed tomography and functional lung imaging