Shortness of breath, medically called dyspnoea, is the subjective sensation of difficult, uncomfortable or insufficient breathing. It is a symptom rather than a diagnosis and can arise from respiratory, cardiovascular, neurological, muscular, metabolic and psychological causes. It may occur suddenly or develop gradually, and it can appear during exertion, at rest, when lying down or in particular environmental circumstances. The underlying mechanism may involve increased respiratory effort, impaired gas exchange, abnormal airway resistance, reduced cardiac output, altered respiratory control or heightened perception of normal breathing signals. Determining the cause requires attention to onset, duration, triggers, associated symptoms and relevant medical history.
Respiratory diseases are among the most common causes of dyspnoea. Asthma can narrow the airways through smooth-muscle contraction, inflammation and mucus production, while COPD can cause persistent airflow limitation, air trapping and impaired gas exchange. Pneumonia can fill alveoli with inflammatory fluid, reducing effective gas exchange, and interstitial lung diseases can thicken or scar the lung tissue through which oxygen must diffuse. Pulmonary embolism can abruptly obstruct pulmonary blood flow and cause sudden breathlessness. These mechanisms are different, so treatment depends on identifying the specific respiratory problem rather than treating breathlessness as a single disease.
Cardiovascular disease can cause breathlessness even when the lungs themselves are initially healthy. Heart failure can increase pressure in the pulmonary circulation and cause fluid accumulation in the lungs, making breathing particularly difficult during exertion or when lying flat. Arrhythmias can reduce effective cardiac output and produce breathlessness, dizziness or palpitations. Ischaemic heart disease can also produce shortness of breath, sometimes with or without obvious chest pain. Because the heart and lungs operate as an integrated oxygen-delivery system, abnormalities in either organ can produce similar symptoms.
Anaemia is another important cause because reduced haemoglobin decreases the blood's capacity to carry oxygen even when the lungs are functioning normally. Metabolic disorders can increase ventilation or alter acid-base balance, producing a sensation of breathlessness. Severe infection, fever and systemic inflammation can increase metabolic demand and respiratory drive. Deconditioning and poor physical fitness can also cause breathlessness during activity that exceeds an individual's current cardiovascular or muscular capacity. These causes demonstrate that dyspnoea does not necessarily mean that the lungs themselves are diseased.
Psychological and neurological factors can contribute to breathlessness. Anxiety and panic can produce rapid breathing, chest tightness, air hunger, dizziness and tingling, particularly when hyperventilation lowers carbon dioxide. This does not mean that breathlessness associated with anxiety is imaginary; the respiratory sensation is real and involves physiological changes. However, anxiety should not automatically be assumed to be the cause of unexplained breathlessness, particularly when symptoms are new, severe, persistent or accompanied by chest pain, fainting or low oxygen levels. Medical causes should be appropriately considered before attributing significant symptoms solely to anxiety.
The circumstances surrounding breathlessness provide important diagnostic information. Exertional breathlessness may point towards cardiopulmonary limitation or deconditioning, while sudden breathlessness can occur with pulmonary embolism, pneumothorax, asthma exacerbation or acute cardiac disease. Breathlessness that worsens when lying flat, known as orthopnoea, is particularly associated with some forms of heart failure. Breathlessness that occurs in attacks at night may have different causes, including asthma or cardiac disease. Associated symptoms such as wheezing, fever, cough, sputum, leg swelling, chest pain, palpitations or haemoptysis can substantially change the differential diagnosis.
Evaluation can involve physical examination, oxygen-saturation measurement, blood tests, electrocardiography, chest imaging, spirometry and other pulmonary or cardiac investigations depending on the clinical context. Pulmonary function testing can distinguish airflow limitation from some restrictive patterns, while diffusion testing assesses movement of gas across the alveolar-capillary interface. Exercise testing can reveal abnormalities that are not evident at rest. The appropriate investigation therefore depends on the suspected mechanism rather than applying the same test to every person with dyspnoea.
Severe or rapidly developing shortness of breath requires urgent assessment because potentially life-threatening conditions can present this way. Sudden breathlessness accompanied by chest pain, coughing up blood, fainting or a very fast heartbeat can indicate pulmonary embolism. Severe breathlessness with chest pressure, collapse or other cardiovascular symptoms can indicate an acute cardiac emergency. Breathlessness associated with blue or grey discoloration, inability to speak normally, severe wheezing or rapidly deteriorating consciousness is also concerning. The symptom should therefore be interpreted in context rather than assumed to be asthma, anxiety, poor fitness or another familiar explanation without assessment.
