Dyspnea, also known as shortness of breath or breathlessness, is a subjective sensation of breathing discomfort. It is a common symptom, present in up to half of patients admitted to acute, tertiary care hospitals and in one quarter of medical outpatients. 
There are also multiple sensations of dyspnea; the best described can be grouped together into: 
Sensation of work or effort
Air hunger/unsatisfied inspiration.
The etiology of dyspnea covers a broad range of pathologies from mild, self-limited processes to life-threatening conditions. Diseases of the cardiovascular, pulmonary, and neuromuscular systems are the most common. Dyspnea may be acute (e.g., acute exacerbation of congestive heart failure, acute pulmonary embolism, acute heart valve insufficiency), subacute (e.g., worsening asthma, exacerbation of chronic obstructive pulmonary disease [COPD]) or chronic (e.g., stable COPD, stable interstitial lung disease).
The evaluation and management of dyspnea is directed by the clinical presentation, findings from the history and physical exam, and preliminary investigation results. 
Clinical Assistant Professor of Medicine
University of Chicago
Division of Pulmonary and Critical Care Medicine
NorthShore University HealthSystem
TJK declares that he has no competing interests.
Dr Tomasz J. Kuzniar would like to gratefully acknowledge Dr Kamilla Kasibowska-Kuzniar and Dr Kaiser G. Lim, previous contributors to this monograph. KGL declares that he has no competing interests, and KKK declares that she has no competing interests.
Senior Lecturer and Consultant Physician
North Bristol Lung Centre
NM declares that he has no competing interests.
MRC Training Fellow and Specialist Registrar, Respiratory Medicine
Oxford Centre for Respiratory Medicine
NR declares that he has no competing interests.
Division of Pulmonary, Allergy, Critical Care and Sleep Medicine
Ohio State University Medical Center
JPP declares that he has no competing interests.
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