Dyspnea, or shortness of breath, is a subjective experience of breathing discomfort marked by distinct sensations of varying intensity. Resulting from interactions among physiologic, psychological, social, and environmental factors, dyspnea may lead to secondary physiologic and behavioral responses. Dyspnea can be caused by cardiac and pulmonary disease (e.g., congestive heart failure, acute coronary syndrome, pneumonia, chronic obstructive pulmonary disease, pulmonary embolism), as well as other conditions (e.g., anemia, mental disorders, lung cancer).