Pathophysiology

Dyspnea can be thought of as difficulty in breathing of which the individual is aware. Thus it is a subjective sensation that is difficult to measure and somewhat poorly understood.

Production of Dyspnea
Primary Causes	Dyspnea occurs when ventilatory demand cannot be met by the body’s ability to respond. Dyspnea develops when there is a mismatch between central respiratory motor activity and incoming afferent information from receptors in the airways, lungs and chest wall structures. (Concept of “length-tension inappropriateness” termed by Campbell and Howell in 1963) The perception of respiratory effort increases whenever the central motor command to the respiratory muscles must be increased; i.e. increase mechanical load or weakened muscles and the increased work of breathing. (See: Corollary Discharge).
CHF	The sense of air hunger, as described in patients with congestive heart failure, has been shown to be associated with increases in ventilatory drive, particularly in the presence of hypoxemia or hypercapnia.
Bronchial Asthma	The sensation of chest tightness is associated with bronchoconstriction. It develops early in the process with mild airway obstruction. With progressive decline in the lung function, increased respiratory effort follows and lastly, air hunger develops.
Perception of Dyspnea
Temporary dyspnea in healthy persons	Healthy persons may perceive dyspnea due to exercise, but it is short lived and well tolerated.
Chronic dyspnea secondary to illness	Dyspnea occurs in patients with chronic disease when ventilatory demands exceed supply. It is multifactorial.
Mood, Stress and Fatigue	Fatigue and mood changes such as anxiety, depression, somatization, and hostility significantly increase with high intensity of dyspnea in asthma (Gift 1991). In an epidemiologic study of healthy individuals, increase in anxiety, anger, depression associated with increase of respiratory symptoms including dyspnea (Dales 1989). Anxiety has been shown to significantly correlate with the intensity of dyspnea, as in cancer patients (Bruera 2000).
Personality	Personality influences perception of dyspnea: in emphysema, nervousness and cyclic tendency are the determinants for dyspnea, in addition to anxiety (Kawakami 1992). in asthma, the severity of disease is linked to psychological disturbances and poor perception of breathlessness, and in hypochondriacs dyspnea is related to severity. (Chetta 1998).

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