The American Thoracic Society defines dyspnea as a “subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity. The experience derives from interactions among multiple physiological, psychological, social, and environmental factors and may induce secondary physiological and behavioral responses.” Dyspnea, a symptom, must be distinguished from the signs of increased work of breathing.
Respiratory sensations are the consequence of interactions between the efferent, or outgoing, motor output from the brain to the ventilatory muscles (feed-forward) and the afferent, or incoming, sensory input from receptors throughout the body (feedback), as well as the integrative processing of this information that we infer must be occurring in the brain (Fig. 5-1). In contrast to painful sensations, which can often be attributed to the stimulation of a single nerve ending, dyspnea sensations are more commonly viewed as holistic, more akin to hunger or thirst. A given disease state may lead to dyspnea by one or more mechanisms, some of which may be operative under some circumstances, e.g., exercise, but not others, e.g., a change in position.
Hypothetical model for integration of sensory inputs in the production of dyspnea. Afferent information from the receptors throughout the respiratory system projects directly to the sensory cortex to contribute to primary qualitative sensory experiences and provide feedback on the action of the ventilatory pump. Afferents also project to the areas of the brain responsible for control of ventilation. The motor cortex, responding to input from the control centers, sends neural messages to the ventilatory muscles and a corollary discharge to the sensory cortex (feed-forward with respect to the instructions sent to the muscles). If the feed-forward and feedback messages do not match, an error signal is generated and the intensity of dyspnea increases. (Adapted from MA Gillette, RM Schwartzstein: Mechanisms of dyspnea, in Supportive Care in Respiratory Disease, SH Ahmedzai and MF Muer [eds]. Oxford, UK, Oxford University Press, 2005.)
Disorders of the ventilatory pump, most commonly increase airway resistance or stiffness (decreased compliance) of the respiratory system, are associated with increased work of breathing or a sense of an increased effort to breathe. When the muscles are weak or fatigued, greater effort is required, even though the mechanics of the system are normal. The increased neural output from the motor cortex is sensed via a corollary discharge, a neural signal that is sent to the sensory cortex at the same time that motor output is directed to the ventilatory muscles.
Chemoreceptors in the carotid bodies and medulla are activated by hypoxemia, acute hypercapnia, and acidemia. Stimulation of these receptors, as well as others that lead to an increase in ventilation, produce a sensation of ...