There is a consensus within the research literature focused on the physiological elements of dyspnea that the degree of perceived breathlessness is proportional to respiratory effort; the greater the unsuccessful respiratory effort exerted by an individual, the greater the sensation of breathlessness experienced [28, 30-35]. Although the evidence suggests a relationship between respiratory effort, chemoreceptors and mechanoreceptors, the precise link to the physical mechanism of dyspnea remains unclear.
In an attempt to better understand the psychological aspect of dyspnea, a number of studies have investigated the relationship between anxiety and levels of dyspnea [36-40]. Indeed, patient descriptors of breathlessness vary depending on the intensity of the dyspnea experience [39]. Often the presenting symptom of acute dyspnea is anxiety [40]. Nurses may focus on anxiety to the exclusion of dyspnea and associated physiological changes [41] rather than recognizing anxiety as an important and initial sign of invisible dyspnea for individuals with acute respiratory distress [42].
Although dyspnea is a subjective experience, it is characterized by observable behaviors [43]. The inability of an individual to self-report dyspnea may result in a failure by nurses to identify this as a change in the patient status from a physiological perspective and appropriately treat this distressing symptom [44]. Campbell [43] suggests that stimulation of the autonomic nervous system produces observational and measurable behaviors. These involuntary responses are elicited sequentially and include: increased heart rate, tachypnea, restlessness, accessory muscle use, end expiratory grunting, involuntary nasal flaring and fearful facial expressions [45]. The Respiratory Distress Observation Scale (RDOS), a reliable and valid observer-rated instrument, that measures these involuntary responses to dyspnea, is available to help nurses in the assessment of dyspnea in individuals who are unable to self-report [46, 47].