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A detailed
discussion of the physiology of dyspnea is not possible here (for
more complete analysis see American
Thoracic Society, 1999, and Manning
and Schwartzstein, 1998). We will highlight the key features
of the physiology that must be understood before embarking on
research in this field.
Mechanical Loads and the Sense of Effort
Most patients with cardiopulmonary disease and respiratory
distress have an increased load on the ventilatory muscles; that
is, there is increased stiffness (decreased compliance) of the
lungs or chest wall, or increased resistance (airway obstruction).
In these circumstances, the ventilatory muscles must work harder
compared to the unloaded state to attain the same tidal volume
or minute ventilation.
To achieve the increased
power output from the ventilatory muscles, the efferent output
from the motor cortex to the muscles must increase. This change
in neurological activity is perceived as a sense of "effort"
or "work" of breathing. This perception is believed
to arise from a "corollary discharge," a neurological
discharge from the motor to the sensory cortex that occurs simultaneously
with the efferent output to the ventilatory muscles (McCloskey,
1981). Muscle strength may alter the perception of respiratory
effort. To generate a given tension in a weak muscle, a greater
effort is required (Gandevia
et al., 1981). Strengthening the diaphragm with an exercise
program reduces the intensity of effort associated with a given
inspiratory load (Redline et
al., 1991). If the intensity of the inspiratory task is expressed
as a fraction of the maximal output of the system, however; effort
is the same before and after training (Redline
et al., 1991).
When asked to rate
the intensity of breathlessness and effort, normal subjects breathing
at normal and increased lung volumes rated the two sensations
in parallel fashion over a range of elastic loads (Killian
et al., 1984). This led to the assertion that "breathlessness
and effort are identical and are mediated by the same mechanism"
(Killian et al., 1984). One
might be skeptical of this claim based on the complexity of "breathlessness"
as a generic term for the breathing discomfort seen in a wide
range of pathophysiological conditions, some of which are not
characterized by an increase in the mechanical load on the respiratory
system, e.g. pulmonary embolism. In addition, one should consider
the fact that the ventilatory muscles receive efferent messages
from both the motor cortex (resulting in voluntary contractions)
and from the respiratory centers in the brainstem (automatic or
involuntary contractions).
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