| |
 | Intensified |
Correct
| Figure
12.1: Relationship between Dyspnea (Breathlessness)
Score and Ventilation Index (V IND) Plotted for Ten
Subjects
|
|---|
|
|
Official
Journal of the American Thoracic Society. © American
Lung Association.
Reprinted with permission. |
|
The data
illustrated here are from a study in which normal subjects were
made hypercapnic and their breathing was targeted at different
levels of ventilation. Dyspnea intensity is plotted as a function
of the ventilation index (the ratio of the targeted ventilation
to that of the ventilation that would have resulted from the hypercapnia
without targeting). Despite the fact that the chemical drive to
breathe is constant throughout, that is, the stimulation of the
chemoreceptors is constant, at levels of ventilation below that
dictated by the chemical drive, the intensity of dyspnea increased.
At levels of ventilation above the chemically determined ventilation,
approximately half of the subjects had an increase in dyspnea
while the remainder had a plateau in their ratings. Similar results
were obtained by Chonan et al.
(1990) and are compatible with the hypothesis that dissociation
between the neural output to the ventilatory muscles and the mechanical
consequences of muscle stimulation intensifies dyspnea. We believe
the brain assesses this dissociation by monitoring the afferent
feedback from mechanoreceptors throughout the respiratory system.
|