Breathlessness in Patients with Chronic Airflow Limitation: Spontaneous Breathing
We have argued that in demonstrably flow-limited patients during spontaneous breathing at rest, expiratory transtho-racic pressures attain, but do not exceed, the critical flow-limiting pressure (Pcrit), thus obviating DC and its deleterious sensory consequences. Similarly, during constant-load submaximal exercise in patients with severe CAL, the results of two studies suggested that transpulmonary pressures appear to be modulated to closely match Pcrit. Furthermore, the application of positive expiratory pressure (+4 cm H2O) at the mouth, an intervention that would be expected to attenuate DC if it existed, did not have any consistent salutary effect on respiratory sensation in exercising patients with CAL.
The close matching of transpulmonary pressures and Pcrit during expiration is possibly an adaptive response that operates at a conscious level in response to sensory information from mechanoreceptors in central airways. This strategy would not only minimize DC, but would also avoid the unpleasant sensation that likely occurs when there is dissociation between expiratory effort (reflected by the change in intrathoracic pressures) and anticipated mechanical consequence (change in expiratory flow). Allergy medications Source However, this adaptation may fail at higher exercise levels prior to breakpoint where transthoracic pressures have been shown in some instances to greatly exceed Pcrit. Under these conditions, unpleasant sensation induced by DC may indeed contribute to the global sensory experience of exertional breathlessness.
In a recent study, we examined qualitative aspects of breathlessness at the breakpoint of incremental exercise in CAL (n=23). It is noteworthy that 65 percent of this sample reported inspiratory difficulty alone, 25 percent reported both inspiratory and expiratory difficulty, and only 10 percent reported predominant expiratory difficulty. Thus, perceived expiratory difficulty, which would be expected to arise when intrathoracic pressures exceed Pcrit, is much less frequently reported than inspiratory difficulty at exercise termination in patients with CAL, supporting the contention that DC per se is not a predominant source of breathlessness in the majority of such patients.
Category: Respiratory Symptoms
Tags: breathlessness, chronic airflow limitation, copd, dynamic compression, dynamic hyperinflation, exercise