Breathlessness in Patients with Chronic Airflow Limitation: Breathlessness
In contrast to normal subjects, EELV in patients with CAL often increases above resting values early in exercise (Fig 1). Acute increases in EELV of 0.31 ±0.55 L and 0.30 ±0.32 L (mean±SD) have been reported with considerable variation noted in the range. In flow-limited patients, EELV is dynamically determined and largely dictated by breathing pattern responses. To the extent that avoidance of DC and the attendant unpleasant sensation is desirable (see above), termination of expiration (or onset of inspiration) before expiratory flow is complete and the resultant DH may be effected at a conscious level in response to sensory input from airway mechanoreceptors generic claritin. Regardless of the mechanism, it is evident that while DH serves to optimize expiratory flows in the setting of increased ventilatory demand, it does so only at the expense of serious negative mechanical and sensory consequences.
The effects of acute DH on ventilatory muscle function have only recently been fully appreciated. Dynamic lung hyperinflation results in elastic loading of the respiratory muscles: tidal volume (Vt) oscillates at a stiffer portion of the pressure-volume relationship of the respiratory system. Dynamic lung hyperinflation also results in inspiratory threshold loading: with each breath, the inspiratory muscles must generate enough pressure to overcome the inward elastic recoil of the respiratory system at end expiration before any inspiratory flow begins; DH further compromises the ability of already disadvantaged respiratory muscles (particularly the diaphragm) to generate pressure, and functional inspiratory muscle weakness is further aggravated by increased velocity of shortening (VT/Ti) as ventilation increases. In contrast to normal subjects, patients with CAL lose the ability to reduce EELV below passive FRC with exercise. Therefore, expiratory muscles cannot share to the same extent in the work of breathing being undertaken by the inspiratory muscles.
Recently we systematically studied the role of DH in exertional breathlessness of CAL using a multiple regression model with Borg ratings as the dependent variable: ABorg=0.16+0.03(AEELVdyn, percent TLC)+0.15(AF)+0.05(AVt, percent VC), r=0.78, p<0.001 (where A=change from rest, EELVdyn-(percent TLC)=dynamic end-expiratory lung volume as a fraction of total lung capacity, VT(percent VC)=tidal volume as a fraction of vital capacity, F=breathing frequency) (Fig 1).
Figure 1 Left (A), Lung volumes during exercise, standardized for ventilation, in CAL (n=23) and; age-matched normal subjects (n=10). Patients with CAL have significantly (p<0.001) greater lung hyperinflation and diminished inspiratory reserve volume (IRV) at any given ventilation. Values are mean±SEM. Right (B), Relationship between actual and predicted changes in Borg breathlessness ratings during exercise in patients with CAL (r=0.78, p<0.001). EILV=end-inspiratory volume; VT=tidal volume; TLC=total lung capacity.
Category: Respiratory Symptoms
Tags: breathlessness, chronic airflow limitation, copd, dynamic compression, dynamic hyperinflation, exercise