Pathophysiology of COPD | allnurses
Pathophysiology of COPD - EzineArticles
EFL is a pathophysiological hallmark of COPD. Patients with COPD are said to be flow limited when the expiratory flow they generate during tidal respiration represents the maximal possible flows that they can generate at that volume. In flow limited patients the time available for lung emptying (expiratory time) during spontaneous breathing is often insufficient to allow end expiratory lung volume (EELV) to decline to its natural relaxation volume leading to lung overinflation. Thus, in flow limited patients, EELV becomes dynamically rather than statically determined, and essentially becomes a continuous variable that fluctuates widely depending on the extent of EFL and the prevailing ventilatory demand. Dynamic hyperinflation (DH) refers to acute and variable increase in EELV above its baseline value. DH occurs during exercise in flow limited patients as inspired tidal volume increases and expiratory time decreases, and is associated with severe mechanical constraints on ventilation and perceived respiratory discomfort. DH also occurs during acute bronchoconstriction in asthma. In this setting, the reduction in inspiratory capacity (IC), which reflects the increase in EELV, correlates strongly with the perception of inspiratory difficulty.,
The pathophysiology of COPD is not completely understood.
The pathophysiology of severe COPD exacerbations requiring mechanical ventilation is now well understood. Increased airways resistance, usually as a result of worsening airway inflammation, results in critical EFL and DH with dramatically increased loading and functional weakness of the inspiratory muscles. The accessory muscles of breathing are maximally recruited and significant thoracoabdominal dysynchrony is often evident. Inspiratory threshold loads increase substantially (values between 13 and 20 cm H2O have been recorded) and this has been suggested to account for nearly 60% of the increased static inspiratory work of breathing during an exacerbation.,,, Dynamic lung compliance becomes precipitously reduced as a result of the accompanying tachypnoea (and reduced inspiratory time) and, in conjunction with the increased airways resistance, also contributes importantly to the increased dynamic inspiratory work.,,,,