The highest values of intrinsic PEEP in stable COPD patients are 7 to 9 cmH 2O. Higher values have been reported in COPD patients with acute respiratory failure—up to 13 cmH2O during spontaneous breathing and 22 cmH2O during mechanical ventilation. Such high values of intrinsic PEEP have profound consequences on the energetics of breathing, as shown fig, 2. Acute ventilatory failure in COPD patients is usually triggered by airway infection. As a result, there is an increase in airway resistance and expiratory flow limitation becomes more severe. The increased resistance causes increased resistive work of breathing and promotes dynamic hyperinflation. The latter is further exacerbated by the tachypnea which is invariably present in acutely ill COPD patients. Dynamic hyperinflation promotes an increase of elastic work of breathing due to intrinsic PEEP (Fig 1). The increased resistive and elastic work of breathing, in association with the impaired inspiratory muscle performance, eventually leads to inspiratory muscle fatigue. As a result, the patient needs mechanical respiratory assistance.
Fig. 2 Scheme of the pathophysiology causing acute ventilatory failure in COPD patients: 7"tot, total breathing cycle duration; T, inspiratory time; Te, expiratory time; Raw, airway resistance; Edyn L, dynamic lung elastance.
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