PATHOPHYSIOLOGY OF FAILURE TO WEAN FROM MECHANICAL VENTILATION

Weaning patients from mechanical ventilation constitutes a major porti on of the workload in an intensive care unit, as over 40% of total ven tilator time is consumed by the weaning process. Several pathophysiolo gical mechanisms may be responsible for weaning failure, but the preci se role of each is incompletely understood. Patients who fail a weanin g trial commonly develop hypercapnia, which appears to be due to decre ased tidal volume rather than a primary decrease in respiratory drive. Respiratory muscle performance is impaired as a result of dynamic hyp erinflation and paradoxic motion of the rib cage and abdomen. Worsenin g of pulmonary mechanics will cause further embarrassment of the respi ratory muscles. However, the clinical importance of respiratory muscle fatigue remains unclear. Afferent stimuli arising in the lung parench yma, respiratory muscles, or as a consequence of impaired gas exchange will be transmitted to the respiratory control centers and result in severe dyspnea in patients who fail a weaning trial.