8 - Respiratory Muscles and Liberation from Mechanical Ventilation

Summary

Introduction

Recovery from the acute respiratory distress syndrome (ARDS) may occur within 2 to 3 days in some patients, and early discontinuation of mechanical ventilation and extubation is possible. Unfortunately, most patients with ARDS require prolonged mechanical ventilation, and its discontinuation poses a greater problem. Although precise figures are not available, at least one-third of patients who display difficulties in being weaned from mechanical ventilation are in the recovery phase after acute lung injury. Weaning such patients from the ventilator presents a considerable clinical challenge. An understanding of the pathophysiologic mechanisms responsible for the inability to resume spontaneous breathing is essential to develop a systematic approach to discontinuation of ventilation.

Pathophysiologic Determinants of Weaning Outcome

The major factors that determine the outcome of a weaning trial are pulmonary gas exchange, respiratory muscle pump failure, and psychological factors.

Pulmonary Gas Exchange

During a weaning trial, hypoxemia may occur as a result of hypoventilation, impaired pulmonary gas exchange, or decreased oxygen (O₂) content of venous blood. In general, severe hypoxemia is uncommon in patients who fail a weaning trial for the simple reason that weaning is not contemplated unless patients display satisfactory oxygenation, such as arterial O₂ tension (PaO₂ > 60 torr with fractional inspired O₂ concentration (FiO₂) of < 0.40 and a positive end-expiratory pressure (PEEP) level of < 10 cm H₂O. Although resumption of spontaneous breathing is associated with worsening of ventilation-perfusion inequality, the degree of impairment does not prevent successful discontinuation of mechanical ventilation.