April 2002 / Volume 47 / Number 4 / Page 454
Weaning from Mechanical Ventilation
IntroductionInvasive mechanical ventilation can be lifesaving for patients with acute respiratory failure, but numerous complications have been identified. Therefore, once clinical improvement has occurred, emphasis is placed on quickly weaning (ie, liberating) the patient from mechanical ventilation. Weaning can be subdivided into 2 components: readiness testing and progressive withdrawal. Traditionally, both clinical factors and weaning predictors have been used to assess readiness for spontaneous breathing trials, which can be carried out using a T-piece or a low level of ventilatory support. The role of weaning predictors is under investigation, and their role in clinical decision making remains poorly defined. Recent insights into the pathophysiology of weaning failure have provided a framework for identifying potentially correctable limiting factors. Randomized controlled trials suggest that several approaches to progressive withdrawal may be acceptable, though only a minority of patients require progressive withdrawal. Emerging evidence indicates that protocol-directed weaning, driven by respiratory therapists and intensive care nurses, can improve outcome.
Definitions and Technical Descriptions
Other Forms of Pressure-Controlled Ventilation
Volume Control Versus Pressure Control: Does It Matter?
Patient Work of Breathing and Comfort
Invasive mechanical ventilation can be lifesaving for patients with acute respiratory failure (ARF). Unfortunately mechanical ventilation is also associated with substantial risks, including ventilator-associated pneumonia, airway injury, barotrauma, gastrointestinal bleeding, and thromboembolism, and some of those complications increase with the duration of intubation and may increase mortality. Once the patient has adequately recovered, efforts should focus on “liberating” the patient from the ventilator as rapidly as possible. The method of liberation, traditionally known as “weaning,” can be divided into 2 distinct phases: readiness testing and progressive withdrawal. Readiness testing is performed by allowing the patient to breathe spontaneously (spontaneous breathing trial or SBT), without assistance (T-piece) or with minimal ventilatory support (continuous positive airway pressure [CPAP] or a low level of pressure support). Progressive withdrawal entails a more gradual process, in which, theoretically, there is a step-by-step shifting of respiratory work from the ventilator to the patient. This conceptual framework, first postulated by Hall and Wood, has been validated by the results of recent large, randomized controlled trials, which found that approximately 75% of patients who satisfied readiness criteria (see below) tolerate the initial SBT. Therefore, the majority of patients who recover from an episode of ARF do not require a gradual process of withdrawal of ventilatory support (ie, weaning is not necessary). Yet, for patients requiring progressive withdrawal, the time devoted to weaning can be substantial, averaging approximately 40% of total ventilator time (60% for patients with chronic obstructive pulmonary disease [COPD]).