Assessing readiness for liberation from mechanical ventilation

Mechanical ventilation is the defining event of intensive care unit (ICU) management. Although it is a life saving intervention in patients with acute respiratory failure and other disease entities, a major goal of critical care clinicians should be to liberate patients from mechanical ventilation as early as possible to avoid the multitude of complications and risks associated with prolonged unnecessary mechanical ventilation, including ventilator induced lung injury, ventilator associated pneumonia, increased length of ICU and hospital stay, and increased cost of care delivery. This review highlights the recent developments in assessing and testing for readiness of liberation from mechanical ventilation, the etiology of weaning failure, the value of weaning protocols, and a simple practical approach for liberation from mechanical ventilation. In 1987, Hall and Wood [1] suggested that weaning from mechanical ventilation, which implies the gradual withdrawal of mechanical ventilation and concomitant resumption of spontaneous breathing, is unnecessary in most patients. They proposed that the ultimate objective is not to wean but rather to liberate the patient from mechanical ventilation. Over the ensuing decades, numerous studies have investigated methods and tools for identifying readiness of mechanically ventilated patients for successful liberation from mechanical ventilation [2-8]. Assessing readiness for liberation from mechanical ventilation The evaluation of patients’ readiness for liberation from mechanical ventilation starts with the resolution of respiratory failure and/or the disease entity that prompted the initiation of mechanical ventilation as well as the presence of a basic level of physiological readiness (Table 1). Prediction based on clinical ‘gestalt’ alone is frequently inaccurate. In a study by Stroetz and Hubmayr [9], intensivists could not accurately predict patient tolerance of a 1-hour period on minimal pressure support. Also, Afessa and colleagues [10] reported that critical care practitioners could not accurately forecast 3-day and 7-day weaning outcome for mechanically ventilated patients in a medical intensive care unit (ICU). Thus, there is a sound rationale that predicting readiness of patients to be successfully liberated from mechanical ventilation needs to be based on objective weaning predictors that can be applied in clinical decision making. A large spectrum of weaning predictors has been studied, which can be divided into simple weaning indices, simple measures of load and capacity, integrative weaning indices, and complex predictors requiring special equipment. A recent expert panel [11] sponsored by the American College of Chest Physicians, Society of Critical Care Medicine, and the American Association for Respiratory Care developed evidence-based weaning guidelines and noted that only eight variables had some predictive capacity: minute ventilation (VE), negative inspiratory force, maximum inspiratory pressure, tidal volume (VT), breathing frequency (f), the ratio of breathing frequency to tidal volume (f/VT), P0.1/PImax (ratio of airway occlusion pressure 0.1 s after the onset of inspiratory effort to maximal inspiratory pressure), and CROP (integrative index of compliance, rate, oxygenation, and pressure) (Table 2). However, another recent study by Conti and colleagues [12] in 2004 showed that vital capacity, VT, P0.1, VE, f, maximum inspiratory pressure, f/VT, P0.1/PImax and P0.1 × f/VT are poor predictors of weaning outcome in a general ICU population. These conflicting results have been attributed to many factors, such as the measurement techniques that differ from one study to another, different timing of measurements made by different investigators, and lack of objective criteria to determine tolerance of the trial [13-16]. Review Clinical review: Liberation from mechanical ventilation Mohamad F El-Khatib and Pierre Bou-Khalil Departments of Anesthesiology and Medicine, School of Medicine, American University of Beirut, Beirut 1107 2020, Lebanon Corresponding author: Pierre Bou-Khalil, [email protected] Published: 6 August 2008 Critical Care 2008, 12:221 (doi:10.1186/cc6959) This article is online at http://ccforum.com/content/12/4/221 © 2008 BioMed Central Ltd ATC = automatic tube compensation; COPD = chronic obstructive pulmonary disease; CPAP = continuous positive airway pressure; CROP = integrative index of compliance, rate, oxygenation, and pressure; f = breathing frequency; f/VT = ratio of breathing frequency to tidal volume; ICU = intensive care unit; NAVA = neurally adjusted ventilation assist; NPPV = non-invasive positive pressure ventilation; P0.1/PImax = ratio of airway occlusion pressure 0.1 s after the onset of inspiratory effort to maximal inspiratory pressure; PAV = proportional assist ventilation; PEEP = positive end expiratory pressure; PSV = pressure support ventilation; RSBI = rapid shallow breathing index; SIMV = synchronized intermittent mandatory ventilation; VE = minute ventilation; VT = tidal volume.