Weaning from mechanical ventilation

For the ventilator-dependent patient, weaning should be accomplished by withdrawing support safely, efficaciously, and efficiently. Success depends largely on physiologic determinants of respiratory system function, avoidance of ventilator-associated complications, and attention to patient readiness. Recent clinical trials, predictors of weaning, current techniques of weaning, the concept of reloading the respiratory pump, and determinants of ventilator dependency are all discussed.     

Adjuncts to mechanical ventilation

Adjunctive ventilatory strategies have been developed to improve oxygenation and carbon dioxide (CO2) removal during mechanical ventilation of critically ill patients. These techniques allow clinicians to attain their clinical goals at lower levels of ventilatory support. In this article, the authors discuss extracorporeal CO2 removal, venovenous intravena caval oxygenator, and tracheal gas insufflation as adjuncts to CO2 removal and nitric oxide, surfactant replacement therapy, perfluorocarbon-associated gas exchange, and prone positioning as adjuncts to oxygenation.     

Weaning from mechanical ventilation: concept development

This article, the first in a series, is written to clarify the process of weaning from mechanical ventilation and to promote the development of a common language for understanding the complex weaning process. The Third National Study Group on Weaning From Mechanical Ventilation proposes a conceptual model and definitions that will provide a framework for future research on this important topic. This conceptual framework describes the preweaning phase, the weaning process, and the outcome phase of mechanical ventilation. Potential outcomes are completion of weaning, lack of completion, and terminal weaning. The weaning decision continuum incorporates: (1) when and how to begin the weaning process, (2) how to select therapies to assist with difficult weaning and chart progress during weaning, and (3) when to stop weaning if progress is no longer being made. An inherent assumption of this model is that each patient will display unique responses to the weaning process. The proposed conceptual framework and definitions provide a foundation for developing clinical practice guidelines and for guiding future ventilator weaning research.     

Weaning from mechanical ventilation in patients with exacerbations of chronic respiratory insufficiency--personal experience

Nineteen patients with exacerbation of chronic respiratory insufficiency treated with mechanical ventilation were included in the study. The mean weaning time from the respirator was 15.9 days (+/- 12.1), ranging from 2 to 49 days. Success was met in 9 patients using inspiratory pressure support (IPS), in 2 synchronized intermittent mandatory ventilation (SIMV), in 8 the simplest model (stepwise prolongation of spontaneous ventilation during continuous mechanical ventilation--CMV/SV). All options were used in the same patients throughout the weaning procedure. The use of IPS led to a successful weaning in those patients in whom other options (CMV/SV, SIMV) were not fortunate. The application of IPS was begun at 40 cm H2O, gradually decreasing the support pressure. The duration of spontaneous breathing in all weaning options was not only depended on gasometric values, continuous SaO2 monitoring, but mainly on the patients' subjective sense of fatigue. The impression of exhaustion preceded the changes of gasometric parameters and fall of SaO2. The basis of effective weaning is proper selection of respirator parameters, providing almost identical gasometric values if individual patient's to those prior exacerbation of chronic respiratory insufficiency. The duration of weaning negatively correlated (0.25) with FEV1 values.     

Neuromuscular disorders associated with failure to wean from the ventilator

OBJECTIVE: To determine, by retrospective chart analysis, the frequency, type and significance of neuromuscular disorders in patients whose clinical features suggested a neuromuscular cause of failure to wean. BACKGROUND: Failure to wean is a common and difficult problem in critical care units. While a neuromuscular cause may be suspected in some patients, the frequency and type has not been determined utilizing comprehensive electrophysiological studies of limbs and the respiratory system. Such knowledge may aid in patient management and prognosis. METHODS: The clinical setting was a critical care/trauma centre that admits 1500 patients per year, approximately 500 being on ventilators for longer than five days. We analyzed the hospital charts of 40 patients admitted to the unit during three years, whose respiratory assessment suggested a neuromuscular cause for failure to wean from the ventilator. To investigate this possibility, we performed electrophysiological studies of the limbs and also of the respiratory system by phrenic nerve conduction and needle electromyography of the chest wall and diaphragm. The results were compared to 25 healthy controls. RESULTS: 38 of 40 patients (95%) had a neuromuscular disorder: 25--critical illness polyneuropathy, 2--Guillain-Barré syndrome, 4--diabetic and critical illness polyneuropathy, 2--uremic and critical illness polyneuropathy, 10--an abnormality of central drive, 5--unilateral phrenic nerve palsy, 3--a neuromuscular transmission defect, and 5--a primary myopathy. Fifteen (38%) had a combination of disorders. Patients with more severe polyneuropathy took longer to wean, a mean of 136 versus 52 days (p = 0.007). The severity of the polyneuropathy had no effect on mortality. CONCLUSIONS: Electrophysiological studies of limbs and the respiratory system are together valuable in confirming the presence, and identifying the specific type of neuromuscular cause for difficulty in weaning from the ventilator. This information is important in patient management and prognosis.     

集束化护理对严重脓毒症患者早期脱机的影响

目的 探讨集束化护理对严重脓毒症患者早期脱机的影响.方法 将2010年收住ICU18岁以上的严重脓毒症患者设为实验组,其采用集束化护理的方法,把2009年收住ICU18岁以上的严重脓毒症患者设为对照组,其采用常规护理方法.观察2组患者28d内机械通气时间及脱机再次应用呼吸机的情况,2组比较采用χ2检验.结果 实验组28d内机械通气时间较对照组缩短,实验组脱机后再次应用呼吸机的人数比对照组少,差异具有统计学意义(P＜0.05).结论 集束化护理能明显缩短脓毒症患者的机械通气时间,减少再次使用呼吸机,提高护理质量,缩短住ICU时间.     

Pulmonary surfactant concentration during transition from high frequency oscillation to conventional mechanical ventilation

OBJECTIVE: To test the hypothesis that conventional mechanical ventilation (CV) provides a greater stimulus to secretion of pulmonary surfactant than high frequency oscillatory ventilation (HFO). METHODOLOGY: Sequential examination of surfactant indices in lung lavage fluid in a group of six infants with severe lung disease (group 1), ventilated with HFO and then converted back to CV as their lung disease recovered. A similar group of 10 infants (group 2) ventilated conventionally throughout the course of their illness were studied for comparison. In groups 1 and 2, two sequential tracheal aspirate samples were taken, the first once lung disease was noted to be improving, and the second 48-72 h later. Group 1 infants had converted from HFO to CV during this time. RESULTS: A marked increase in concentration of total surfactant phospholipid (PL) and disaturated phosphatidylcholine (DSPC) was seen in group 1 after transition from HFO to CV; the magnitude of this increase was significantly greater than that sequentially observed in group II (total PL: 9.4-fold increase in group 1 vs 1.8-fold in group 2, P = 0.006; DSPC: group 1 6.4-fold increase vs. group 2 1.7-fold, P = 0.02). CONCLUSION: These findings suggest that intermittent lung inflation during CV produces more secretion of surfactant phospholipid than continuous alveolar distension on HFO, and raise the possibility that conservation and additional maturation of surfactant elements may occur when the injured lung is ventilated with HFO.     

Gastric intramucosal pH: an indicator of weaning outcome from mechanical ventilation in COPD patients

The aim of this study was to determine whether gastric intramucosal pH (pHim) and/or gastric intramucosal carbon dioxide tension (PCO2,im) measured by tonometry can be used to predict the success of weaning in chronic obstructive pulmonary disease (COPD) patients. Twenty six consecutive COPD patients, undergoing mechanical ventilation for acute respiratory failure and satisfying the criteria of weaning from mechanical ventilation with nasogastric tonometer in place, were studied. Arterial blood gas values and PCO2,im were measured 24 h before (H-24), just before (H0), and after 20 min of a weaning trial on T-piece (H20min). Weaning failure was defined as the development of respiratory distress and/or arterial blood gas impairments during the first 2 h of spontaneous breathing on T-piece, or reintubation within 24 h after extubation. Between the weaning failure (n = 6) and weaning success (n = 20) groups, there were no differences in blood gas analysis readings at H-24 and H0 before the weaning period, age, Simplified Acute Physiology Score (SAPS) on admission, SAPS on the day of weaning trial, and duration of ventilation. Clinical status, tonometric and arterial gasometric data were similar at H-24 and H0 in all patients. During mechanical ventilation, pHim was < or = 7.30 in patients who failed weaning and > 7.30 in patients who were successfully weaned (p < 0.001; 100% sensitivity and specificity). The threshold value for PCO2,im of 8.0 kPa (60 mmHg) represents a clear demarcation with respect to outcome before the weaning trial. PCO2,im values during mechanical ventilation are significantly different (p < 0.001) between patients who were successfully weaned and those who were not (6.9 +/- 0.9 vs 9.9 +/- 1.1 kPa (51.9 +/- 6.7 vs 74.3 +/- 8.0 mmHg, respectively)). At H20min, pHim and PCO2,im were still statistically different between the weaning failure and the weaning success group. We conclude that measurement of gastric intramucosal pH (or gastric intramucosal carbon dioxide tension) represents a simple and accurate index to predict weaning outcome in chronic obstructive pulmonary disease patients before attempting weaning.     

Liberation from mechanical ventilation: a decade of progress

Multiple complications associated with mechanical ventilation mandate that clinicians expeditiously define and reverse the pathophysiologic processes that precipitate respiratory failure and then, detect the earliest point that a patient can breathe without the ventilator. Over the past decade, numerous laboratory and clinical studies have been reported that may inform transformation of the "art of weaning" to the science of liberation. We review these studies and use them to formulate a systematic approach to assure early, safe, and successful liberation of patients from mechanical ventilation.     

Volume and pressure modes of mechanical ventilation in pediatric patients

There are few controlled pediatric studies comparing the various modes of ventilation in terms of patient outcomes. Thus at this time the choice of ventilator mode depends largely on the apparatus available, the patient's disease state, and personal preference based on one's experience. The next generation of ventilators may well allow the use of the best of both modes, setting both pressure and volume minimums and maximums, safely meeting ventilation targets. Today's challenges are to become familiar with the various modes of ventilators available, understand the developing physiology of the lung and lung disease pathophysiology, and incorporate all this into proper ventilator strategies to prevent ventilator-induced lung injury.     

Discontinuation of mechanical ventilation

Since 1967, about 40 cases of digital necrosis associated with neoplasia have been reported. We report a new case of digital necrosis associated with an ovarian carcinoma and with a lupus-like syndrome. Immunologic cross-reactivity to tumoral antigen could explain the lupus-like syndrome.     

Vasodilators in mechanical ventilation

Vasodilators that affect the pulmonary vasculature are appealing adjuncts in many cardiopulmonary conditions that require mechanical ventilation such as ARDS, COPD, PPHN, and cardiothoracic surgery. The adverse systemic effects of parenteral PGE1 and parenteral prostacyclin limit their usefulness in critically ill patients. Liposomal PGE1 has few systemic effects, but thus far has not resulted in a significant clinical benefit in patients with ARDS. Inhaled NO and aerosolized prostacyclin offer the advantage of selective pulmonary vasodilation with minimal systemic effects. Both agents decrease PAP and in many clinical situations improve oxygenation; however, the physiologic effects of inhaled NO and aerosolized prostacyclin have not convincingly led to improved clinical outcomes. Currently, use of vasodilators in mechanically ventilated patients remains investigational.     

Monitoring during mechanical ventilation

Approximately half of the patients admitted to an ICU are admitted for the purposes of monitoring rather than interventional therapy. In the last decade, significant technologic advances have enhanced monitoring capacities, and the understanding of the pathophysiology of respiratory failure has improved pari passu, allowing clinicians to employ monitors in a more intelligent manner. This article deals with new developments in arterial blood gas monitoring, pulse oximetry, capnometry, and monitoring of neuromuscular function and pulmonary mechanics, emphasizing issues most relevant to mechanical ventilation.     

A comparison of noninvasive positive-pressure ventilation and conventional mechanical ventilation in patients with acute respiratory failure

BACKGROUND AND METHODS: The role of noninvasive positive-pressure ventilation delivered through a face mask in patients with acute respiratory failure is uncertain. We conducted a prospective, randomized trial of noninvasive positive-pressure ventilation as compared with endotracheal intubation with conventional mechanical ventilation in 64 patients with hypoxemic acute respiratory failure who required mechanical ventilation. RESULTS: Within the first hour of ventilation, 20 of 32 patients (62 percent) in the noninvasive-ventilation group and 15 of 32 (47 percent) in the conventional-ventilation group had an improved ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2:FiO2) (P=0.21). Ten patients in the noninvasive-ventilation group subsequently required endotracheal intubation. Seventeen patients in the conventional-ventilation group (53 percent) and 23 in the noninvasive-ventilation group (72 percent) survived their stay in the intensive care unit (odds ratio, 0.4; 95 percent confidence interval, 0.1 to 1.4; P=0.19); 16 patients in the conventional-ventilation group and 22 patients in the noninvasive-ventilation group were discharged from the hospital. More patients in the conventional-ventilation group had serious complications (66 percent vs. 38 percent, P=0.02) and had pneumonia or sinusitis related to the endotracheal tube (31 percent vs. 3 percent, P=0.003). Among the survivors, patients in the noninvasive-ventilation group had shorter periods of ventilation (P=0.006) and shorter stays in the intensive care unit (P=0.002). CONCLUSIONS: In patients with acute respiratory failure, noninvasive ventilation was as effective as conventional ventilation in improving gas exchange and was associated with fewer serious complications and shorter stays in the intensive care unit.     

Physiologically based indices of volumetric capnography in patients receiving mechanical ventilation

Several indices of ventilatory heterogeneity can be identified from the expiratory CO2 partial pressure or CO2 elimination versus volume curves. The aims of this study were: 1) to analyse several computerizable indices of volumetric capnography in order to detect ventilatory disturbances; and 2) to establish the relationship between those indices and respiratory system mechanics in subjects with normal lungs and in patients with acute respiratory distress syndrome (ARDS), both receiving mechanical ventilation. We studied six normal subjects and five patients with early ARDS mechanically ventilated at three levels of tidal volume (VT). Respiratory system mechanics were assessed by end-expiratory and end-inspiratory occlusion methods, respectively. We determined Phase III slopes, Fletcher's efficiency index, Bohr's dead space (VD,Bohr/VT), and the ratio of alveolar ejection volume to tidal volume (VAE/VT) from expiratory capnograms, as a function of expired volume. Differences between normal subjects and ARDS patients were significant both for capnographic and mechanical parameters. Changes in VT significantly altered capnographic indices in normal subjects, but failed to change ventilatory mechanics and VAE/VT in ARDS patients. After adjusting for breathing pattern, VAE/VT exhibited the best correlation with the mechanical parameters. In conclusion, volumetric capnography, and, specifically, the ratio of alveolar ejection volume to tidal volume allows evaluation and monitoring of ventilatory disturbances in patients with adult respiratory distress syndrome.