Weaning failure in critical illness
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Weaning failure has been defined as failure to discontinue mechanical ventilation, as assessed by the spontaneous breathing trial, or need for re-intubation after extubation, so-called extubation failure. Both events represent major clinical and economic burdens, and are associated with high morbidity and mortality. The most important mechanism leading to discontinuation failure is an unfavourable balance between respiratory muscle capacity and the load they must face. Beyond specific diseases leading to loss of muscle force-generating capacity, other factors may impair respiratory muscle function, including prolonged mechanical ventilation, sedation, and ICU-acquired neuromuscular dysfunction, potentially consequent to multiple factors. The load depends on the mechanical properties of the respiratory system. An increased load is consequent to any condition leading to increased resistance, reduced compliance, and/or occurrence of intrinsic positive-end-expiratory pressure. Noteworthy, the load can significantly increase throughout the spontaneous breathing trial. Cardiac, cerebral, and neuropsychiatric disorders are also causes of discontinuation failure. Extubation failure may depend, on the one hand, on a deteriorated force-load balance occurring after removal of the endotracheal tube and, on the other hand, on specific problems. Careful patient evaluation, avoidance and treatment of all the potential determinants of failure are crucial to achieve successful discontinuation and extubation.Keywords:
Discontinuation
Spontaneous breathing trial
Patients with chronic airflow obstruction and difficulty in weaning from mechanical ventilation are at increased risk for intubation-associated complications and mortality because of prolonged invasive mechanical ventilation. Non-invasive ventilation (NIV) may avert most of the pathophysiologic mechanisms associated with weaning failure in these patients. Several randomised controlled trials have shown that the use of NIV in order to advance extubation in difficult patients can result in reduced periods of endotracheal intubation, complication rates and survival. The published data with the outcome as a primary variable are nearly exclusively from patients who had pre-existing lung disease. In addition, the patients were hemodynamically stable, with a normal level of consciousness, no fever and a preserved cough reflex. The use of NIV in the management of respiratory failure after extubation did not show clinical benefits, although clinical trials included a small proportion of chronic respiratory patients. In contrast, NIV immediately after extubation is effective in avoiding respiratory failure after extubation in patients at risk for this complication, particularly those with chronic respiratory disorders and hypercapnic respiratory failure.
Spontaneous breathing trial
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Positive pressure ventilation
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Le vieillissement de la population s'accompagne d'une augmentation du nombre de patients souffrant de maladies chroniques. À la différence du patient jeune, les maladies chroniques sont souvent multiples chez le patient âgé. Par exemple, il est fréquent chez les patients de plus de 75 ans de cumuler une maladie chronique respiratoire, une insuffisance cardiaque et une insuffisance rénale chronique. Ce tableau polypathologique complexifie la prise en charge médicale de la personne âgée. La prise en charge de la personne âgée s'intègre donc dans une approche globale du patient en particulier en réanimation où les questions éthiques sont fréquentes. Cette vision globale s'applique à la prise en charge d'une décompensation de bronchopneumopathie chronique obstructive (BPCO) en réanimation. En effet, la qualité de vie, le degré de handicap respiratoire et le nombre de comorbidités du patient sont des notions extrêmement importantes dans la décision d'admission en réanimation et sur le degré d'invasivité des thérapeutiques proposées. Néanmoins, l'évolution récente des techniques de ventilation mécanique en réanimation et tout particulièrement de la ventilation non invasive (VNI) ont permis de progresser dans le pronostic et la prise en charge des personnes âgées admises en réanimation pour une décompensation d'une insuffisance respiratoire chronique telle que la BPCO. En effet, la VNI constitue désormais une option thérapeutique incontournable et permet fréquemment d'éviter le recours à l'intubation chez ces patients âgés [1, 2].
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Respiratory failure may result from inadequate central respiratory drive, excessive respiratory workload, or inadequate respiratory muscle endurance. With the exception of drug overdoses, central causes of respiratory failure are uncommon in the adult, and respiratory failure can be considered an imbalance between workload and endurance. Excessive workload can result from airway obstruction or chest wall or lung restriction. Anything that increases the required minute ventilation will increase the workload proportionately. Inadequate endurance results from neuromuscular disease, malnutrition, and a variety of metabolic factors. In most cases, treatment of the precipitating cause allows weaning from mechanical ventilation. However, when respiratory failure persists, often because the precipitating cause cannot be treated, all possible contributing conditions must be identified and corrected to the greatest possible extent. In that way, many patients with apparently intractable respiratory failure can be weaned. Four new approaches are showing some promise in the treatment of persistent respiratory failure: pharmacologic therapy to strengthen respiratory muscles, periodic respiratory muscle rest, sedation, and inspiratory muscle training.
Neuromuscular disease
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Pressure support ventilation
Acute respiratory failure
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Exacerbations of chronic obstructive pulmonary disease (COPD) are a major problem worldwide and are usually the main indication for mechanical ventilation (MV), especially in the intensive care unit (ICU). The rate of weaning failure is also high and prolonged MV leads to complications of intubation. The goal is to wean these patients as soon as possible.To determine the optimal time necessary to start the weaning process.In an attempt to determine the length of MV and stay in the ICU, we compared the length of MV, weaning, reintubations and discharge during a 10 month period. This study included 122 patients on MV due to severe exacerbation of COPD who were not suitable for non-invasive ventilation. For each patient serial arterial blood gases were measured at admission and during hospitalization. PeCO2 (mixed expired CO2) was tested using a Datex S/5 instrument at follow-up.The study population comprised all patients who required MV; of these 122, 108 were ventilated from 6 to 140 hours (mean 48 +/- 42), 9 needed more than 168 hours, and 5 died due to severe ventilation-associated pneumonia. No correlation was found between pH, PCO2 and length of MV; these findings did not contribute to evaluation of the patient's condition nor did they enable us to predict the length of treatment necessary.Most of the patients (93%) ventilated for acute respiratory failure due to COPD required MV for only 6-90 hours.
Arterial blood
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Weaning from the respirator is obviously the most time-consuming phase in mechanically ventilated patients. Major problems still arise in the early detection and elimination of risk factors which may impair adequate spontaneous breathing. Detailed physiological consideration of the respiratory disability should distinguish between defects in lung mechanics and inspiratory muscle failure. Thus, proper mechanical ventilation can be established in order to restore or to train the respiratory muscles (IPPB versus IMV/CPAP principle) during the weaning period. Prior to any extubation, the data on clinical state, results of blood gas analysis and pulmonary mechanics, as well as special tests for estimation of inspiratory muscle strength, are of definite value.
Respirator
Respiratory physiology
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During partial ventilatory support modes both the ventilator and the patient are performing mechanical work. These modes are currently used as mechanical ventilatory support to provide a better synchronism between the patient and the machine and to avoid a complete rest of the respiratory muscles, or as modes of weaning from mechanical ventilation. Assist control ventilation allows the patient to regulate his own respiratory rate by triggering preset ventilatory cycles. Synchronized intermittent mandatory ventilation includes both controlled cycles and spontaneous breathing cycles. Lastly, during inspiratory pressure support, each spontaneous cycle is assisted by pressurization of the circuit. This latter modality alone seems efficiently to decrease the patients' breathing work while allowing complete synchronism between patient and ventilator.
Synchronism
Work of breathing
Pressure support ventilation
Respiratory Rate
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