Mechanical ventilation is an established supportive treatment for patients with various forms of respiratory failure. Despite the widespread use and clear benefits of mechanical ventilation, it is not a risk-free intervention. Prolonged mechanical ventilation increases the risk of pneumonia, barotrauma, tracheal injuries and musculoskeletal deconditioning. At the same time, delayed weaning is associated with increased morbidity, mortality, hospital stay and risk of long-term care facility discharge. 
For the majority of patients (70%), weaning from mechanical ventilation is a simple and straightforward process. That usually entails extubation after the passage of first spontaneous breathing trial. The remaining 30% of patients represent a challenge for ICU physicians. Difficulties usually arise in patients with chronic obstructive and restrictive pulmonary disease, heart failure, neuromuscular disorders among other potential causes. In our discussion, we review the most common barriers to successful ventilator weaning and the various extubation readiness assessment tools. 
Arguably the most common mechanism underlying failure to wean patients off the ventilator. In its simplest form, the problem stems from an imbalance between respiratory pump capacity and demands.
Reduced Ventilatory Capacity
Prolonged duration of mechanical ventilation, especially when accompanied by the use of passive modes of ventilation can lead to diaphragmatic weakness and atrophy. Other factors that contribute to respiratory muscle weakness include excessive steroids, sedatives, and paralytics use, critical illness myopathy, a systemic inflammatory response associated with sepsis, malnutrition, and immobility. All of these factors are inherent to the ICU patient population; together they trap the patient in a vicious circle where more weakness leads to more difficulty to wean off the vent, leading to prolonged ICU stay and so forth.
Heart failure is another risk factor that can compound the process of ventilator weaning. Important physiological changes occur during the process of transition from mechanical ventilation to spontaneous breathing. The most notable of which is a loss of positive intra-thoracic pressure. That results in an increase in venous return to the right ventricle and increase in both preload and afterload. This is particularly relevant in the ICU patient population, most of whom have variable degrees of positive fluid balance. That increase in cardiac work can raise myocardial oxygen demand and precipitate ischemia in patients with coronary artery disease.
Examining Readiness to Wean From Mechanical Ventilation
Despite the known hazards associated with prolonged mechanical ventilation, many patients remain unnecessarily intubated for longer periods than needed. This knowledge stems from the fact that patients who accidentally self-extubate have 31-78% risk of reintubation. That means the same patient population has 22-69% chance of successfully weaning off mechanical ventilation. Even the most experienced clinicians may underestimate a patient’s readiness for ventilator weaning.
It is for this reason that major critical care societies strongly encourage the implementation of protocols for daily sedation interruption and spontaneous breathing trials. However, before a patient is considered for ventilator weaning, the following questions are a well worth consideration to ensure maximum chances of successful ventilator weaning:
Note that presence of any of the above parameters doesn’t guarantee weaning failure. The opposite also holds true. Because clinical judgment may sometimes over or underestimate patient’s readiness, a need for objective measures exists. Those indices should ideally be easily measurable and widely applicable.
Some of the suggested indices correlate directly with ventilatory parameters, such as the minute ventilation (VE) or vital capacity(VC). Other indices correlate with the degree of oxygen requirements such as the ratio of arterial to alveolar oxygen ratio (Pao2/PAo2), the ratio of arterial oxygen to the fraction of inspired oxygen (PaO2/FiO2) ratio or the alveolar-arterial oxygen gradient (A-a gradient). Also, some parameters examine the respiratory muscle strength, such as the maximal inspiratory pressure (MIP).
The reason there are so many indices is that none of them is perfect. There is a wide degree of variability in the performance of the above tests, and their predictive value is far from optimal. The reason being is that these indices only measure one aspect of the respiratory function, while the weaning process is complex and multifactorial.
Rapid Shallow Breathing Index (RSBI)
Defined as the ratio of respiratory rate (f) to tidal volume (VT), initially described by Yang and Tobin 1991. In their trial, they found that RSBI <105 correlated with weaning success, while a score >105 correlated with weaning failure. The reported sensitivity, specificity, positive predictive value and negative predictive values were 97, 64, 78, and 95 percent, respectively. Since the initial description, their results were confirmed in multiple subsequent trials, and their test has gained wide popularity and is integrated into ventilator liberation protocols in many ICUs.The test is ideally conducted over 30 minutes period while the patient is on minimal pressure support ventilation (PSV) and/or small PEEP value.
Spontaneous Breathing Trial (SBT)
Patients should be assessed daily for readiness to wean off mechanical ventilation. Trials are often part of an ICU protocol for ventilator liberation, and usually, start after patient fulfills many of the criteria mentioned previously.
Trials should be conducted with patient off sedation, on minimal ventilator support (pressure support ventilation PSV of 5-8 mmHg or less and/or a small amount of PEEP. There is some data to suggest using zero PSV and zero PEEP (known as ZEEP trials), which seems to correlate with higher chances of successful ventilator weaning.
For a spontaneous breathing trial to be considered successful, a patient should be able to breathe for at least 30 minutes without signs of hemodynamic derangement (respiratory rate <35, no significant elevation or drop in blood pressure, oxygen saturation >90%), and with the absence of signs of distress or anxiety. a is usually combined with one of the readiness indices such as the RSBI calculated at the beginning and end of each trial. Other factors to consider include respiratory secretions burden, the presence of a strong cough, and ability to maintain adequate wakefulness. A successful trial is usually followed by removal of the endotracheal tube and discontinuation of mechanical ventilation. Failure to fulfill one or more of the aforementioned parameters usually correlate with higher chances of weaning failure and reintubation. Those patients should be placed back on comfortably, controlled mode with appropriate sedation. A careful work-up should be done to examine underlying barriers to a successful weaning process.
It is worth to mention that clinician’s judgment is an important consideration when considering extubation. An experienced clinician is often able to combine all the data together and see through an apparently failed weaning parameter.
Extubation to Non-invasive Ventilation (NIV)
Although previously thought of as a failed weaning attempt, it is currently endorsed by American College of Chest Physicians (CHEST) and American Thoracic Society (ATS) to extubate patient to preventive non-invasive ventilation. Multiple studies correlated extubation to NIV with shorter ICU length of stay and short-term mortality. Patients who should be considered for such intervention include high risk intubated patients, including those who repeatedly fail their SBT, patients with advanced COPD or CHF, hypercapnic patients and those >65 years of age.
an interprofessional approach to weaning
Weaning off mechanical ventilation can often become a daunting task for the clinician caring for critically ill patients. It requires a meticulous daily assessment and collaborative effort between the ICU care team. The ultimate decision should be based on a multitude of clinical factors, combined with predictive scores after careful clinician assessment that often influenced by experience. It is vital to work as a team that includes a pulmonologist, intensivist, anesthesiologist, respiratory therapist, and an ICU when it comes to weaning. No one person should make the decision to wean a patient; there are countless cases of solo weaning cases gone bad. Every weaning that fails is associated with a higher morbidity for the patient. It is important to know and understand that at any given moment, there are more mechanically ventilated patients than should be. Finally, weaning from a mechanical ventilator should never be an emergency and preferably should be done during the day time.
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