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:
- Has the disease process that led to mechanical ventilation resolved or improved?
- Is the patient hemodynamically stable? Absence of shock or requirement for pressors or significant arrhythmias
- Is patient adequately oxygenated? (Fraction of inspired oxygen <50% and/or low PEEP requirements)
- Is patient adequately awake and communicative? (absence of encephalopathy, agitation or overtly altered mental status)
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.
Enhancing Healthcare Team Outcomes
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.