Definition :

Weaning or discontinuation of mechanical ventilation can be defined as the process of gradual or sudden ventilatory support withdrawal in critically ill patients. It is usually done when the Intensivist or clinician considers that the patient who has been intubated may tolerate a reduction of ventilatory support.
Weaning failure is defined as either failure of a Spontaneous Breathing Trial or the need for reintubation within 48 hours following extubation. It is estimated from studies from 1994 to 2001 that total failed weaning is around 31.2 %.

Importance :

It is estimated that 40% of the time, the patient who has been intubated and ventilated will be dedicated to the process of weaning. In one study, the incidence of accidental or unplanned extubation ranged from 0.3-16% [1]. Almost 50% of these patients were not requiring reintubation suggesting that ventilation was unnecessarily prolonged. This delay in weaning will cause an increase in the length of ICU stay, length of hospital stay, number of ventilatory days, overall cost of care, and increased mortality [2].

Delay in weaning can be a risk for Ventilator-induced lung injury, Hospital acquired infection especially Ventilator Associated Pneumonia, airway trauma due to the endotracheal tube, and prolonged sedation with its complications. On the other hand, early weaning or extubation can cause respiratory muscle fatigue and risk of reintubation. This reintubation in general is associated with mortality [3] Scott K et al showed the reintubation rate to be approximately 15% in the ICU [1].

Weaning Process :

Sixth International Consensus Conference on Intensive care Medicine [4] presented schematic representation of different stages occurring in mechanically ventilated patients with

  • Acute Respiratory Failure
  • Suspicion for weaning
  • Assessing readiness to wean
  • Spontaneous Breathing Trial (SBT)
  • Extubation or Discontinuation of ventilatory support
  • Reintubation in case of weaning failure

Classification of Weaning :

  • Easy weaning – Extubation after a successful first attempt of Spontaneous Breathing Trial, which includes 30-50% of the cases.
  • Difficult Weaning – Failing to wean after the first attempt of SBT requires up to three SBTs or up to seven days for successful extubation which includes 26-40% of the cases.
  • Prolonged Weaning – Weaning requires more than seven days after the failure of the first attempt of SBT which includes 6-30%.

When the patient enters into difficult or prolonged weaning there is also a risk of need for tracheostomy as well as increase in mortality. Boles JM et al in their study mention that prolonged weaning have a risk of ICU mortality of 13-22% [4] .

Criteria for Weaning :

Patient should meet the following criteria for us to initiate weaning. These recommendations from ACCP – SCCM – AARC Evidence based weaning guidelines taskforce 2001 [12] , are as follows:

  1. The pathology for which we have intubated should be resolving.
  2. Adequate oxygenation – PiO2:FiO2 ratio more than or equal to 150-200 with lesser PEEP (<5-8), lesser FiO2 (0.4-0.5) and pH > 7.25.
  3. Stable hemodynamics – either no vasopressors or therapy with low doses of vasopressors.
  4. Patient should have ability to initiate an inspiratory effort.

Intensivists, Critical care residents, or clinicians should always look for these criteria to initiate weaning for all intubated patients daily.

Weaning predictor tests :

There are several parameters and tests which have been used as a predictor of successful weaning. They are as follows,

  1. Negative inspiratory pressure ( threshold -20 to -30 cm H2O)
  2. Maximal inspiratory pressure Pmax ( threshold -50 to -30 cm H2O)
  3. Minute Ventilation ( Threshold 10-15 L/min)
  4. P 0.1 / P max ( Mouth Occlusion Pressure 0.1 second after the onset of Inspiratory effort)
    Threshold values 0.30
  5. CROP index ( Compliance, Rate, Oxygenation, Pressure – threshold value 13 )
  6. Rapid Shallow Breathing Index(RSBI) – ( F/ VT ratio – Threshold value 60-105 /L)

Even though all these predictors will be helpful in predicting weaning failure but will not be of much help to predict successful weaning.

Tanios MA et al [5] did a comparison of outcomes between two study groups with or without Rapid Shallow Breathing Index. He found that there was no difference between weaning time, days on mechanical ventilation, ICU days, hospital days, Reintubation rate and mortality. There are also confounding factors like Pressure support of 5-10 cm of H2O decreased RSBI by 20 to 80% and CPAP of 5 cm H2O reduces RSBI by 20 to 50 % as compared to unassisted breathing [6]. Recently, diaphragm ultrasound has also been used as a predictor of successful extubation.

Spontaneous Breathing trials (SBT) :

There are three methods of trials being used:

  1. T piece trial – Abrupt removal of support and placement of T piece
  2. CPAP with Pressure support – Providing low pressure support 5-8cm H2O and CPAP
    of 5cm H2O
  3. Synchronized Intermittent Mandatory Ventilation – Mandatory rates are reduced

Brochard et al [7] and Esteban et al [8], demonstrated that T piece trial of 2 hours is tolerated by 76% of study groups when patients were ready to wean and T piece trial was used as initial trial of weaning.

In 1997, Esteban et al [9] compared Pressure Support with T piece trial as initial trial of weaning found that Pressure Support weaning increased the chances of extubation (86% vs 78%) compared to T piece trial for 2 hours without increasing the frequency of reintubation. There was no difference in the length of stay and mortality.
In another study, Esteban et al [8] compared T piece trial, Pressure Support and SIMV. In patients who failed initial Tpiece trial, found that SIMV may not be a very good modality in these groups.
So, in recent guidelines, liberation from mechanical ventilation in critically ill adults from American College of Chest Physicians and American Thoracic Society clinical practice guidelines suggest that for acutely hospitalized patients ventilated more than 24hours, the initial SBT be conducted with Inspiratory pressure augmentation (5-8 cm H2O) rather than without ( T piece or CPAP) [10].

Duration of Spontaneous breathing trial :

Earlier, clinicians used to try SBT for longer durations which adversely affected the weaning process as patient had to breath with minimal support or without support against increased respiratory load. However, while weaning, most of the weaning failures can be identified within 30 minutes. So Esteban et al [11] compared duration of 2 hour T piece trial versus 30 minute T piece trial, found that there is no difference in mortality and reintubation rate. So, initial SBT with duration of 30 minutes conducted with Pressure Support ventilation of 5-8 cm H2O can be used as major diagnostic test to predict weaning failure.

Tolerance of SBT :

While conducting SBT, the following criteria can be used to identify tolerance of the SBT [12].

Subjective criteria – Patient should be comfortable without any signs of increased work of breathing like paradoxical breathing, accessory muscle usage, intercostal retractions, nasal flaring, diaphoresis, and agitation. No change in mental status – Drowsy, comatose, agitation.

Objective criteria
1.Respiratory rate < 30-35 cpm
2.Gas exchange – *sPO2 > 88-92%
*PaO2 > 50-60
*pH > 7.32
*Increase in PaCo2 < 10 mm Hg

*Heart rate < 120 BPM or change in HR < 20%
*SBP < 180 and change in BP < 20%

If the patient is not tolerating the weaning as per the above criteria, the patient should be put back to adequate ventilatory support to rest the muscles. Repeating unsuccessful attempts at weaning will be harmful unless the cause of weaning failure is identified and corrected. Hence, repeating SBT after 24 hours will be appropriate.

Causes of weaning failure :

  1. Increase in respiratory load –
    There may be increase in respiratory demand due to hypoxia (unresolved pneumonia), metabolic acidosis, bronchospasm, dynamic hyperinflation, pleural and chest wall diseases or abdominal distension. There may be also reduced neuromuscular capacity due to dyselectrolytemia and medications like excessive sedation, malnutrition and Central nervous system diseases.
  1. Increase in cardiac load –
    There may be masked latent left ventricular failure which will be unmasked due to switching from full support to SBT. There is also occult ischemia which interferes with weaning. Right ventricular failure is very common and undetected in ICU settings.
  1. Neuromuscular insufficiency –
    Central Nervous system diseases like Stroke, Intracerebral hemorrhage, Subarachnoid hemorrhage, brainstem dysfunction, peripheral nervous system diseases like Critical Illness Polyneuropathy and Myopathy, GB syndrome, Myasthenia gravis will cause difficulty in weaning and prolong the ventilation. The level of coma is an independent predictor of weaning success [13]. Neuropsychological factors also will contribute in difficult weaning.
  1. Metabolic and endocrine disorders –
    Hypothyroidism, Steroid Myopathy, Metabolic acidosis, Hypoxia, Hypocapnia, Hypercapnia, Hypothermia and Hyperthermia, Hyponatremia and Hypernatremia, Hypokalemia, Hypomagnesemia, Hypophosphatemia will be the major contributing factors in difficult weaning.
  1. Nutritional factors –
    Nutrition support is the most important and neglected area in critically ill patients which will contribute to difficult weaning due to significant muscle wasting. Overfeeding with carbohydrates and increasing CO2 concentration will add to the respiratory load.

Managing difficult Weaning :

  1. Reduce the respiratory load –
    Identify and correct the causes of increased respiratory load like treating Pneumonia, Pulmonary edema, reducing the intrinsic PEEP, adequate bronchodilation, draining large pleural effusions, adequate steroid therapy for Asthma, treating Post extubation stridor, correcting metabolic acidosis and electrolyte abnormalities by replacing Potassium, Magnesium, Phosphate, treating sepsis, adequate nutritional support, giving adequate respiratory muscle rest in between SBT, avoiding or reducing sedation use, early cessation of neuromuscular blockers, aminoglycoside therapy and decompressing abdominal distension. Non Invasive Ventilation during persistent weaning failure: NIV in case of weaning intubated COPD patients has clearly demonstrated survival benefit with shorter ICU length of stay and reduced complications. But, in general ICU population, extubating all patients to NIV irrespective of COPD factor has shown worsening mortality [14], which is explained by delay in reintubation in these patients by prolonging NIV. Miquel Ferrer et al [15] in his study, patients who had risk factors for reintubation which is defined as Age > 65 years, cardiac failure as a cause of respiratory failure or an APACHE score > 12 on the day of extubation, applying NIV in these groups post extubation had reduced reintubation rates and ICU mortality. So, ACCP/ATSCPG in 2017 recommended patients at risk for extubation failure like patient with Hypercapnia, COPD, Congestive heart failure or other serious comorbidities who have been receiving mechanical ventilation for more than 24 hours and who have passed an SBT, get extubated to preventive NIV [10] .

2. Managing Weaning induced Pulmonary oedema –
Weaning Induced Pulmonary Edema (WIPO) is responsible for 59% of weaning failure [16]. Switching from mechanical ventilation to spontaneous ventilation will cause increase in negative intrathoracic and pleural pressure leading to worsening of preload and left ventricular afterload. This can unmask the latent left ventricular failure and cause pulmonary edema. In this set of patients, BNP driven fluid management strategy [17] may be advisable. If there is a rise in BNP levels equal or greater than 200 pg/ml, compared to pre SBT, fluid restriction and administration of diuretics will improve weaning success. And
applying NIV post extubation as discussed earlier also improves weaning success.

3. Improving neuromuscular competence –
Managing electrolyte disturbances (Potassium, Magnesium, Phosphate), early cessation of neuromuscular blockers, amino glycosides, reducing or avoiding sedative drugs, early physical and occupational therapy all improve neuromuscular competency. Patient with CNS disease, non convulsive seizures should be identified early and treated appropriately. Tracheostomy is one of the important options for patients requiring prolonged ventilation in CNS diseases which improves comfort for the care giver, less resistance and reduction in ventilator days even though there is no difference in infection rates and mortality [18].

4. Reducing and avoiding sedation –

In 2000, one RCT showed daily interruption of sedative drug infusions which is called Wake up test decreased the duration of mechanical ventilation and length of stay in ICU [19] . Recent studies also suggest minimizing sedation without causing any harm like accidental extubation.

5. Physical and Occupational therapy to aid weaning –
A strategy of whole body rehabilitation was considered safe and had better outcomes at hospital discharge by reducing the risk of delirium and providing more ventilator free days compared to standard care [20] . So, early mobilization in ICU will be helpful in weaning.

6. Managing Prolonged Weaning –
Patient with repeated weaning failure can be transferred to weaning facilities if patient medically stable to transfer. Here, in these groups, weaning should be slow paced and should include gradual lengthening of SBT. Early physiotherapy, occupational therapy, whole body rehabilitation should be considered in these groups.

Conclusion :

  1. Delay in weaning has been shown to increase the length of ICU stay, hospital stay, infective complications, cost of overall care and mortality.
  2. So, early identification for readiness of weaning using weaning criteria and initiating Spontaneous Breathing Trial with Pressure Support 5-8 cm H2O, CPAP of 5 cm H2O i advisable.
  3. Patients who fail first SBT should undergo systematic identification of the cause for weaning failure and correction of the same before resuming further trials.
  4. SIMV may not be a very good weaning mode in difficult weaning groups.
  5. In patients at high risk for extubation failure, extubation to preventive NIV is suggested.
  6. Weaning induced Pulmonary edema is very common and BNP driven fluid strategy along with NIV will be helpful in weaning.
  7. Early physical and occupational therapy is safe and well tolerated, resulting in better functional outcomes.


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