1 Unity Pulmonary, Critical Care, and Sleep Physicians, Rochester, NY, USA
2 White Plains Hospital, White Plains, NY, USA
Background
Mechanical ventilation can be a life‐saving intervention in the critically ill patient. However, intubation and mechanical ventilation are not without risk.
Complications of mechanical ventilation can be serious and include hemodynamic disturbances, VAP, tracheal injury, gastrointestinal stress ulcers with bleeding, skin breakdown and pressure ulcers, muscle wasting and acquired weakness, barotrauma, pain and discomfort.
The aim of the intensivist, therefore, should be to utilize mechanical ventilation for whatever duration necessary and not a day longer.
While focus should appropriately be on reversing the cause of respiratory failure, it must also be on maintaining homeostasis of the other systems, e.g. fluid balance, mental status, muscle strength.
About 20–40% of mechanically ventilated patients fail their first attempt at weaning. More than half of the total duration of mechanical ventilation can be spent in the weaning process.
Premature extubation requiring reintubation has been shown to increase mortality from 2.5 to 10 times compared with patients who do not require reintubation.
Definition
Ventilator weaning, also known as liberation, refers to the process of discontinuing mechanical ventilation and involves two components:
Assessment of readiness to wean from mechanical ventilation: determine whether the patient meets prespecified weaning readiness criteria to assess if he/she can be safely removed from the ventilator.
Weaning: this is the process of structured decrease in mechanical ventilation support and liberation from the mechanical ventilator. Some patients can be immediately shifted to spontaneous ventilation or a spontaneous breathing trial, for others it may be a progressive transition.
Incidence/prevalence
Approximately 800 000 hospitalizations require mechanical ventilation per year in the USA. This accounts for roughly 3% of hospital admissions.
It is estimated that there are 2.7 episodes of mechanical ventilation per 1000 population.
Assessment of readiness to wean
Before weaning can begin, the causes of respiratory failure must be addressed.
Respiratory failure, simplified, is an imbalance in strength versus load, where the work of breathing (load) exceeds the capacity of the patient to maintain it (strength).
Failure is often due to a combination of changes in both strength and load; each can be comprised of multiple factors.
Weaning will be successful when the balance is restored in favor of strength.
The cause of respiratory failure must be treated and resolving.
Antibiotics for pneumonia, systemic corticosteroids/bronchodilators for asthma/COPD exacerbation, diuresis for heart failure, and allowing sufficient time for the underlying acute process to improve.
Such measures will lead to improved compliance and resistance via mechanisms such as reducing pulmonary edema, resolving atelectasis and pleural effusions, dilating airways, and reducing auto‐PEEP and dynamic hyperinflation.
What precipitates respiratory failure is not necessarily what maintains it. An example would be muscle atrophy, in particular the diaphragm, which occurs over the course of intubation. This has been shown to occur within hours of the onset of controlled mechanical ventilation.
When the inciting event stops progressing and begins to regress, assessment for weaning should begin.
Maintaining homeostasis of the other systems
Maintenance of homeostasis of systems outside the respiratory system is essential to successful ventilator weaning. These other systems include but are not limited to the neurologic, cardiovascular, and neuromuscular systems.
In the past, patients had been kept sedated, immobile (sometimes paralyzed), and on full ventilator support for the duration of their acute illness.
We now understand that we need to be as aggressive with weaning (off) as we are with supportive interventions (on). The following are now standard best practices, a number of which have been shown to reduce the duration of mechanical ventilation.
Daily sedation interruption
Continuous sedation deprives the physician of a vital assessment tool in the evaluation of ventilator weaning: the patient’s mental status. This can lead to overestimation of the patient’s level of illness, underestimation of strength, as well as prolonged immobilization.
In the vast majority of patients, a daily sedative hold with re‐initiation only if necessary (and at half the dose) has been shown to decrease the duration of intubation.
In a landmark study, a daily sedative hold reduced the duration of mechanical ventilation by an average of 2.4 days.
Daily spontaneous breathing trial
A spontaneous breathing trial (SBT) should be performed daily on ventilated patients who meet prespecified weaning readiness criteria; this SBT is ideally performed during a sedation hold.
In addition to allowing for assessment of weaning readiness, a SBT may prevent the atrophy of the diaphragm and chest wall muscles.
Fluid restriction
Positive fluid balance can lead to pulmonary congestion, decreased lung compliance, and increased respiratory load.
In hemodynamically stable patients, a net‐even fluid balance should be targeted. This approach was shown to reduce mechanical ventilation duration without increasing the incidence of circulatory shock or the need for dialysis.
Early initiation of physical therapy
With each passing day immobilized in bed, a patient becomes weaker. The muscles of the respiratory system are not spared.
Immobilization can also perpetuate the idea that a patient is not capable of unsupported breathing, i.e. they look sicker than they are.
Early mobilization was shown to reduce the duration of mechanical ventilation as well as improving physical and cognitive functions.
Mechanically ventilated patients, who are at baseline functional independence, and meet criteria for cardiopulmonary stability, should participate in physical and occupational therapy as early as possible.
Nutrition
Malnutrition plays a role in patient weakness, and becomes cumulative over the course of an ICU stay. This is potentiated by the hypercatabolic state of the critically ill patient.
While there is ongoing debate as to the timing of initiation of nutrition, maintaining caloric intake, including fat and protein, is necessary to prevent further weakness.
Hypokalemia, hypophosphatemia, and hypomagnesemia all cause muscle weakness and should be corrected.
Acid–base status
Some patients have a chronic component of respiratory failure, such as chronic respiratory acidosis in COPD or obesity hypoventilation syndrome. These patients will never be able to tolerate physiologically normal bicarbonate levels, as they cannot maintain the necessary minute ventilation to keep a normal serum pH.
For these patients, trying to achieve a ‘normal PaCO2’ will lead to wasting of appropriately retained bicarbonate. Therefore, the physician should accept bicarbonate retention and hypoventilation at baseline levels and not target a normal arterial blood gas prior to extubation.
Assessment of readiness for extubation
Once weaning has commenced, patients should be assessed for possible extubation each day. The assessment includes:
Respiratory parameters – demonstrating the ability to breath spontaneously with minimal ventilator support and supplemental oxygen requirements.
Hemodynamic status – shock resolving, not requiring high doses of vasopressors.
Mental status – able to protect the airway.
The standard for assessing the patient’s readiness for extubation is an SBT.
An SBT should be attempted when the above criteria are met. Waiting to initiate SBTs when the physician feels confident that extubation will be successful will delay extubation.
A patient who completes a successful SBT should be assessed for extubation if he/she can protect his/her airway without an endotracheal tube in place.
Spontaneous breathing trials
The SBT is an unassisted to minimally assisted period of breathing while the patient remains intubated.
The SBT consists of placing the patient on a T‐piece, CPAP (typically 5 cmH2O), or low level pressure support (5–8 cmH2O or less above PEEP) for a period of 30 minutes to several hours daily.
These modes have been shown to be superior to higher levels of pressure support or synchronized intermittent mandatory ventilation (SIMV).
Circumstances may require that an SBT is extended, but neither increased duration nor increased frequency of the SBT has been shown to add predictive power when compared with at least 30 minutes once a day.
Note that even low levels of pressure support and PEEP can significantly augment work of breathing, and therefore mask ongoing respiratory failure.
Arguments have been made for the T‐piece as the ‘truest’ assessment of a patient’s capacity for spontaneous breathing at atmospheric pressure.
The physician should be aware of the cardiac augmentation that CPAP or pressure support provides, and account for this in patients with heart failure.
Methods of SBTs
Continuous positive airway pressure
CPAP provides a minimal static pressure that prevents atelectasis while allowing for relatively unassisted breathing. Proponents of CPAP argue that it increases functional residual capacity and maintains small airway patency, while not masking work of breathing.
A CPAP trial occurs while the patient is connected to the ventilator, providing a means of monitoring with alarms in place.
Spontaneous breathing trials using CPAP or a T‐piece have been shown to decrease the duration of intubation, as compared with a graduated weaning of pressure support and SIMV.
CPAP will not compensate for the resistance of the endotracheal tube, and can lead to a failed SBT in an otherwise capable patient if the tube is narrow.
Pressure support ventilation
Pressure support ventilation (PSV) is a mode which is patient triggered, flow cycled, and pressure limited, usually combined with CPAP.
Like CPAP, it does not require disconnection from the ventilator, and apnea alarms and pressure monitors remain in place.
PSV can also overcome the increased resistance of narrower endotracheal tubes.
The drawback of pressure support is that it augments the respiratory muscles of inspiration in direct proportion to the amount of support employed, which can mask ongoing respiratory failure.
For example, pressure support of 5 cmH2O over PEEP 5 cmH2O is equivalent to bilevel non‐invasive pressure support IPAP 10/EPAP 5 (assuming adequate seal), a level that is often used to rescue patients from respiratory failure.
T‐piece/T‐tube
A T‐piece trial involves disconnecting the endotracheal tube from the ventilator and attaching it to a tube that is connected to humidified oxygen. This tubing extends beyond the endotracheal tube, forming a ‘T’ and allowing for a reservoir of oxygen (Figure 25.1).
The T‐piece trial is simple, well tested, and imposes a pulmonary workload that is comparable to that encountered after extubation.
Initially, there were concerns that the T‐piece led to an increase in the work of breathing as compared with the extubated airway due to the resistance provided by the endotracheal tube.
These studies did not account for the airway inflammation and edema that persist after extubation, however, which results in little difference between the pre‐extubation and post‐extubation airway diameter and thus workload.
Unlike positive pressure, a T‐piece does not reduce venous return or reduce the left ventricle afterload and therefore will not mask heart failure.
A drawback of the T‐piece is that the patient needs more frequent monitoring as they are disconnected from the ventilator without its associated alarms.
With smaller diameter endotracheal tubes (≤7 mm) resistance can exceed even that of the post‐extubation airway.
After 30–120 minutes of an SBT the patient should be evaluated for work of breathing. To be considered a success, the patient should be breathing comfortably without signs of respiratory distress (excessive tachypnea or use of accessory muscles of respiration).
Other important criteria for extubation include:
Hemodynamic stability.
Patients who develop significant hypertension or hypotension or significant tachycardia or bradycardia should not be extubated.
Ability to protect the airway.
Patients with subdued level of consciousness, poor gag reflex, or excessive respiratory secretions are likely to aspirate and require reintubation.
Extubation in such patients should be attempted on a case‐by‐case basis, taking into account the reversibility of the underlying process and engaging in patient/family education as to the risk of reintubation.
In these patients, tracheostomy is an alternative option.
Oxygen requirements that can be provided by non‐invasive modalities.
Cause of respiratory failure is improving Mental status awake, alert, or easily arousable and following commands No signs of accessory muscle use, abdominal breathing, diaphoresis, sensation of dyspnea, or general distress
Objective measures
Respiratory: Respiratory rate less than 35/min, greater than 8/min SpO2 >90% on FiO2 ≤50% RSBI <105
Cardiovascular: Hemodynamic stability or on low dose vasopressor Blood pressure and heart rate within 20% of pre‐SBT No evidence of myocardial ischemia or onset of arrhythmia
* These criteria are individualized, as some patients may not be expected to follow commands. Some patients may have chronic respiratory failure and not fulfill usual respiratory parameters.
Resolution of the obstruction, if upper airway obstruction was the indication for intubation.
In these patients, a cuff leak test should be performed. This involves deflating the endotracheal tube cuff and listening for laryngeal airflow, as well as looking for a discrepancy between measured inspiratory and expiratory tidal volumes on the ventilator. The cuff leak test (with a leak of >110 mL) can be utilized as a means to predict lack of post‐extubation stridor. This leak test has been applied to patients with airway edema. Although not definitive as a criterion for extubation, absence of a leak can indicate that obstruction has not resolved.
Manageable secretions.
Voluminous secretions, especially in a patient with decreased level of alertness, can lead to inability to clear the airways and reintubation.
RSBI <105.
In addition to bedside clinical evaluation, a scoring system has been developed that is predictive of extubation failure. The RSBI has been used both in assessing readiness to wean and in evaluating the success of the weaning trial itself.
RSBI is the respiratory rate divided by the tidal volume (in liters).
A number higher than 105 (breath/min/L) is highly predictive of failure (sensitivity 0.97).
A number lower than 105 is moderately predictive of success (specificity 0.64). The lower the number, the greater the likelihood of success. This correlates with a lower respiratory rate and a larger tidal volume which simulates normal, relaxed breathing.
The trend, rather than an individual value, of RSBI may be a better predictor of weaning success.
Most patients will require minimal to no weaning off ventilatory support and are extubated without difficulty after their first SBT.
Diaphragm atrophy has been demonstrated in an autopsy study of patients receiving mechanical ventilation. Thinning of the diaphragm (based on ultrasound studies) has been shown during the course of mechanical ventilation. For this reason, some clinicians favor a mode of breathing that preserves spontaneous respiratory effort.
Examples would include patients with upper airway obstruction (e.g. angioedema) or patients who breathe comfortably on higher levels of pressure support.
Such patients can be maintained on a spontaneous mode of ventilation as long as they are breathing comfortably.
Many patients who meet some but not all of the criteria for weaning will still be liberated successfully. Clinicians frequently underestimate a patient’s likelihood of success, and failure of an SBT in most patients is less injurious than not attempting one at all.
The SBT is ideally done during a sedation hold, when the patient is awake and interactive. This is not always feasible.
Weaning protocols
Weaning protocols are instituted in many ICUs to ensure that all intubated patients are evaluated on a daily basis for readiness to wean.
Implementation of a daily weaning protocol has been shown to reduce the duration of mechanical ventilation, ventilator‐associated pneumonia, self‐extubation rates, tracheostomy rates, and cost.
Weaning protocols are often used in combination with sedation protocols. The goal is daily interruption of sedatives or maintaining patients in an alert, comfortable state so that weaning can progress and extubation performed at the earliest moment.
A respiratory therapist‐led or nurse‐led protocol generally incorporates the following:
Daily early assessment of each patient for weaning readiness, based on specific criteria for respiratory status, hemodynamic stability and mental status.
Initiate weaning mode for specified time.
Assess tolerance of SBT.
ICU team makes decision for extubation.
Lack of a weaning protocol and delays in advancing the weaning process are associated with increased morbidity and mortality.
Extubation
Once the decision is made for extubation, the process proceeds as follows:
Enteral tube feedings should ideally be held for 2 hours prior to extubation.
Explain to the patient what is going to happen.
Supplemental oxygen and humidified air set up.
Position the patient appropriately in the sitting‐up position, if possible.
NPPV on standby if indicated for a patient at high risk of extubation failure (heart failure, obesity).
Suction the mouth and trachea to remove retained secretions.
Deflate the endotracheal tube cuff.
Remove the endotracheal tube.
Examine the patient for increased work of breathing and stridor. Ongoing monitoring of vital signs.
Extubation failure
Approximately 10–20% of patients fail extubation despite a successful SBT. A failed extubation does not mean the decision to extubate was wrong.
Factors shown to increase the risk of failed extubation despite passing an SBT include: net positive fluid balance in the prior 24 hours, pneumonia as the cause of respiratory failure, inadequate cough, poor mental status, and a higher RSBI score.
Failure typically manifests as increased work of breathing as evidenced by accessory muscle use, tachypnea, tachycardia, and anxiety.
Prolonged post‐extubation failure can lead to shallower breathing and decreasing alertness.
Patients should be examined for stridor; inspiratory stridor indicates a narrow upper airway, increased airway resistance, and turbulent flow.
Extubation failure requiring reintubation is associated with worse outcomes, including increased mortality, length of stay, and transfer to a long‐term care facility.
Delaying reintubation in patients who are failing has been shown to further increase mortality.
Management of extubation failure
The patient showing signs of failure after extubation should be considered for reintubation as early as possible. Patients at high risk of extubation failure should be considered for NPPV or high flow oxygen therapy after extubation.
Good candidates for a trial of NPPV include patients with COPD as the cause of respiratory failure, a patient who is awake, strong, and able to protect their airway, patients who have a rapidly reversible cause of persistent increased work of breathing (residual pulmonary vascular congestion responsive to a diuretic), or those with mild work of breathing that easily resolves with NPPV.
Regardless of the underlying factor(s), if the patient is not rapidly rescued by non‐invasive ventilation, reintubation should not be delayed, as prolonged failure prior to reintubation increases mortality.
For stridor: nebulized racemic epinephrine (0.5 mL of a 2% solution diluted in a volume of 2–4 mL), methylprednisolone 40 mg IV, and a trial of NPPV can improve airflow and may prevent reintubation. For severe stridor or deteriorating vital signs, reintubation should be done immediately.
Risk factors for extubation failure
Older patients.
Severity of illness on ICU admission.
Prolonged ventilation.
Weak cough.
Failure of multiple SBTs.
Pre‐existing left ventricular dysfunction.
Positive fluid balance (in the prior 24 hours)
Pneumonia as the cause of respiratory failure.
Altered mental status.
Higher RSBI score.
Upper‐airway obstruction.
Managing the difficult to wean patient
Not all patients will pass their SBT and be ready for extubation on their first attempt. For these patients, ongoing management of the cause of failure, continued work on improving strength/load parameters, and frequently just passage of time will eventually lead to a successful SBT and extubation. Alternatively, a subset of patients may benefit from early extubation to NPPV.
Role of non‐invasive ventilatory support in weaning
Non‐invasive positive pressure ventilation (BIPAP or CPAP) can be used immediately after extubation in selected patients who show borderline parameters during their SBT but are deemed likely to tolerate extubation with pressure support.
Such patients include those with COPD, resolving heart failure, and patients with marginal RSBI scores.
These patients should have good mentation, be capable of protecting their airway, and should be able to clear their secretions.
Role of tracheostomy
A subset of patients has no hope of weaning (e.g. persistent neurologic injury) or has a prolonged course of endotracheal intubation. In these patients, tracheostomy may be indicated.
Tracheostomy can be an integral part of the weaning process and is not necessarily an endpoint in itself.
Tracheostomy can facilitate weaning by reducing dead space and decreasing airway resistance (which improve work of breathing), augmenting secretion clearance, and improving patient comfort (which reduces the need for sedation). Studies have shown reduced rates of ventilator‐associated pneumonia.
Tracheostomy provides a more secure airway and allows for greater participation in physical therapy and mobilization.
Superiority of early versus late (at 2 weeks) tracheostomy has not been shown.
The timing of tracheostomy remains controversial and no clear consensus has been reached on early versus delayed tracheostomy in most ICU patients.
Other factors to consider
Agitation
Patients frequently suffer from disorientation that occurs upon sedative withdrawal, delirium from a prolonged ICU course, and pain from catheters and the endotracheal tube. This can lead to anxiety, tachypnea, and shallow and dyssynchronous breathing.
In this case, a small amount of sedation can relieve the agitation and allow the physician to differentiate what is true work of breathing from mere agitation. If the patient tolerates the SBT with this sedation, then the physician should proceed with extubation.
Obesity
Obesity decreases chest wall compliance and functional residual capacity. When intubated and passively ventilated, this can lead to atelectasis and hypoxia. For this reason, obese patients may require increased PEEP for adequate oxygenation despite resolution of the cause of respiratory failure. This can exaggerate their ventilator and oxygen requirements.
Once extubated and actively breathing, they will often recruit lung and oxygen saturation will improve.
NPPV can ameliorate post‐extubation atelectasis.
Persistent hypoxia despite decreased work of breathing
Some patients pass an SBT, yet continue to have significant hypoxemia with a high FiO2 requirement, indicative of shunt physiology. An example would be resolving pneumonia.
In such patients extubation to HFNC will allow for a high FiO2 without requiring invasive ventilation. HFNC is capable of providing FiO2 at levels as high as 80–90%, even with patients who have a high minute ventilation.
Older and smaller individuals often breathe at lower tidal volumes. This decreases the sensitivity of the RSBI for predicting successful extubation.
Ventilator factors
Problems with the ventilator and endotracheal tube can lead to a patient failing their SBT despite adequate respiratory function.
Examples include kinked tubing, endotracheal tube narrowing secondary to precipitated secretions (Figure 25.2), fluid in tubing, and equipment dead space. This should be investigated in patients without a clear reason for failing their breathing trial.
Indicators of an obstructed endotracheal tube include difficulty passing the suction catheter and ventilator flow waveforms of a fixed upper airway obstruction.
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