Mar 132013
Weaning From Mechanical Ventilation Update
(See More PulmCCM Reviews)

by Brett Ley, MD

Nearly 800,000 patients require mechanical ventilation yearly. There’s no doubt it is a life-saving intervention, but it is one that is fraught with the potential for iatrogenesis, especially if continued for longer than necessary. That is the main message of this review in the NEJM by John F. McConville and John P. Kress (father of the sedation holiday) from the University of Chicago. They describe how to reduce the duration of intubation and mechanical ventilation to the absolute minimum through aggressive and evidence-based strategies to be implemented before, during, and after the tube goes in. Hence, the authors prefer the term "liberation" over "weaning" from the ventilator, encouraging readers to see mechanical ventilation as a burden that should be rapidly removed in those who no longer need it. Liberation in general terms requires that the condition that lead to mechanical ventilation be improving and that the patient has adequate respiratory muscle strength to sustain spontaneous breathing and adequate gas exchange.

The authors point out evidence that the process of discontinuing mechanical ventilation after the underlying cause for intubation has been addressed takes up half of the time of mechanical ventilation, and that delaying discontinuation after demonstrated readiness for extubation is associated with increased mortality, higher pneumonia risk, and longer hospital stays. Further, nearly three-quarters of patients who fail their first spontaneous breathing trial (SBT) will be successful on subsequent trials. In sum, most patients can resume breathing on their own if we only let them.

The typical way in which mechanical ventilation is discontinued goes like this:
  1. Determine if the patient is ready to demonstrate he/she can breathe spontaneously: Patients who are ready for an SBT should be hemodynamically stable, have adequate oxygenation (e.g. partial pressure of arterial oxygen to fraction of inspired oxygen greater than 200 on a PEEP of 5 or less), and the underlying condition that lead to intubation should be improving.
  2. If deemed ready, perform an SBT: A trial is performed to demonstrate that the patient can breathe on minimal or no support, typically on low-level pressure support, continuous positive airway pressure, or a T-piece for at least 30 minutes. A successful trial is one in which the patient does not develop respiratory distress (excessive tachypnea, desaturation), hemodynamic distress (excessive tachycardia, hypertension, hypotension), or apparent distress (anxiety, diaphoresis).
  3. If the trial is successful, and the patient demonstrates he/she can likely breathe without the ventilator, then the next step is to evaluate whether the patient can protect his/her airway without the endotracheal tube by assessing the quantity of airway secretions, cough strength, and mental status. If the patient can breathe unassisted AND protect the airway, then extubate as soon as possible. If not, then mechanical ventilation at the previous support level should be reinitiated, the underlying reason for failure should be investigated and treated, and then start back at step 1.
Strategies to Reduce the Duration of Mechanical Ventilation

These can be broken down into strategies to reduce the need for intubation and mechanical ventilation, strategies to reduce the duration of mechanical ventilation once intubated, earlier appreciation of readiness for an SBT, and a shorter process of discontinuation of mechanical ventilation after passing an SBT and demonstrating adequate airway protection.

Some evidence-based strategies that reduce the need for mechanical ventilation include early goal-directed therapy for sepsis and use of non-invasive positive pressure ventilation for COPD and acute cardiogenic pulmonary edema.

Some evidence-based strategies for reducing the duration of mechanical ventilation once the patient is intubated (i.e. strategies to speed up "readiness") include lung-protective ventilation in ARDS, daily interruption of sedation (although this was challenged by a recent article in JAMA in patients who are appropriately sedated) or even no sedation, early physical and occupational therapy, conservative fluid management in ARDS, and ventilator-associated pneumonia prevention.

A rapid shallow breathing index (RSBI) (respiratory rate divided by tidal volume in liters) greater than 105 during 1-minute of breathing on a T-piece has shown a reasonable ability to predict who will and will not pass an SBT (PPV 78%, NPV 95%). However, expert consensus recommends just placing patients on an SBT once pre-defined readiness criteria have been met.

Weaning protocols (or should I say liberation protocols) that enforce daily evaluation of readiness (often involving assessments by non-physician providers such as nurses and respiratory therapists) with pre-specified criteria, structured SBTs, and extubation criteria probably reduce the duration of mechanical ventilation and ensure we’re giving all patients in the ICU a fair chance to demonstrate they can breathe on their own.

Failed Spontaneous Breathing Trials: Try and Try again

Patients may fail SBTs due to an imbalance between the respiratory load of spontaneous breathing and their respiratory capacity. Respiratory load may be increased due to increased airway resistance (e.g. asthma and COPD), decreased lung compliance (e.g. pulmonary fibrosis, pulmonary edema, ARDS), air trapping, chest-wall disease (e.g. abdominal compartment syndrome), and cardiac dysfunction. Respiratory capacity may be reduced due to respiratory muscle weakness, diminished respiratory drive, and impaired neuromuscular function.

The difficulty in transitioning patients to spontaneous breathing may be categorized as simple transition (successful first SBT followed by extubation), difficult transition (3 or fewer SBTs and 7 or fewer days until extubation), and prolonged transition (3 or more SBTs or 7 or more days of mechanical ventilation after the first failed SBT).

Mortality is increased in patients requiring prolonged transitions and 10-20% of patients fall into this category. Data are lacking on the best approach to prolonged respiratory failure. These are the patients that are truly “weaned” from the ventilator, commonly through progressive decreases in pressure support and increased SBT duration. This is also the group in whom to think about tracheostomy, primarily for ease of suctioning, patient comfort, and communication; but the timing of trachoestomy is still controversial.

How to Identify Those Who Will Still Fail Extubation Despite Passing an SBT

In spite of passing an SBT with flying colors, some patients still don’t fly after extubation. In fact, about 15% overall require reintubation within 48 hours (this rate is higher in medical and neurology compared to surgical ICUs). And again, reintubated patients appear to have higher mortality, prolonged hospitalization, and decreased likelihood of returning home. So how do we identify those at risk for reintubation?

One study found that all patients with a combination of inadequate cough, excessive secretions, and impaired mental status were reintubated, whereas nearly all (97%) without any these problems did fine after extubation. Another study demonstrated that increased RSBI at the end of an SBT, positive fluid balance, and pneumonia were at higher risk for reintubation. Other risk factors include failure of two previous SBTs, heart failure, hypercarbia, other co-morbidities, upper airway stridor after extubation, older age, and high APACHE II score (>12) on the day of planned extubation. A provocative editorial by Dr. Tobin recently argued that patients haven’t truly passed an SBT unless done on a T-piece.

Does Post-Extubation Noninvasive Positive Pressure Ventilation Prevent Reintubation? 

Studies have shown that non-invasive positive pressure ventilation (NIPPV) for patients who develop respiratory distress within 48 hours after extubation does not prevent reintubation, and in fact may be dangerous. Conversely, preemptive use of NIPPV for patients with risk factors (see above) for reintubation (i.e. placing these patients on NIPPV immediately after extubation) does seem to prevent reintubation. In other words, placing patients at higher risk for reintubation on NIPPV immediately after extubation may reduce rates of reintubation, but waiting until post-extubation respiratory distress develops is probably too late and patients should just be reintubated at that point.

Authors' Approach to Mechanical Ventilation "Weaning"

The authors evaluate all hemodynamically stable, improving patients for readiness to extubate, and then perform an SBT on CPAP 5 cmH2O for 30 minutes (off sedation of course). Their definitions of “hemodynamically stable” and “improving” are more lenient than most. For example, a septic patient with a clear source (controlled and on appropriate treatment) and decreasing (but not necessarily discontinued) dose of norepinephrine might qualify. Additionally, they don’t necessarily follow the rule of “extubate in the daylight.” They would put the above patient on an SBT at 8PM, and if the patient passes the SBT at 8:30PM, is awake and has minimal airway secretions, they would generally extubate. This is in contrast to a more conservative approach of continuing to wean off norepinephrine overnight and waiting for the cavalry in the morning to try an SBT. If they extubate a patient at increased risk for post-extubation respiratory distress, then they institute preemptive NIPPV  and assess the patient within 30 minutes, reintubating if there are any signs of distress.

The authors acknowledge that there is no clear evidence to support a universally aggressive approach, but they tend to accept the possible higher risk of reintubation in order to reduce the risks associated with one more day of intubation and mechanical ventilation. Whether your unit can employ such an aggressive approach clearly depends on staffing, expertise, monitoring capabilities, and equipment necessary to safely extubate (and reintubate) a higher risk patient. And while an aggressive approach to evaluation for readiness to extubate may be beneficial, remember that each ICU patient is unique, and individual patient circumstances must be considered before acutely extubating.

(See More PulmCCM Reviews)

With appreciation:

John F. McConville and John P. Kress. Weaning patients from the ventilator. NEJM 2012;367(23):2233-2239.

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Weaning from Mechanical Ventilation Update (Review)