Discontinuation of Mechanical Ventilation

1 What proportion of patients can be readily removed from mechanical ventilation?

The majority of patients (75%) supported with mechanical ventilation are able to resume unsupported breathing within 7 days of intubation if the illness that resulted in respiratory failure resolves or improves. One of the clinician’s challenging tasks is to determine when the patient is ready for ventilator discontinuation. Continuing mechanical ventilation beyond the time that is necessary exposes the patient to risks for nosocomial infection and ventilator-induced lung injury. Conversely, removing ventilatory support from a patient prematurely can lead to severe stress from respiratory and cardiovascular decompensation and exposes the patient to the risks associated with reintubation including increased mortality rate, increased time in the intensive care unit (ICU), and need for long-term care in a rehabilitation facility.

2 When should patients receiving mechanical ventilation be assessed for ventilator discontinuation?

Every patient receiving mechanical ventilation should be assessed for ventilator discontinuation on a daily basis as long as his or her medical status meets the following criteria:

Evidence indicates that systematic daily weaning assessments improve patient outcomes, reduce the number of days patients are dependent on the ventilator, and reduce the number of patients who require tracheostomies.

3 How, exactly, should this assessment be done?

As of yet, no systematic weaning protocol has been agreed on. However, most protocols have a stepwise assessment that varies in the details. The above criteria should be assessed daily as a wean screen. For the patients who pass the daily wean screen, there is first an initial brief trial or readiness assessment during which patients are closely observed for 1 to 5 minutes while receiving minimal or no support (continuous positive airway pressure [CPAP] ≤ 5 cm H₂O, T-piece trial, 5 to 7 cm H₂O pressure support from the ventilator, or automatic tube compensation) to assess their ability to undergo a formal spontaneous breathing trial (SBT). If the patient does well during the readiness assessment, an SBT is performed for between 30 and 120 minutes. During this time patients are closely monitored for signs of respiratory insufficiency, hemodynamic deterioration, problems with gas exchange, or patient discomfort. Full ventilatory support is promptly reinitiated if problems develop. Successfully completing an SBT is highly predictive of successful ventilator discontinuation. These steps are illustrated in Figure 10-1.

Figure 10-1 A protocol for daily assessment for extubation. RSBI, Rapid shallow breathing index calculated by frequency of breaths per minute divided by tidal volume in liters; SBT, spontaneous breathing trial.

4 What is the rapid shallow breathing index (RSBI)? What does it predict?

The RSBI is calculated by using the following formula:

Higher values for the RSBI signify a pattern of breathing often seen in patients with respiratory muscle fatigue who tend to have weak inspiratory efforts and consequently higher respiratory rates. Values greater than 105 to 120 predict patients who may not tolerate an SBT. However, the observed RSBI value can be increased by recent suctioning, anxiety, fever, and the size of the endotracheal tube. Clinicians should be aware that the RSBI will generally be lower during use of minimal support modes (pressure support, CPAP) in comparison with when patients are just using a T-piece. The RSBI is most often measured during the readiness assessment to identify patients who may go forward with an SBT. The RSBI does not predict the ability for the patient to have the endotracheal tube removed on its own.

5 To which mode should the ventilator be set during the SBT?

The specific ventilatory mode during the SBT is not critical. The choices usually include one of the following:

A disadvantage of T-piece trials is that the patient is not connected to the ventilator’s alarm systems that supplement clinician monitoring for apnea or tachypnea. The choice among these options will depend on each provider’s experience and preferences.

6 What are the traditional weaning parameters, and how are they used?

Traditional weaning parameters include maximal inspiratory pressure, minute volume, vital capacity, maximum voluntary ventilation, thoracic compliance, and respiratory resistance. In the past, they were used to predict the likelihood of success with weaning trials. It is now known that they do not discriminate well between patients who will have success and those who will have failure after extubation. Assessment during a carefully monitored SBT appears to provide the most clinically useful information regarding ventilator discontinuation. Measurement of traditional weaning parameters is generally not necessary.

7 Describe what to do about sedation and analgesia with such a patient

Patients are often medicated with sedatives and analgesics while receiving mechanical ventilation to reduce patient pain and discomfort and to limit patient movements that could lead to accidental extubation or other injuries. Continuous sedation may provide a more constant level of sedation, but this has been associated with a longer duration of mechanical ventilation, days spent in the ICU, and hospitalization compared with intermittent sedation protocols. Patients randomly assigned to undergo a planned interruption of continuous sedation on a daily basis had reduced days of mechanical ventilation and days in the ICU compared with those who were randomly assigned to receive continuous sedation. No adverse effects of interruption of sedation were apparent. The optimal method of providing sedation and analgesia for these patients is not known. However, minimizing sedatives to the level that achieves a specified sedation target and attempting to awaken the patient daily appear to be important aspects of patient management during mechanical ventilation. It is also important to have the daily sedation vacation correspond with the weaning trial to ensure an accurate assessment of the patient’s ability to breathe. Systematic improvements in sedation practice are associated with improvements in outcomes including shorter ICU and hospital length of stay, duration of mechanical ventilation, and costs.

8 What do you do with patients who have failure during the SBT?

Two actions are necessary:

To sustain spontaneous ventilation successfully, patients must have an intact respiratory center drive and adequate neuromuscular function and not have excessive loads on the respiratory muscles. Box 10-1 provides one method of systematically reviewing possible causes of failure during an SBT. Patients often have more than one cause for failure to wean, and correction of these factors may require multiple interventions. In general, it is recommended to wait 24 hours before attempting another SBT.

Box 10-1 Factors to consider when tests of inspiratory efforts or spontaneous breathing trials fail in patients

1. The patient has an increasing PaCO₂ without increases in respiratory effort or rate.

(a) Inadequate respiratory center drive because of excessive narcotics, sedatives, hypothyroidism, or brain injury

(b) Appropriate compensation for metabolic alkalosis because of excessive diuresis or nasogastric suctioning

(c) Return to a chronic hypercapnic state after inappropriate overventilation in patients with COPD or sleep apnea

2. The patient has tachypnea, tachycardia, or distress.

(a) Impaired neuromuscular function

Fatigue due to prolonged high loads, inadequate rest, or ventilator dyssynchrony

Hypothyroidism

Electrolyte deficiencies (e.g., hypokalemia, hypophosphatemia, hypomagnesemia)

Critical illness myopathy or polyneuropathy

Steroid myopathy

Effects of drugs (e.g., aminoglycosides, neuromuscular antagonists)

Sepsis

Diaphragmatic paresis or paralysis due to phrenic nerve injury resulting from cold cardioplegia or thoracic or neck surgery

Prolonged malnutrition

(b) Excessive respiratory load

Increased airway resistance (e.g., asthma, COPD, excessive secretions, small endotracheal tube)

Air trapping and increased threshold load due to positive residual pressures (particularly in patients with COPD)

Decreased respiratory system compliance (e.g., pulmonary edema, fibrosis, pneumonia, abdominal distention, thoracic cage abnormalities, pleural effusions)

High minute ventilation requirements (e.g., fever, sepsis, metabolic acidosis, high physiologic dead space, excessive caloric intake, pulmonary embolism)

(c) Impaired left ventricular function

(d) Psychological dependence: a diagnosis of exclusion but not rare in patients in ICUs

COPD, Chronic obstructive pulmonary disease.

9 What criteria are important when considering removal of an artificial airway?

Successful completion of an SBT does not necessarily indicate that the patient is ready for extubation. Reintubation for respiratory failure occurs in approximately 10% to 15% of patients in most well-run ICUs. This rate is higher among those who have had endotracheal tubes in place for longer than 48 hours, who are older, or who have increased severity of illness, anemia, or cardiac failure. Unfortunately, reintubation is associated with a significantly increased mortality compared with patients not requiring reintubation, even when controlling for the severity of illness among these patients. Patients should be able to protect their airway, should demonstrate good cough effort, and should not have copious secretions. Patients should be responsive and able to follow commands. The difficulty of reintubating the patient’s airway should be taken into account; the threshold for extubation in someone with a difficult airway should be higher. A cuff leak test can be performed if there is concern about postextubation upper airway obstruction. Though the presence of a leak is reassuring, the absence of a leak does not necessarily predict stridor after extubation.

10 What about using noninvasive ventilation (NIV) for patients who have respiratory failure after extubation?

The initial use of NIV—ventilators that interface with the patient through a full face or nasal mask rather than an endotracheal tube—has improved outcomes in subsets of patients who are seen with acute respiratory failure, particularly patients with chronic obstructive pulmonary disease (COPD) and cardiogenic pulmonary edema. However, the situation may be different in patients who have respiratory failure after extubation.

A recent Cochrane review found that in patients with COPD in whom extubation failed, NIV may be a reasonable option. NIV had a positive effect on mortality and ventilator-associated pneumonia, length of stay in the ICU and hospital, and total duration of ventilation in COPD patients in whom extubation failed. Patients need close monitoring if this approach is tried. NIV may also be useful as a prophylactic measure in patients who are thought to be high reintubation risks. The full utility of NIV after failure of extubation needs to be further elucidated. The majority of positive trials enrolled exclusively or predominately patients with COPD.

11 Define prolonged mechanical ventilatory support (PMV)

A recent consensus conference defined patients receiving PMV as those who need at least 6 hours/day of ventilatory support for ≥ 21 days. These patients generally require a tracheostomy for optimal care. It is estimated that approximately 3% to 7% of patients receiving mechanical ventilatory support meet this definition. One-year survival rates among these patients range from 23% to 76%, with older age and poor functional status before the acute illness predicting a worse prognosis. In patients receiving PMV, the criteria used in the weaning protocols previously described for acutely ill patients do not apply. Many of these patients are managed in long-term care units outside the ICU.

12 Should these patients be managed with different modes of ventilation?

Patients who are ventilator dependent after 14 to 21 days despite improvement in disease state may require different management strategies. Multidisciplinary rehabilitation with focus on ventilatory support, nutrition, physical therapy, and psychosocial support are all important aspects. Gradual reduction in ventilatory support may be used in PMV patients. Many clinicians wean patients to approximately 50% of their maximal support levels without using SBTs. Once at the 50% level, daily SBTs are started. Ventilatory support should be withdrawn gradually during the day, with progressively longer SBTs, allowing rest and sleep on full support modes at night. Once the patient tolerates spontaneous ventilation throughout the day, withdrawal of nocturnal ventilation may proceed relatively quickly. The success rate of ventilator discontinuation is only 50% to 60%. As in acutely ill patients, a therapist-driven protocol approach to weaning appears to improve outcomes in comparison with clinical judgment alone. Clinicians should continue efforts to identify and correct physiologic reasons for the patient’s inability to resume spontaneous ventilation (see Box 10-1).

13 Why is there such an emphasis on protocols?

In most recent studies, systematic weaning protocols, protocols to minimize or interrupt sedation, and raising the head of the bed have been associated with improved patient outcomes and reductions in the cost of care. Protocols run by nonphysicians produce faster liberation from the ventilator unless the physician-run usual care aggressively follows guidelines, in which case there is no difference. A recent Cochrane review found that protocolized weaning in critically ill adults resulted in a reduced shorter duration of mechanical ventilation by 25%, weaning duration was reduced by 78%, and ICU length of stay was reduced by 10%.