Mechanical Ventilation: Discontinuation
Scott E. Kopec
Richard S. Irwin
I. GENERAL PRINCIPLES
A. Outcome for patients with respiratory failure.
1. Mechanical ventilation (MV) can be discontinued in 80% to 90% of patients within 3 weeks. Of this group, 77% can be extubated within 72 hours of the start of MV.
2. Ten to twenty percent require prolonged MV > 21 days. Weaning from MV in this group may take >3 months. One-year survival can be as high as 93%. Survivor quality of life may be minimally to moderately impaired when assessed 2 years later.
B. Four potentially reversible causes of prolonged MV.
1. Inadequate respiratory drive due to
a. Nutritional deficiencies.
b. Sedatives.
c. Central nervous system (CNS) abnormalities.
d. Sleep deprivation.
2. Inability of the lungs to carry out gas exchange without MV if the underlying cause of respiratory failure has not significantly improved.
3. Inspiratory respiratory muscle fatigue or weakness due to
a. CNS and neuromuscular diseases or dysfunction.
b. Active inflammatory processes (e.g., sepsis).
c. Nutritional and metabolic deficiencies.
d. Medications (e.g., corticosteroids).
e. Chronic renal failure.
f. Increased work of breathing (WOB) from intrinsic lung disease or extravascular lung water, chest wall disorders, or cardiovascular failure
g. Hypoxia and hypercapnia.
4. Psychological dependency.
C. Pump failure from inspiratory respiratory muscle fatigue is probably the primary etiology for failure of discontinuation of MV in most patients on prolonged MV.
II. INDICATIONS
A. When to initiate discontinuation trials.
No objective data exist on when to begin the weaning process, so clinical judgment is necessary. A monitored spontaneous breathing (SB) screening trial is recommended when the following criteria are met.
1. The underlying reason for MV has been stabilized, and the patient is improving.
2. Hemodynamically stable and on no or minimal and unchanging doses of pressors.
3. Adequate oxygenation (PaO2/FIO2 > 200, positive end-expiratory pressure [PEEP] < 7.5 cm H2O, FIO2 < 0.5).
4. The patient is able to initiate spontaneous inspiratory effort.
B. Principles of weaning.
1. Breathing is a form of continuous muscular exercise, and MV discontinuation should reflect principles of muscle training that include stressing respiratory muscles to early fatigue and then resting them. Maintain a structured, progressive program because benefit is transient.
2. Sudden increased WOB with MV discontinuation can cause harmful effects. Monitor closely during the first 5 minutes and return to MV if there is deterioration.
3. Because physiologic failure can cause tachycardia, tachypnea, and hypertension as can anxiety, do not assume anxiety alone is the cause.
4. Screening patients daily may reduce intensive care unit (ICU) stay and time of MV.
5. Studies have shown that when a standardized, hospital-based protocol is used that incorporates a team approach between physicians, nurses, and respiratory therapists, success rates for weaning are significantly improved.
C. Predictive indices for successful discontinuation.
1. It does not appear that any single parameter can consistently and accurately predict success in weaning. The following parameters have the highest accuracy.
a. Spontaneous respiratory rate (RR) < 38 breaths per minute (sensitivity 88%, specificity 47%).
b. Rapid shallow breathing index (RSBI) < 100 breaths/min/L. RSBI is the RR divided by tidal volume (Vt) in liters averaged over 1 minute. RSBI should be measured while the patient is breathing spontaneously.
i. RSBI < 100 has a PPV of 0.78 and NPV of 0.95.
c. Maximal inspiratory pressure (MIP) less than 15 cm H2O had a negative predictive value of 100%.
2. The above predictive results for successful discontinuation of MV are even less accurate the longer a patient is dependent on MV.
3. Clinical observation of respiratory muscles is not reliable in predicting failure. Both muscle fatigue and any increase in respiratory muscle load cause a change in rate, depth, and pattern of breathing. Nevertheless, close monitoring is necessary because discontinuation failure is inevitable if these signs are due to fatigue. If these signs never appear, successful discontinuation is likely.
III. PROCEDURE
A. Modes of discontinuation from MV.
Four modes of weaning are typically used: SB trial, synchronized intermittent mechanical ventilation (SIMV), pressure support (PS), and noninvasive positive-pressure ventilation (NIPPV). Successful discontinuation of MV is less determined by the mode of weaning than by identification and correction of medical barriers to weaning. However, when compared to other modes, SIMV has nearly consistently performed the worst in clinical trials and is not recommended.
1. SB discontinuation trial.
