Ventilator Weaning

 
Ventilator Weaning: 
 
In most patients, mechanical ventilation can be discontinued as soon as the underlying reason for acute respiratory failure has been resolved. Hence, the first step in ventilator weaning is to reverse the process that caused the respiratory failure to begin with. However, 20-30% of patients are considered difficult to wean from ventilator. Weaning failure is defined as the failure to pass a spontaneous-breathing trial or the need for reintubation within 48 hours following extubation. 
 
Most common reasons for failure to wean are :
  1. Lungs​/Airway
    1. ​Increased resistance : from worsening bronchoconstriction, pooled secretions in ET tube, tracheal stenosis, tracheomalacia , upper airway edema, ARDS ( due to edema of bronchial walls). 
    2. Decreased compliance with increased elastic workload: due to ARDS, atelectasis, chest wall restriction like kyphosis, pleural effusion, pneumothorax, massive ascites
    3. Decreased diffusion with abnormal gas exchange : alveolar edema, pneumonia , pulmonary fibrosis, diffuse alveolar hemorrhage
  2. Cardiac : severe systolic dysfunction, arrythmias, high vasopressor use
  3. Neurological : stroke, delirium, anxiety, sleep deprivation, sedatives , impaired respiratory drive 
  4. Musculoskeletal : diaphragm weakness from prolonged mechanical ventilation, critical illness myopathy, GBS 
  5. Endocrine / Metabolic : severe hypothyroidism induced myopathy and impaired ventilatory drive, adrenal insufficiency, severe hypokalemia
Work of breathing increase if patient has to overcome any of the above mentioned lung or cardiac issues. For example, transition from mechanical ventilation to spontaneous breathing imposes an additional load on the cardiovascular system because of intrathoracic pressure changes, which increases ventricular afterload and increased oxygen consumption by the respiratory muscles. If the heart can't cope up with this increased load, patient may go into pulmonary edema with resultant weaning failure. 
 
A methodical approach as mentioned above will provide an opportunity to address the underlying issues for weaning failure much faster. For example, one of the most common causes of failure to wean is agitation with tachypnea, tachycardia and hypertenion during an SBT. Some of us sense this as failed attempt at weaning and put the patient back on full ventilator support with sedation. If other underlying causes like lungs and cardiac issues are ruled out, then anxiety could be the biggest reason for those signs and symptoms. If we don't follow a methodical approach, instead of addressing the underlying anxiety, we are just putting the patient back on the ventilator due to wrong assessment, resulting in more sedation with prolonged stay on the ventilator with development of new complications and an ongoing vicious cycle. 
 
 
Reasons for Reintubation following successful SBT and extubation
  • Upper airway resistance (supraglottic edema) – A good marker for severe upper airway obstruction is the absence of air leakage when the endotracheal tube cuff is deflated. Intensive Care Med. 2009 Jul;35(7):1171-9. The cuff-leak test is extremely useful because methylprednisolone therapy at least 12 hours before extubation might reduce the incidence of stridor and the rate of reintubation due to upper airway obstruction. 
  • Poor cough and excessive secretions 
  • Poor airway reflexes leading to aspiration 
  • vocal cord dysfunction
  • laryngospasm
  • Respiratory weakness masked by pressure support 
  • Increased cardiac load induced by removal of CPAP , especially with severe systolic heart failure 
  • Respiratory muscle fatigue, especially if they have a h/o prolonged weaning. 
  • Neurological impairment
  • Higher positive fluid balance (Chest. 2006 Dec;130(6):1664-71)
  • Onset of new pathology 
 
Criteria for initiating SBT:
Reversal of primary pathology, P/F ratio >150, PEEP <8, Hemodynamic stability , ability to initiate spontaneous breaths. 
 
Methods of weaning:
Different techniques have been used for weaning which include gradual reduction in mandatory rate as in SIMV, gradual reduction in pressure support, spontaneous breathing through a T-piece, proportional assist ventilation, NAVA and adaptive support ventilation. 
 
Duration of weaning:
30 mins vs 120 mins: An SBT with PSV of 7 cmH(2)O lasting 30 min is equally effective in recognizing the successfully extubated patients as a 120-min trial. Intensive Care Med. 2002 Aug;28(8):1058-63 , Am J Respir Crit Care Med. 1999 Feb;159(2):512-8 . However, in patients with prolonged weaning, some recommended that 120 mins of SBT is much safer. Am J Respir Crit Care Med 158:1855–1862. Also, Conventional weaning parameters did not predict extubation outcome in intubated subjects requiring prolonged mechanical ventilation. Absent cough was the best predictor of extubation failure. Respir Care. 2013 Aug;58(8):1307-14
 
Signs and symptoms of failed weaning trial:
Tachypnea, low tidal volumes, tachycardia, hypotension or hypertension, arrythmias, poor NIF, low FVC, 
 
 

Criteria for extubation after an SBT:

Several criteria has been used with variable sensitivity and specificity. The most common ones that are used in practice are Rapid shallow breathing index (RSBI), negative inspiratory force (NIF) , P0.1, Maximal vital capacity. Other methods include ScVO2 measurements, CROP index, minute ventilation, RSBI rate . Refer to Table 3 : Respir Care. 2012 Oct;57(10):1649-62
 
Rapid shallow breathing index ( Tobin Index) : It is measured by RR/Tidal volume in litres. Values >105 indicates weaning failure. However, when tobin devised this index, it was designed to measure the patient readiness for weaning trial and was done before a SBT, not after SBT. In his original study, he put patients on T-piece for 1 min and measure the RSBI. If its more than 105, it indicates their readiness of weaning trial and hence, they were placed on SBT. If its less than 105, they were placed back on full support.  N Engl J Med 1991; 324:1445-1450 . In a study, RSBI cut-off value proved inaccurate, predicting only 20% of extubation failure. J Bras Pneumol. 2009 Jun;35(6):541-7.
 
RSBI rate : 
The RSBI Rate is calculated by obtaining the difference between the initial RSBI and the final RSBI, and then dividing the result by the initial RSBI. The resulting number is then multiplied by 100. It was shown that RSBI Rate of less than 20% was over 90% sensitive and 100% specific for predicting weaning success. It had a positive predictive value of 100% and a negative predictive value of over 81%.
 
The mathematical formula is as follows: RSBI Rate = [(RSBI 2– RSBI 1)/RSBI 1] x100.
 
RSBI rate, which is a dynamic measure of lung mechanics, is an accurate predictor of weaning outcome and even more reliable than other weaning parameters such as lung static and dynamic compliance and single RSBI determination.​ Chest. 2016;150(4_S):303A Intensive Care Med. 2010 Mar;36(3):487-95
 
P0.1: Negative pressure generated by the patient during first 100 m.sec of inspiration against an occluded airway. It is a measure of how quickly the patient is trying to inspire. It is a very efficient way of predicting patient effort as well as evaluating the respiratory drive. It not only depends on respiratory drive but also on inspiratory muscle capacity. If it is too high i.e. >-8, it means that patient is working too hard to breathe in or to suck the air in at the beginning of inspiration. When on weaning trial, the P0.1 should be the same at the end of the trial for successful weaning. If it is increasing (becoming more negative), it predicts that patient had to generate that much negative inspiratory force for the same tidal volumes and it predicts weaning failure. Please note that in this case the tidal volumes and RR may be the same but P0.1 is increasing. Threshold P0.1 is less than -4. Since P0.1 is dependent on patient effort and willingness, a decreased value may not mean anything. However, a normal P0.1 indicated intact respiratory drive.
 
NIF: Normal values in males are > -75 and in females are > – 50. This is patient effort dependant, and hence, a low value value doesn't indicate respiratory muscle weakness. The threshold value is -20 to -30. 
 
P0.1/PImax : P0.1 and P0.1/PImax ratio provide the best means of predicting extubation success, and they are not influenced by tracheal tube resistance. This has been found to be the best predictor of weaning success with a sensitivity of 98%, specificity of 100%, positive predictive value of  100% and negative predictive value of 92%. The threshold value was less than 0.1. Chest. 1995 Aug;108(2):482-9
 
CORE index (compliance, oxygenation, respiration, and effort)=[Cdyn x (PImax/P0.1)x(PaO2/PAO2)]/f Compared to RSBI, P0.1, and CROP index ( compliance, rate, oxygenation, and pressure ), CORE index was the most accurate predictor of SBT success/failure. It is also a very good predictor of weaning success with a sensitivity of 100%, specificity of 95%, positive predictive value of  96% and negative predictive value of 100%. The threshold value was more than 8. Respir Care. 2011 Oct;56(10):1500-5
 
Integrative weaning index = (CRSSaO2)/(f/VT), where CRS is static compliance. It has a sensitivity of 97%, specificity of 94%, positive predictive value of  99% and negative predictive value of 86%. The threshold value was more than 25. Crit Care. 2009; 13(5): R152
 
Methods of ventilator weaning:
 
ACCP and ATS gave a conditional recommendation in their 2017 guidelines that the initial SBT be conducted with inspiratory pressure augmentation (5-8 cm H2O) rather than without (T-piece or CPAP). Chest. 2017;151(1):166-180 , Am J Respir Crit Care Med. 1997 Aug;156(2 Pt 1):459-65 , Acta Anaesthesiol Scand. 2002 Sep;46(8):973-9 , Croat Med J. 2004 Apr;45(2):162-6Am J Med Sci. 2014 Oct;348(4):300-5.
 
T-piece vs pressure support: In another study, COPD patients who failed T-piece study were successfully extubated when they were placed on pressure support of 7 for 30 mins. Intensive Care Med. 2006; 32(1):165-169.
 
T-piece vs pressure supportAmong patients requiring prolonged mechanical ventilation and treated at a single long-term care facility, unassisted breathing through a tracheostomy, compared with pressure support, resulted in shorter median weaning time. JAMA. 2013 Feb 20;309(7):671-7.
 
ATC vs CPAP: SBT with  ATC were safe but did not hasten liberation from mechanical ventilation, when compared to CPAP. Respir Care 2010;55: 549–554.
 
T-Piece vs ATC: In another study of T-piece trials, spontaneous breathing through an endotracheal tube well mimics the work of breathing performed after extubation and hence, there is no need for automatic tube compensation. Am J Respir Crit Care Med. 1998 Jan;157(1):23-30.
 
However, Cabello and colleagues compared the SBT on a T-piece and the low-PS test (7 cm H2O) with or without PEEP in patients with heart failure and difficult weaning. Patient effort was lower during the low-PS test than during the T-piece test and decreased further when PEEP was added to PS. These findings are consistent with previous evidence that PS and PEEP can reduce patient effort by about 30 to 40%. Intensive Care Med. 2010 Jul;36(7):1171-9
 
Pressure support reduces both the resistive and elastic loads as opposed to automatic tube compensation which reduces only resistive load. .
 
Also, Lemaire and colleagues showed the consequences of positive pressure withdrawal in patients with chronic obstructive pulmonary disease and cardiovascular disease with weaning failure. During spontaneous ventilation, these patients rapidly developed right ventricular dilatation due to increase in venous return, reduction in left ventricular compliance, increase in left ventricular afterload, and marked increases in pulmonary artery occlusion pressures.  Anesthesiology. 1988 Aug;69(2):171-9
 
Switching a patient from positive pressure ventilation to spontaneous breathing re-establishes negative inspiratory intra-thoracic pressures thus increasing venous return (left ventricular preload), central blood volume and left ventricular afterload. This normal condition, often an effort test for the patient, can decompensate the cardiorespiratory function in case of volume overload and left ventricular systolic or diastolic dysfunction. Intensive Care Med. 1997 May;23(5):493-503
 
My take home message: For most patients in a MICU, there was no difference in the extubation rates using a T-piece or CPAP of 5 or pressure support of 5-8 or ATC. However, in patients who are on prolonged mechanical ventilation or who has severe heart failure, their cardiopulmonary status may not be optimal. A 30– 60% increase in respiratory load immediately after extubation sufficient to cause a cardiorespiratory catastrophe in a vulnerable patient. Hence, a PS or CPAP may over estimate their ability to cope with increased work load after extubation. Hence, i use T-Piece trials without ATC in those selected patients. However, even the patients who failed T-pice trial can be placed on CPAP/PS and if they do well, they could be extubated to BiPAP/CPAP. 
 
In patients at high risk of reintubation, the early use of prophylactic non invasive ventilation seems to efficiently prevent reintubation. These high rsik patients include CHF, prolonged mechanical ventilation, upper airway obstruction, critical illness myopathy and hypercapnea on extubation. Crit Care Med. 2005 Nov;33(11):2465-70Am J Respir Crit Care Med. 2006 Jan 15;173(2):164-70Intensive Care Med. 2012 Oct;38(10):1599-606
 
 
The concept of ‘minimal ventilator settings’ is controversial: 
  • Martin Tobin has argued that adding either 5 cm H2O as “physiologic” PEEP or pressure support of 7 cm H2O to overcome the resistance in an endotracheal tube (or both, as is usually done) may actually reduce the “spontaneously” breathing patient’s workload by >40% .Am Rev Respir Dis. 1991 Mar;143(3):469-75
  • Some clinicians believe that insertion of an endotracheal tube leads to the loss of “physiologic PEEP,” which is thought to result from intermittent narrowing of the vocal cords. The concept of physiologic PEEP, however, is a myth. Lung volume at end expiration generally approximates the relaxation volume of the respiratory system, which is determined by the static balance between the opposing elastic recoil of the lung and chest wall. Accordingly, static recoil pressure of the respiratory system is zero at end-expiration in a healthy adult. The addition of 5 cm H2O of PEEP can decrease work of breathing by as much as 40% in ventilated patients. 
  • PEEP also produces a substantial increase in cardiac output in patients with left-ventricular failure. Chest. 2005 Nov;128(5 Suppl 2):592S-597SIn patients with heart or lung disease, the elimination of PEEP at the moment of extubation can lead to rapid cardiopulmonary decompensation. As when assessing patients on low levels of pressure support, observing a patient breathe on CPAP 5 cm H2O hampers the ability of a physician to predict the patient’s capacity to handle an increase in cardiorespiratory load following extubation. 
  • It has been shown that the work of breathing through an endotracheal tube, compared to the work of breathing following extubation, is almost identical due to upper airway edema resulting from an ETT being in place for several days. Am J Respir Crit Care Med. 1998 Jan;157(1):23-30
  • Tobin argues for wider use of true T-piece spontaneous breathing trials, especially in those at high risk of failed extubation and when the consequences of failed extubation may be catastrophic. An alternative is to have the ventilator set on “flow-by,” with pressure support and PEEP set at zero. 
  • The level of pressure support necessary to eliminate the work imposed by endotracheal tubes and ventilator circuits varies considerably (from 3 to 14 cm of water). A pressure support of 5 can decrease the WOB by as much as 40% in some patients.  
  • Studies showed that CPAP of 5 significantly decreased the RSBI. Chest. 2002 Feb;121(2):475-9 , Respir Care. 2009 Nov;54(11):1462-6
  • In general, it is a misconception that, after extubation, upper airway resistance decreases. Hence, lot of us provide automatic tube compensation to counteract the ET tube resistance, with the belief that once that tube comes off, the increased tube resistance would go away. In fact, the work of breathing in patients mechanically ventilated for ± 5.5 days increased after extubation. This increase was not shown to be due to tracheal or laryngeal edema but was probably due to edema of the oropharynx and velopharynx. Chest. 1995 Jan;107(1):204-9
     
    Four methods of weaning patients from mechanical ventilation – Esteban, 1995 
     
    Study groups: 
    1. SIMV 
    2. Pressure Support in which PS was initially set at 18 ± 6 cm of water and then reduced, if possible, by 2 to 4 cm of water at least twice a day. 
    3. Intermittent trials of spontaneous breathing, conducted two or more times a day if possible  
    4. Once-daily trial of spontaneous breathing 
     
    Protocol
    After patients were enrolled in the study, assist–control ventilation was stopped and the patients breathed spontaneously for three minutes through a T-tube circuit. During this trial, patients received humidified oxygen. 
     
    Patients who met at least two of the following criteria underwent a trial of spontaneous breathing lasting up to two hours:  
    1. Maximal inspiratory pressure below 20 cm of water. (Maximal inspiratory pressure was measured three times in succession, and the most negative value was selected). 
    2. Tidal volume above 5cc/kg 
    3. Respiratory rate of less than 35 breaths per minute. 
     
    The primary physician terminated the trial if a patient had any of the following signs of distress: 
    1. a respiratory frequency of more than 35 breaths per minute 
    2. oxygen saturation below 90 % 
    3. heart rate above 140 beats per minute  
    4. a sustained increase or decrease in the heart rate of more than 20 percent 
    5. systolic blood pressure above 180 mm Hg or below 90 mm Hg 
    6. Agitation, diaphoresis, or anxiety.  
     
    Patients who had none of these features at the end of the trial were extubated. For all four methods, weaning was considered to have failed if reintubation was necessary within 48 hours after extubation.  
     
    If a patient had signs of poor tolerance at any time during the trial, assist–control ventilation was reinstituted. Even if there were no signs of distress by the end of this trial, extubation could be postponed for a maximum of 24 hours if the primary 
    physician thought that a patient might not be able to clear secretions or protect the airway against aspiration. Patients continued to breathe spontaneously through the T-tube circuit. 
     
    Methods
    1. Intermittent Mandatory Ventilation 
    The initial rate was 10.0 ± 2 breaths per minute and mechanical breaths were synchronized with inspiratory effort. We attempted to decrease the ventilator rate, usually by 2-4 breaths per minute, at least twice a day but were decreased more rapidly if tolerated by the patient. Patients who tolerated a ventilator rate of five breaths per minute for two hours without signs of distress were extubated. A continuous positive airway pressure of 5 cm of water was permitted. 
    1. Pressure-Support Ventilation 
    Pressure support was titrated to achieve a frequency of 25 breaths per minute. Pressure support was initially set at 18 ±6 cm of water, and we attempted to reduce this level of support by 2 to 4 cm of water at least twice a day. The pace was increased if the patient did not have signs of distress (the same criteria were applied as in the initial trial of spontaneous breathing, except that a respiratory frequency of 25 breaths per minute was required). Patients who tolerated pressure support at a setting of 5 cm of water for two hours were extubated. A continuous positive airway pressure of 5 cm of water was permitted. 
    1. Intermittent Trials of Spontaneous Breathing 
    Patients were disconnected from the ventilator and allowed to breathe spontaneously through either a T-tube or a continuous-flow circuit designed to provide a continuous positive airway pressure of 5 cm of water. The duration of the trials was gradually increased, and they were attempted at least twice a day. Between the trials, assist–control ventilation was provided for at least one hour. Patients able to breathe on their own for at least two hours were extubated. 
    1. Once-Daily Trial of Spontaneous Breathing 
    Patients were disconnected from the ventilator and allowed to breathe spontaneously through a T-tube for up to two hours each day. If signs of intolerance developed, assist–control ventilation was reinstituted for 24 hours, at which time another trial was attempted. Patients who tolerated a two-hour trial were extubated. 
     
    Results: 
    A once-daily trial of spontaneous breathing led to extubation about three times more quickly than intermittent mandatory ventilation and about twice as quickly as pressure-support ventilation. Multiple daily trials of spontaneous breathing were equally successful. 
     
     
    Three methods of ventilator weaning- Brochard, 1994  
    Study groups
    1. SIMV 
    1. Pressure Support  
    1. T-piece trials intermittently 
     
    Protocol
    After patients were enrolled in the study, assist–control ventilation was stopped and the patients breathed spontaneously for 2 minutes through a T-tube circuit.  
     
    At least three of the following criteria were required to continue with the procedure:  
    1. maximal inspiratory pressure equal to or lower than -25 cm H20 
    2. RR below or equal to 35 breaths/min 
    3. vital capacity equal to or above 10 ml/kg of body weight 
    4. O2 sat greater than 90% for an inspired oxygen fraction of 40%. 
     
    When at least three criteria were present, the ability of the patient to sustain spontaneous breathing was evaluated during disconnection from the ventilator with supply of supplemental oxygen and humidification of gas on a T piece for a maximum of 2 h. Note that in this trial all patients are subjected to 2 hour T-piece trial after an intial 2 min T-piece trial. If they pass this 2 hour trial, they are extubated and if they fail this 2 hour trial, they are randomized to one of the 3 groups. This is unlike the esteban trial where patients were randomized after an intial 3 min T-piece trial. 
     
    When clinical signs of poor tolerance developed at any time before or at the end of the 2-h trial, the patient was put back on the ventilator and considered for randomization into the study. Randomization and initiation of the weaning trial were performed within 24 h after failure of the T-piece trial. In all modes of assisted ventilation , a positive end-expiratory pressure of up to 4 cm H20 could be applied in every patient. 
     
    Poor clinical tolerance was diagnosed when breathing frequency was above 35 per minute or increased by 50% or more, when heart rate or systolic blood pressure rose by 20% or more, or when agitation, depressed mental status, or diaphoresis was present.  
     
    Methods: 
    1. T-piece trial.  
    The T-piece method is based on the principle of gradually lengthening the periods of disconnection from the ventilator.  
    Patients assigned to the T-piece groups had undergone several attempts at disconnection from the ventilator during the day. Between each period, at least 1 h of mechanical ventilation was reinstituted in the assist-control mode. Durations of these periods could be 5, 15, 30, 60, or 120 min in progressive steps.  
    Because none of the patients had sustained a period longer than 120 min, the duration of the first attempt was selected as equal or immediately shorter than the duration of the initial tolerance trial. When a preset duration was sustained without evidence of poor tolerance, a longer duration was tried on the next time. Incremental durations of the T-piece periods were recommended two times per day. When the duration of disconnection had reached 2 h, tracheal extubation was decided. 
    1. SIMV: 
    The initial ventilator rate was set at half the total frequency used previously during controlled mechanical ventilation or assist-control ventilation, keeping tidal volume and inspiratory flow constant.  
     
    Two times per day, a decrease in the ventilator rate by two to four breaths per minute was recommended when the patient breathed at the preceding rate with no sign of poor tolerance. When one or more signs of poor tolerance was present, the ventilator rate was increased back to the preceding level.  
     
    When the patient was able to tolerate a mechanical rate of four per minute or less over 1 d, tracheal extubation was decided.  
    1. PSV: 
    The initial level of pressure support was adjusted until the breathing frequency of the patient ranged between 20-30 breaths per minute. Average intial pressure support level was 18 ±4. Two times per day, in a systematic fashion, a decrease in the level of pressure by 2 or 4 cm H20 was recommended. When one or more signs of poor tolerance appeared, the pressure support level was increased back to the preceding level.  
     
    When the patient was able to tolerate spontaneous breathing with 8 cm H20 of pressure support or less during a 24-h period, tracheal extubation was performed. (In esteban trial, they extubated at PS of 5).  
     
    Results: 
    The weaning duration was significantly shorter with pressure support than with the two other modalities pooled together. Total length of stay in the intensive care unit was significantly shorter with PSV than with the two other modalities.  
     
     
     
    PEARLS: 
    • Increased airway resistance in ARDS with out a history of reactive airway disease indicates bronchial wall edema. They need diuretics but not bronchodilators.
    • Tracheomalacia : Tracheal support cartilage is soft such that the trachea partly collapses especially during increased airflow. Thus , it leads to increased airway resistance leading to weaning failure. It can be diagnosed by bronchoscopy. Tracheomalacia may be treated with nocturnal) non-invasive ventilation or placement of endotracheal stents. 
    • Weaning trial using ScVO2 : A normal heart should be able to increase cardiac output when breathing spontaneouly. Hence, ScVO2 should remain the same or only decrease slightly before and after SBT. However, SvO2 decreases below 50% even in healthy subjects with moderate exercise.  Weaning failure without a huge decrease in ScVO2 indicates that cardiac etiology is highly unlikely
    • Extubation in patients with brain injury: Timely extubation of brain-injured patients who meet standard weaning criteria appears to be safe (no increased risk of reintubation or subsequent tracheotomy, even when GCS scores were less than 8. Am J Respir Crit Care Med. 2000 May;161(5):1530-6
    • Difficult weaning” refers to patients who fail the first weaning test and require up to three tests or 7 days to achieve successful weaning, and “prolonged weaning” refers to patients who require more than 7 days of weaning after the first test.
    • Average reintubation rate in medical ICU is 17%.
    • In patients with upper airway adema and absent cuff leak, steroid administration several hours before extubation reduce laryngeal oedema and reduce the incidence of extubation failure. Am J Respir Crit Care Med. 2013 Jun 15;187(12):1294-302Intensive Care Med. 2009 Jun;35(6):977-86
    • Ventilator asynchrony can cause prolonged mechanical ventilation resulting in failure to wean. Compared to PS, both NAVA and PAV+ increased the probability of remaining on spontaneous breathing, while it considerably reduces the incidence of patient-ventilator asynchronies. Crit Care. 2015; 19(1): 56 , Intensive Care Med. 2011 Feb;37(2):263-71Intensive Care Med. 2008 Nov;34(11):2026-34Crit Care Med. 2007 Apr;35(4):1048-54
    • Disuse atrophy of the diaphragm has been shown to occur very rapidly following diaphragmatic inactivity due to controlled mechanical ventilation even after only18 h . N Engl J Med 2008; 358:1327-1335J Appl Physiol (1985). 2002 Jun;92(6):2585-95. Hence, as soon as controlled mechanical ventilation is not necessary, try to switch to assisted modes like pressure support, ASV or PAV. 
    • Prolonged weaning: Short daily cuff-down trials with a speaking valve are performed to induce vocal cords to exert their original function during expiration. The tracheostomy cannula has to be progressively downsized and the tube kept uncuffed to increase the airway diameter. 
    • Patients with unplanned extubations ( incidence of 2%) doesn't have any worse mortality than controls. Crit Care. 2011; 15(1): R19
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