Mechanical Ventilation

Introduction
Patients with reduced ventilatory capacity due to respiratory distress require ventilatory support. Mechanical ventilation can help to cope with the current increased demand for ventilation due to acute pathology. By treating the underlying cause with appropriate therapy, early cessation of ventilatory support becomes critical in determining further prognosis. Prolonged mechanical ventilation may cause complications such as ventilator-related pneumonia, critical illness myoneuropathy and malnutrition, increasing the economic burden. Once the disease process has worsened or stabilized, freedom from mechanical ventilation should become a priority. However, premature withdrawal of mechanical ventilation in these patients may also have adverse effects and higher mortality. Therefore, when attempting evacuation, it should be initiated as early as possible, with attention to emergencies in which premature withdrawal may lead to difficult artificial airway replacement and hypoxic injury.

This chapter focuses on assessing the feasibility of mechanical ventilation withdrawal, comorbidities and clinical conditions that may adversely affect withdrawal, long-term freedom from mechanical ventilation, and the management of patients who cannot be removed from mechanical ventilation.
Assessment of Mechanical Ventilation Withdrawal
Ventilator support should be tapered as the patient's ventilatory efforts are restarted. Clinical criteria should be used to assess the appropriateness of the test. Test preparation can determine which patients requiring ventilatory support can be withdrawn and help prevent complications and even death associated with mechanical ventilation. In addition, premature withdrawal may also lead to cardiovascular dysfunction and respiratory muscle weakness in patients already undergoing withdrawal, which can be avoided by assessing readiness for withdrawal.
All patients receiving mechanical ventilation should undergo withdrawal readiness testing. Studies have shown that patients receiving mechanical ventilation for >24 hours should undergo a daily withdrawal readiness test. In addition, physicians often underestimate the patient's ability to withdraw from mechanical ventilation, and tailoring the withdrawal protocol to each patient has been found to be beneficial.
Intensive care unit (ICU) admissions can lead to early and rapid muscle loss, and plans to relieve neuromuscular disease after discharge are often futile. Early withdrawal should be attempted in every patient, as it promotes movement in ICU patients and further prevents critical care muscular neuropathy and other complications, such as pressure sores.
In the era of evidence-based medicine, readiness for withdrawal should be assessed using clinical guidelines based on evidence-based treatment; however, a subset of patients still undergo successful withdrawal even if they do not meet these criteria.
The following are clinical criteria that warrant consideration for an autonomic breathing test (SBT):
▪ Improvement in the cause of respiratory failure.
■ Inhaled oxygen concentration (FiO2) ≤ 40% and positive end-expiratory pressure (PEEP) ≤ 5 cm H2O, maintaining arterial partial pressure of oxygen (PaO2)/FiO2 ≥ 150 or peripheral oxygen saturation (SpO2) ≥ 90%
▪ pH > 7.25
▪ Presence of respiratory effort
▪ Absence or need for low dose of antihypertensive agents to maintain hemodynamic stability
A state of hemoglobin ≥ 7 mg/dL, absence of fever, alertness and ease of awakening are considered optional criteria for readiness testing.
Patients who meet these criteria may still fail. This demonstrates the imperfect nature of withdrawal readiness testing and the uncertainty that exists in some patients as to whether readiness criteria are successful. For these patients, withdrawal predictors can be used to distinguish between patients who can withdraw and those who cannot. One of the most commonly used predictors of withdrawal is the rapid superficial breathing index (RSBI), which is the ratio of respiratory rate to tidal volume (f/VT).VT can be calculated from the ventilator during which pressure support and positive end-expiratory pressure are set to 0 cm H2O.The RSBI may underestimate frequency due to respiratory rate not triggered by insufficient patient effort, so respiratory rate should always be Manual counting. RSBI >105 breaths/min/L may predict failure to withdraw, whereas patients with RSBI <105 breaths/min/L have a better chance of successful withdrawal. RSBI ≥105 breaths/min/L leads to failure to withdraw, whereas RSBI <105 breaths/min/L is associated with successful withdrawal with a sensitivity and specificity of 97% and 64%, respectively. A negative RSBI (>105 breaths/min/L) has been shown to be more valuable than a positive RSBI (<105 breaths/min/L). However, the use of RSBI has not been found to reduce the time to disengagement or mechanical ventilation. Other predictors used for deconditioning included PaO2/FiO2, PaO2/PAO2, A-a gradient, dead space fraction, respiratory system compliance, total minute ventilation, p0.1, ventilatory work, diaphragmatic ultrasound, and gastric mucosal pH, but they were not as effective as RSBI.

Which deconditioning method do we follow?
Once ready, deconditioning can be performed using both automated and manual protocols. Manual protocols are usually done manually by a respiratory therapist or nurse to change ventilator parameters and are more laborious and error-prone in a busy ICU setting. Manual protocols have been shown to be more effective and more sensitive to patient needs.
Although automated systems use proprietary computer software to automatically adjust pressure support to maintain the patient within the normal range of respiratory rate, tidal volume and exhaled CO2, these systems cannot be used for certain conditions, such as type II respiratory failure with CO2 >60 mm Hg, which is the upper limit in this procedure.

What are the different off-ramp protocols?
Autonomic breathing test
An autonomic breathing trial should be performed once a day, during which the patient can breathe spontaneously through a tracheal tube and the ventilator can apply airway pressure support. It can usually be performed for 30 minutes to 2 hours, where 30 minutes of SBT is usually sufficient for patients receiving a ventilator for less than 10 days, but 30 minutes is also sufficient for patients receiving a ventilator for longer periods of time; however, studies have shown that SBT failure after 120 minutes has also occurred in patients with COPD. It has been observed that 50-75% of patients will pass SBT and be successfully ventilated from mechanical ventilation
The American College of Chest Physicians recommends the use of pressure support ventilation (PSV) during SBT, usually maintained at 5-8 cm H2O and a baseline PEEP of 5 cm H2O and FiO2 ≤ 0.4. Other modes of airway support, such as automated tube compensation and continuous positive airway pressure (CPAP), are usually preferred over SBT without airway support like a T-tube. PSV-SBT can overcome the increased respiratory load caused by resistance to airway intubation, and tube size has no effect on outcome. However, in patients with pulmonary edema and COPD, PSV-SBT may produce spuriously reassuring results because CPAP will help address the underlying pathology. In these cases, T-tubes may provide more realistic results. Patients who fail initial SBT may receive extended-duration off-tubes.
Successful or unsuccessful deconditioning
Vital signs and vital parameters should be monitored during the deconditioning trial. The decision to extubate needs to be made whether the extubation trial is performed using automated or manual mode. In addition to many parameters such as respiratory distress, altered mental status, dyspnea, chest pain, and ECG changes, clinical judgment should be used to draw conclusions about a failed or successful deconditioning trial.

Various parameters that may predict failure of deconditioning include
▪ Respiratory rate > 35 breaths/min
▪ Use of respiratory assist muscles
Heart rate > 140 beats/min or < 50 beats/min
Systolic blood pressure > 180 mm Hg or <90 mm Hg
■ SpO2 <90% or <88% for patients with chronic hypoxia
▪ PaO2 <50 mm Hg
▪ Increase in partial pressure of carbon dioxide (PaCO2) > 10 mmHg or decrease in pH > 0.1 compared to baseline
▪ Altered neurological status, such as drowsiness, agitation, irritability, and delirium
▪ Other, such as sweating.
After successful completion of the deconditioning trial, the patient's clinical condition and safety of extubation should be assessed. This includes assessment of airway secretion volume, cough reflex, and mental status.
Difficult withdrawal and prolonged withdrawal
A patient is considered difficult to extubate if he or she fails the first SBT and requires up to three SBTs or 7 days to pass the SBT. A patient is considered offline extended if he or she fails three SBTs or takes >7 days to pass the SBT.
If a patient fails an SBT, the clinician should attempt to determine the cause of the failure.
Increased respiratory causes of dyspnea

▪ Increased ventilator requirements

▪ Hypoxemia
▪ Elevated dead space
■ Excess carbon dioxide
■ Metabolic acidosis
■ Neuropsychiatric causes
■ Increased respiratory pathway resistance
■ Airway disease, e.g. COPD, asthma
■ Secretions
■ Ventilator lines
■ Increased elastic load

■ COPD, asthma
■ Pulmonary edema
■ Infections
■ Obesity and chest wall disease
■ Reduced neuromuscular function

■ Electrolyte abnormalities
■ Medications
■ Hypothyroidism
■ sepsis
■ Critical illness polyneuropathy and myopathy
■ Reduced respiratory drive

■ Excessive sedation
■ Metabolic alkalosis
■ Central nervous system disorders
■ Central sleep apnea
■ Obesity hypoventilation syndrome
▪ Other respiratory causes

■ Hyperventilation
▪ Automated PEEP
Cardiac .
■ Myocardial ischemia
▪ Pulmonary edema/fluid overload
Psychological factors.
■ Anxiety
■ Depression
■ Delirium
■ Hypersedation
Ventilator .
■ Dead space
■ Ventilation line compliance
■ Increased resistance to ventilation and tracheal intubation
■ Expiratory valve dysfunction
Nutrition .
▪ Decreased ingestion
▪ Increased feeding
Once the etiology has been determined by all available investigations, every effort should be made to correct it. If the etiology is not corrected, the patient is likely to fail multiple weaning trials.

In some patients, even after correcting the underlying cause of respiratory distress and optimizing the factors contributing to the failure of deconditioning, they remain difficult to disengage. For these patients a tracheotomy can be planned or they can be transferred to a long-term acute care facility (LTAC). These facilities focus on deconditioning and pulmonary rehabilitation programs.

Extubation and post-extubation management
Once the medical situation is stable and the deconvolution trial is successful, extubation can be planned. Extubation is the final step in removing mechanical ventilation. It should be performed during the day, although in exceptional cases the patient may be extubated at night.
Once the clinical situation that led to the patient's tracheal intubation has improved and the patient meets the criteria for decannulation and has successfully passed the decannulation trial, extubation can be planned. Prior to extubation, cough strength and level of consciousness should be assessed. This helps determine the patient's ability to protect the airway when extubated. If the patient has a large or thick secretion, the likelihood of aspiration after extubation is high. If the patient has insufficient coughing power or a Glasgow Coma Score (GCS) <8, extubation must be delayed. The ability of the patient receiving ventilation to clear secretions should also be analyzed. This is usually estimated based on the frequency of aspiration. Ideally, extubation should not be performed if the patient requires aspiration more than every 2-3 hours. Formal and informal assessments of coughing power should also be performed. The formal assessment consists of measuring the peak expiratory flow rate (PEF) during coughing using a peak expiratory flow meter. If PEF ≤60 L/min, extubation failure is likely to occur. Placing the card 1-2 cm from the tracheal tube to check if the patient's cough moistens the card is another way to assess cough force.
The patient's mental status needs to be assessed prior to extubation. Ideally, patients who are awake with their eyes open, able to follow instructions, and not taking any sedatives are most appropriate. However, in exceptional cases, patients who are mildly sedated may be extubated and monitored closely to prevent aspiration.

Post-extraction wheezing is another important complication that can lead to reintubation. This occurs in less than 10% of patients and can lead to increased reintubation rates, prolonged mechanical ventilation, and increased ICU stay. This is more common in patients with prolonged intubation, age >80 years, larger catheters, during traumatic intubation, GCS <8, female patients, history of obstructive airway disease, inadequate sedation, poor tube fixation, tube lumen to laryngeal diameter ratio >45%, elevated APACHE II scores, and patients with small height to tube diameter ratios.

A balloon leak test may be performed to assess the risk of wheezing after extubation. When the balloon is deflated, the absence of a leak indicates a narrowing of the space between the ETT and the larynx. This may be due to secretions, strictures, secretions and laryngeal edema, and the absence of balloon leakage indicates a high risk of wheezing after extubation. The sensitivity and specificity ranged from 56% to 92%, respectively. Glucocorticoids may also be useful if the patient is at high risk for postextraction wheezing.

Oral and endotracheal suction catheters, as well as tubing and oxygen delivery devices, should be prepared at extubation, and the patient should be monitored continuously. The patient should be placed in an upright position and the procedure should be explained to him/her. Remove all straps holding the ETT in place and ask the patient to take a deep breath or cough. As the patient exhales or coughs, the balloon is deflated and the ETT is removed.

After extubation, 15-20% of patients are at high risk of reintubation due to post-extubation respiratory failure. Initial reintubation occurs between minutes and 12 hours after extubation, and delayed reintubation occurs more than 12 hours after extubation. It is characterized by decreased oxygen saturation, increased respiratory rate and sometimes wheezing.

All post-extubation patients should be treated with oxygen. The decision to place a patient on high-flow equipment should be based on the needs of each patient. A high-flow nasal cannula (HFNC) has the added advantage of providing PEEP and better compliance compared to a high-flow mask.

The role of noninvasive ventilation in evacuation
In a meta-analysis by Lin et al. of 10 randomized controlled trials (RCTs) consisting of 1382 patients with respiratory failure after extubation, noninvasive ventilation (NIV) failed to reduce ICU mortality and reintubation rates compared with standard medical therapy. Applying NIV before the onset of respiratory failure reduced hospital and ICU mortality. Patients with neuromuscular disease, high oxygen demand, previous failed extubation, and hypercapnic COPD may be extubated with NIV. Heart failure patients with a history of pulmonary edema also benefit from NIV. in a Cochrane review of 16 trials, noninvasive extubation reduced mortality and pneumonia morbidity. mortality was significantly reduced in COPD patients without an increased risk of reintubation or extubation failure. If, despite HFNC or NIV, a patient develops respiratory distress after extubation, the upper airway and vocal cords should be carefully examined for edema, erosions, secretions, and masses during reintubation. Patients who have failed multiple extubations should be planned for tracheostomy. Reintubation is associated with increased mortality, prolonged ICU stay, and increased odds of hospital-acquired infection.

Conclusion
All patients intubated for more than 24 hours and in improved clinical condition will be planned for extubation. Early withdrawal should always be a priority, as it has been shown to have multiple benefits. It should be tailored to the patient's requirements on the basis of underlying etiology and comorbidities. For each patient, withdrawal criteria should be evaluated and a trial of withdrawal should be performed. Protocolized withdrawal combined with clinician judgment should identify patients who are suitable for extubation.