DISCONTINUATION OF MECHANICAL VENTILATION AND TRACHEAL EXTUBATION

Mechanical ventilation may be instituted for a variety of reasons, including impaired respiratory drive, increased mechanical load, impaired respiratory motor function, and impaired pulmonary gas exchange (see Chapter 75 ). Endotracheal intubation is usually performed to facilitate mechanical ventilation but may also be instituted for maintenance of a patent upper airway, to enable suctioning of secretions, for bronchoscopy, and to provide airway protection. Development of criteria for the successful liberation of a patient from mechanical ventilation and extubation may be highly specific to the clinical situation.

The most common reason for mechanical ventilation during and after anesthesia is impaired respiratory drive. However, components of increased mechanical load and impaired neuromuscular function may contribute to an abnormally low ventilatory function. Patients with severe chronic obstructive pulmonary disease usually are mechanically ventilated because of transient increases in mechanical load superimposed on a chronic increase in airways resistance. However, impaired respiratory drive

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Discontinuation of ventilatory support (i.e., weaning) from a patient whose only reason for mechanical ventilation is residual anesthesia is generally straightforward. A different approach is often required for a patient who has had prolonged ventilatory support for chronic respiratory disease.^[262] Screening criteria are listed in Table 36-5 . Commonly used discontinuation criteria include hemodynamic stability, a vital capacity not less than 10 mL/kg, maximum inspiratory pressure less than (i.e., more negative) -25 cm H₂ O, respiratory rate not above 20 breaths/min, and V̇E not more than 10 to 20 L/min in the face of normal PaCO₂ , and adequate arterial oxygenation (e.g., PaO₂ /FIO₂ ratio > 150 to 200) maintained with an inspired O₂ concentration not above 40% to 50%, with less than 5 to 8 cm H₂ O of PEEP. An extremely useful, easily measured variable is the ratio of ventilatory frequency to tidal volume (f/VT) during a 1- to 2-minute

TABLE 36-5 -- Screening criteria used in weaning trials to determine whether patients receiving high levels of ventilatory support can be considered for discontinuation

Criteria	Description
Objective	Adequate oxygenation (e.g., PO2 ≥ 60 mm Hg on FIO2 ≤ 4; PEEP ≤ 5–10 cm H2 O; PaO2 /FIO2 ≥ 150–300 mm Hg)
	Stable cardiovascular system (e.g., HR ≤ 140 beats/min, stable BP; no [or minimal] pressors)
	Afebrile (temperature < 38°C)
	No significant respiratory acidosis
	Adequate hemoglobin (e.g., Hb ≥ 8–10 g/dL)
	Adequate mentation (e.g., arousable, GCS ≥ 13, no continuous sedative infusions)
	Stable metabolic status (e.g., acceptable electrolytes)
Subjective clinical assessments	Resolution of disease acute phase; physician believes discontinuation possible; adequate cough
BP, blood pressure; GCS, Glasgow Coma Scale; Hb, hemoglobin; HR, heart rate; PaO2 /FIO2 (P/F ratio), arterial oxygen tension/fractional inspired oxygen concentration; PEEP, positive end-expiratory pressure; PO2 , partial pressure of oxygen.
From MacIntyre NR, Cook DJ, Ely EW Jr, et al: Evidence-based guidelines for weaning and discontinuing ventilatory support: A collective task force facilitated by the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine. Chest 120:375S, 2001.

trial of spontaneous ventilation.^{[263] [264]} An f/VT ratio greater than 100 predicts unsuccessful weaning. A partial list of published weaning criteria is shown in Table 36-6 .

Criteria used for patients being weaned from mechanical ventilation in the postoperative period are not applicable to patients who have been ventilator dependent for several days or weeks, in whom respiratory muscle atrophy, malnutrition, and peripheral neuropathy have often supervened. In the latter situation, any single criterion for discontinuation of ventilation has been shown to have poor predictive value.^[262]

Extensive reviews and comments on weaning criteria and techniques have been published.^{[262] [263] [265] [266] [267] [268]} A task force of the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine has published a set of evidence-based guidelines for discontinuation of mechanical ventilation.^[266] The first seven of these recommendations pertain to monitoring and weaning individual patients:

Recommendation 1: In patients requiring mechanical ventilation for more than 24 hours, a search for all the causes that may be contributing to ventilator dependence should be undertaken. This is particularly true for the patient who has failed attempts at withdrawing the mechanical ventilator. Reversing all possible ventilatory and nonventilatory issues should be an integral part of the ventilator-discontinuation process.

Recommendation 2: patients receiving mechanical ventilation for respiratory failure should undergo a formal assessment of discontinuation potential if the following criteria are satisfied:

1. Evidence for some reversal of the underlying cause for respiratory failure
2. Adequate oxygenation (e.g., PaO₂ /FIO₂ ratio > 150 to 200, requiring positive end-expiratory pressure [PEEP] = 5 to 8 cm H₂ O and FIO₂ = 0.4 to 0.5) and pH (e.g., pH = 7.25)
3. Hemodynamic stability, as defined by the absence of active myocardial ischemia and the absence of clinically significant hypotension (i.e., a condition requiring no vasopressor therapy or therapy with only low-dose vasopressors such as dopamine or dobutamine [<5 µg · kg^-1 · min])
4. The capability to initiate an inspiratory effort

The decision to use these criteria must be individualized. Some patients not satisfying all of the above criteria (e.g., patients with chronic hypoxemia values below the thresholds cited) may be ready for attempts at the discontinuation of mechanical ventilation.

Recommendation 3: Formal discontinuation assessments for patients receiving mechanical ventilation for respiratory failure should be performed during spontaneous breathing rather than while the patient is still receiving substantial ventilatory support. An initial brief period of spontaneous breathing can be used to assess the capability of continuing onto a formal spontaneous breathing trial (SBT). The criteria with which to assess patient tolerance during SBTs are the respiratory pattern, the adequacy of gas exchange, hemodynamic stability, and subjective comfort. The tolerance of SBTs

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lasting 30 to 120 minutes should prompt consideration for permanent ventilator discontinuation.

Recommendation 4: The removal of the artificial airway from a patient who has successfully been discontinued from ventilatory support should be based on assessments of airway patency and the ability of the patient to protect the airway.

Recommendation 5: Patients receiving mechanical ventilation for respiratory failure who fail an SBT should have the cause for the failed SBT determined. After reversible causes for failure are corrected and if the patient still meets the criteria listed in Table 36-5 , subsequent SBTs should be performed every 24 hours.

Recommendation 6: Patients receiving mechanical ventilation for respiratory failure who fail an SBT should receive a stable, nonfatiguing, and comfortable form of ventilatory support.

Recommendation 7: Anesthesia or sedation strategies and ventilator management aimed at early extubation should be used in postsurgical patients.

TABLE 36-6 -- Possible criteria for ventilator discontinuation

Test	Criterion for Weaning
Mechanical Function
Forced vital capacity (FVC)	>10–15 mL/kg
Forced expiratory volume in 1 second (FEV1 )	>10 mL/kg
Tidal volume during spontaneous breathing (VT) *	≥325–408 mL (4–6 mL/kg)
Maximum inspiratory pressure (PImax) *	≤ -15 – -30 cm H2 O
Negative inspiratory force (NIF) *	≤ -20 – -30 cm H2 O
Maximum voluntary ventilation (MVV)	>2 × resting V̇E
Respiratory rate during spontaneous breathing (VF) *	<30–38/min
Respiratory minute volume (V̇E)	<10–15 L/min
VF/VT (i.e., f/VT or rapid shallow breathing index [RSBI]) during spontaneous breathing *	≤60–105 breaths/min per 1 L
VF/(VT/kg) during spontaneous breathing †	<11 breaths/min per 1 mL/kg
Thoracic compliance
Static	≥33 mL/cm H2 O
Dynamic	≥22 mL/cm H2 O
Work of breathing
Per minute	≤1.6 kg · m/min
Per L/min V̇E	≤0.14 kg · m/min
(0.75 VT/Cdyn ) · (TI/Ttot )/(MIP) ‡	<0.15
Functional residual capacity (FRC)	>50% of predicted
Gas Exchange Function
A-a gradient (FIO2 = 1.0)	<350 mm Hg
PaO2 /PAO2 ratio	≥0.35
Shunt fraction (S/T)	<0.2
Dead space/tidal volume ratio (VD/VT)	<0.6
Minute ventilation (V̇E) with normal PaCO2	<180 mL/kg/min
Respiratory quotient (R)	<0.9
PaO2 /FIO2	>200
Respiratory Drive
Pressure obtained 100 msec after temporary airway occlusion (P100 )	<6 cm H2 O
P100 /PImax *	<0.3
Other Tests
Urine output	500 mL/6 hr
Arterial pH	>7.30
Gastric intramural pHi during pressure support trial	>7.30 or a change of <0.09
[Cdyn · MIP · (PaO2 /PAO2 )]/VF §	≥13 mL per 1 breath/min
[(Cdyn /kg) · MIP · (PaO2 /PAO2 )]/VF †	≥0.1 mL/kg per 1 breath/min
Cdyn , dynamic thoracic compliance; PImax, maximum inspiratory pressure measured in an occluded airway after 20 seconds starting from residual volume; MIP, maximum inspiratory pressure; NIF, negative pressure measured after at least 1 second of inspiratory effort against an occluded airway, with the most negative value of three attempts recorded; TI, inspiratory time; Ttot , total respiratory cycle length.
Data from references [18] [200] [202] [262] [263] and [306] [307] [308] [309] [310] [311] [312] [313] [314] [315] [316] [317] .