Anesthesiology APPLIED Exam › Thoracic and respiratory

Postoperative mechanical ventilation and weaning

T1Thoracic and respiratory·795 words · ~4 min read·v0.1DRAFT

Why examiners ask this

The post-op section of the SOE often starts with "the patient is still intubated; when will you extubate?" The examiner wants an organized readiness assessment, a structured trial, and a clear plan for failure. It stresses judgment (stay intubated or extubate) and application of knowledge about lung-protective ventilation and why weaning fails.

The 60-second answer

I keep a patient intubated when the reason for intubation persists: a compromised airway, inadequate gas exchange, hemodynamic instability, a large ongoing resuscitation, or a depressed level of consciousness. While ventilated I use lung-protective settings: tidal volume 6–8 mL/kg predicted body weight, PEEP of at least 5 cmH2O, plateau pressure below 30 and driving pressure below 15 cmH2O. I lighten sedation daily and, when the patient meets readiness criteria, run a spontaneous breathing trial of 30–120 minutes on low pressure support or a T-piece. If the patient passes and can protect the airway, I extubate, often to high-flow nasal oxygen or noninvasive ventilation in high-risk patients. If the patient fails, I find out why before trying again.

Decision points and the "why"

Readiness criteria — the cause is improving, FiO2 is 0.4–0.5 or less, PEEP is 8 cmH2O or less, PaO2/FiO2 is above 150–200, the patient is hemodynamically stable on minimal vasopressor, awake enough to follow commands, and has a cough. Each one addresses a common reason for failure.
Spontaneous breathing trial — pressure support 5–8 cmH2O with PEEP 5, or a T-piece, for 30–120 minutes; failure is tachypnea above 35, SpO2 below 90%, tachycardia, hypertension, diaphoresis or agitation.
Rapid shallow breathing index — frequency divided by tidal volume in liters; below 105 predicts success, but I treat it as one data point, because the trial itself predicts better.
Paired sedation and breathing trials — daily awakening combined with a breathing trial shortens ventilation and ICU stay; I use propofol or dexmedetomidine rather than benzodiazepines.
Airway protection — strong cough, manageable secretions and an intact gag; a patient who passes the trial but cannot clear secretions fails extubation.
Cuff leak test — after prolonged intubation, airway surgery or a traumatic intubation, a leak below about 110 mL or 10–15% of tidal volume predicts stridor; I give methylprednisolone 20 mg every 4 hours starting 12 hours before extubation.
Post-extubation support — high-flow nasal oxygen for moderate risk and noninvasive ventilation for obese or COPD patients reduce reintubation; I do not use noninvasive ventilation to rescue established failure, because it delays reintubation.

The complication they will inject

Thirty minutes into a trial, the patient's rate is 38, SpO2 88%, heart rate 125 and blood pressure 175/95. I stop the trial and return to full support. I then work through the causes: weaning-induced pulmonary edema from the rise in afterload and venous return on spontaneous breathing, myocardial ischemia, fluid overload, residual neuromuscular block, oversedation, bronchospasm, pneumonia, metabolic alkalosis, electrolyte losses such as phosphate, and fatigue. I get an ECG, troponin, echo and a chest radiograph, and I diurese if the ventricle is filling high. I repeat the trial after 24 hours of rest, because fatigued diaphragms do not recover in minutes.

Pitfalls that cost points

Extubating because the patient is "awake" without assessing cough, secretions and gas exchange.
Running the trial repeatedly the same day after a failure.
Using noninvasive ventilation to delay reintubation in a patient who is clearly failing.
Ignoring cardiac causes of weaning failure.
Ventilating with 10 mL/kg tidal volumes because the patient "came from the OR".

Rapid-why

Q: Why can weaning cause pulmonary edema? A: Spontaneous breathing lowers intrathoracic pressure, raising venous return and left ventricular afterload, which unmasks a stiff or ischemic ventricle.

Q: Why is driving pressure important? A: Plateau minus PEEP reflects strain relative to functional lung size, and it predicts mortality better than tidal volume alone.

Q: What does NIF tell you? A: A negative inspiratory force more negative than −20 to −30 cmH2O suggests adequate muscle strength, but it is effort-dependent.

Q: Why extubate an obese patient to noninvasive ventilation? A: It prevents atelectasis and upper airway collapse in the first hours, when reintubation risk is highest.

Numbers to know

Item	Value
Tidal volume	6–8 mL/kg PBW
Plateau and driving pressure	<30 and <15 cmH2O
Readiness FiO2 and PEEP	≤0.4–0.5 and ≤8 cmH2O
Breathing trial	30–120 min, PS 5–8, PEEP 5
RSBI	<105 breaths/min/L
NIF	More negative than −20 to −30 cmH2O

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