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Difficulty: Very hardPulmonary and Critical Care

A 2929-year-old man is intubated and placed on mechanical ventilation in the intensive care unit for refractory status asthmaticus. His current ventilator settings on volume assist-control mode are: tidal volume 88 mL/kg of ideal body weight, respiratory rate 1818/min, positive end-expiratory pressure (PEEP) 55 cm H 2_2 O, and fraction of inspired oxygen (FiO 2_2) 1.01.0. Ten minutes after intubation, the ventilator's high-pressure alarm sounds. His blood pressure is 76/4276/42 mm Hg and heart rate is 142142/min. The peak inspiratory pressure is 7272 cm H 2_2 O, and the plateau pressure (measured via a 0.50.5-second end-inspiratory hold) is 2424 cm H 2_2 O. The ventilator's flow-time waveform indicates that expiratory flow does not return to zero before the initiation of the next breath. Which of the following are the most appropriate immediate interventions? (Select all that apply)

  1. Briefly disconnect the patient from the mechanical ventilatorAnswer
  2. Decrease the set respiratory rateAnswer
  3. C
    Increase the set respiratory rate
  4. D
    Administer intravenous furosemide
  5. E
    Initiate empiric intravenous broad-spectrum antibiotics

Answer

The most appropriate interventions are to briefly disconnect the patient from the mechanical ventilator and to decrease the set respiratory rate.
This patient is experiencing life-threatening obstructive shock due to dynamic hyperinflation (auto-PEEP). In severe asthma, profound bronchospasm significantly prolongs the time needed to fully exhale a delivered tidal volume. When the ventilator delivers the next breath before exhalation is complete (indicated by the flow-time waveform not returning to zero), air becomes trapped in the lungs. Over successive breaths, this trapped air drastically increases intrathoracic pressure, which decreases venous return to the right heart, resulting in severe hypotension. The primary diagnostic clue for airway resistance (rather than an alveolar compliance issue like ARDS) is the markedly elevated peak inspiratory pressure (PIP) with a normal plateau pressure (Pplat < 3030 cm H 2_2 O).

The immediate life-saving maneuver is to briefly disconnect the patient from the ventilator circuit. This removes the circuit's external resistance and allows the patient to passively exhale the trapped air, instantly lowering intrathoracic pressure and restoring venous return. After stabilizing the patient's hemodynamics, the ventilator settings must be adjusted to maximize expiratory time (TE). This is most effectively achieved by decreasing the respiratory rate, which allows more time for the patient to empty their lungs before the next breath is delivered.

Step-by-Step Solution

1
Analyze the ventilator mechanics.
Peak inspiratory pressure (PIP) is extremely high (7272 cm H 2_2 O), while plateau pressure (Pplat) remains normal (2424 cm H 2_2 O).
The large difference between PIP and Pplat (>5>5 cm H 2_2 O) isolates the problem to increased airway resistance (severe bronchospasm in asthma), rather than decreased lung compliance.
2
Interpret the flow-time waveform and hemodynamic changes.
Expiratory flow does not reach zero before the next breath, confirming dynamic hyperinflation (auto-PEEP). The patient is severely hypotensive (76/4276/42 mm Hg).
Air trapping leads to progressively increasing intrathoracic pressure, which compresses the vena cava and decreases venous return, causing obstructive shock.
3
Determine the necessary immediate interventions.
Disconnect the patient from the ventilator to allow exhalation, and decrease the set respiratory rate to prevent recurrence.
Disconnecting provides an immediate physical release of the trapped intrathoracic air, restoring cardiac output. Lowering the respiratory rate extends expiratory time, addressing the root cause of the mechanical air trapping.

Key Concept

In mechanically ventilated patients with severe asthma, auto-PEEP can cause life-threatening obstructive shock. Management requires immediate disconnection from the ventilator to allow complete exhalation, followed by adjusting ventilator settings to maximize expiratory time (such as decreasing the respiratory rate).
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Topics

Pulmonary and Critical Care

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