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Difficulty: HardPulmonary and Critical Care

A 61-year-old male with a history of moderate chronic obstructive pulmonary disease is evaluated in the intensive care unit for worsening hypoxemia. He was admitted two days ago with severe community-acquired pneumonia and was intubated yesterday due to progressive respiratory failure. Sputum cultures from admission grew *Streptococcus pneumoniae* sensitive to penicillin, and he is currently receiving intravenous aqueous penicillin G.

His temperature is 38.3°C (100.9°F), blood pressure is 112/68 mmHg, and heart rate is 108/min. Physical examination reveals diffuse bilateral crackles. A chest radiograph demonstrates new, diffuse bilateral alveolar opacities.

He is receiving mechanical ventilation in volume assist-control mode with the following settings:
- Tidal volume: 550 mL (approximately 8 mL/kg of predicted body weight)
- Respiratory rate: 14/min
- FiO₂: 0.70
- Positive end-expiratory pressure (PEEP): 5 cm H₂O

Arterial blood gas analysis on these settings reveals:
- pH: 7.18
- PaCO₂: 38 mmHg
- PaO₂: 52 mmHg
- HCO₃⁻: 14 mEq/L

Ventilator mechanics show a peak inspiratory pressure of 45 cm H₂O and a plateau pressure of 34 cm H₂O.

Which of the following interventions are appropriate for this patient? (Select all that apply)

  1. Decrease the set tidal volume to approximately 6 mL/kg6 \text{ mL/kg} of predicted body weightAnswer
  2. Increase the positive end-expiratory pressure (PEEP)Answer
  3. C
    Administer intravenous furosemide to manage fluid overload
  4. D
    Decrease the respiratory rate as the patient is overcompensating for the metabolic acidosis
  5. E
    Broaden antimicrobial coverage by initiating meropenem and linezolid

Answer

The appropriate interventions are to decrease the set tidal volume to approximately 6 mL/kg of predicted body weight and to increase the positive end-expiratory pressure (PEEP).
The patient has developed Acute Respiratory Distress Syndrome (ARDS), characterized by severe hypoxemia, bilateral alveolar opacities, and a plateau pressure > 30 cm H₂O in the setting of severe pneumonia. The cornerstone of mechanical ventilation in ARDS is lung-protective ventilation, which requires lowering the tidal volume to 6 mL/kg of predicted body weight to prevent alveolar overdistension (volutrauma) and keeping the plateau pressure < 30 cm H₂O. Additionally, the patient has severe hypoxemia (PaO₂ 52 mmHg) despite a high FiO₂ (0.70). Increasing PEEP helps recruit collapsed alveoli, thereby decreasing intrapulmonary shunting and improving oxygenation.

Step-by-Step Solution

1
Diagnose the cause of respiratory worsening
The patient has developed Acute Respiratory Distress Syndrome (ARDS) secondary to pneumonia.
Indicated by new bilateral opacities, severe hypoxemia (PaO₂/FiO₂ ratio < 100), and elevated plateau pressure (> 30 cm H₂O) in the absence of clinical heart failure.
2
Evaluate ventilator mechanics and adjust settings
Identify plateau pressure of 34 cm H₂O and decrease tidal volume to 6 mL/kg.
Lung-protective ventilation strategy dictates keeping plateau pressures < 30 cm H₂O to prevent barotrauma and volutrauma.
3
Address severe hypoxemia
Increase PEEP.
The patient is severely hypoxemic on FiO₂ 0.70. Higher PEEP is needed to recruit alveoli and decrease intrapulmonary shunting.
4
Evaluate acid-base status using Winter's formula
Identify a mixed severe metabolic acidosis and concomitant respiratory acidosis.
Expected PaCO₂ = 1.5(14) + 8 ± 2 = 27-31 mmHg. Actual is 38 mmHg, confirming inadequate ventilation (respiratory acidosis) compounding the metabolic acidosis.

Key Concept

Management of ARDS involves lung-protective ventilation (low tidal volume to keep plateau pressure < 30 cm H₂O) and optimization of PEEP for refractory hypoxemia.
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Topics

Pulmonary and Critical Care

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