A 42-year-old woman is receiving continuous mechanical ventilation in the critical care un… — USMLE Step 2 CK

A $42$ -year-old woman is receiving continuous mechanical ventilation in the critical care unit due to severe acute respiratory distress syndrome (ARDS) secondary to viral pneumonitis. Over the past $12$ hours, her oxygen saturation has declined to $84\%$ despite aggressive optimization of sedation and initiation of a continuous cisatracurium infusion.

Currently, she is receiving norepinephrine at $0.15$ $\mu\text{g/kg/min}$ . Vital signs show a blood pressure of $105/65$ mmHg and a heart rate of $98$ /min.

Ventilator parameters (Volume Assist-Control mode) are set as follows:
- Tidal volume: $380$ mL ( $6$ mL/kg predicted body weight)
- Respiratory rate: $28$ /min
- $\text{FiO}_2$ : $0.95$
- PEEP: $16$ cm $\text{H}_2\text{O}$

The ventilator display indicates a peak airway pressure of $48$ cm $\text{H}_2\text{O}$ and a plateau airway pressure of $38$ cm $\text{H}_2\text{O}$ .

An arterial blood gas demonstrates:
- pH: $7.21$
- $\text{pCO}_2$ : $58$ mmHg
- $\text{pO}_2$ : $52$ mmHg
- $\text{HCO}_3^-$ : $23$ mEq/L

Which of the following interventions should be implemented immediately? (Select all that apply)

Decrease tidal volume to $4$ mL/kg predicted body weightAnswer
Place the patient in the prone positionAnswer
C
Administer intravenous sodium bicarbonate
D
Administer intravenous furosemide
E
Initiate empiric intravenous broad-spectrum antibiotics

Answer

Decrease tidal volume to 4 mL/kg predicted body weight AND Place the patient in the prone position

The most appropriate immediate steps are decreasing the tidal volume and placing the patient in the prone position. This patient is experiencing refractory hypoxemia and dangerously elevated airway pressures due to severe ARDS. Lung-protective ventilation strategies dictate that plateau pressure must be maintained at ≤

30

\text{H}_2\text{O}

to prevent ventilator-induced lung injury (barotrauma). Because her plateau pressure is

38

\text{H}_2\text{O}

, the tidal volume must be lowered to

4

5

mL/kg predicted body weight. This adjustment takes precedence even if it exacerbates respiratory acidosis (permissive hypercapnia). Additionally, her

\text{PaO}_2/\text{FiO}_2

ratio is

54.7

mmHg. For patients with severe ARDS (

\text{PaO}_2/\text{FiO}_2 < 150

mmHg) who remain hypoxemic despite optimized PEEP and neuromuscular blockade, prone positioning is strongly indicated as it recruits dependent posterior alveoli, improves ventilation-perfusion (V/Q) matching, and has a proven mortality benefit.

Step-by-Step Solution

Calculate the PaO2/FiO2 ratio to stratify ARDS severity.

PaO2/FiO2 = 52 / 0.95 = 54.7 mmHg (Severe ARDS).

This calculation determines eligibility for advanced interventions like prone positioning.

Evaluate the ventilator mechanics, specifically the plateau pressure.

The plateau pressure is elevated at 38 cm H2O.

Lung-protective ventilation protocols mandate maintaining a plateau pressure ≤ 30 cm H2O to prevent alveolar overdistension and barotrauma.

Determine the necessary ventilator adjustments.

Decrease tidal volume to 4 mL/kg PBW.

When plateau pressure exceeds 30 cm H2O on a 6 mL/kg tidal volume, the tidal volume must be decreased stepwise (down to 4 mL/kg) until the plateau pressure normalizes.

Determine the intervention for refractory hypoxemia.

Initiate prone positioning.

Prone positioning is strongly indicated for severe ARDS (PaO2/FiO2 < 150 mmHg) to recruit dependent lung regions and improve mortality.

Key Concept

Management of refractory hypoxemia and high plateau pressures in severe Acute Respiratory Distress Syndrome (ARDS)

Estimated Time:2m 0s

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