A 62-year-old man is evaluated in the intensive care unit for worsening hypoxemia. He unde… — USMLE Step 2 CK

A $62$ -year-old man is evaluated in the intensive care unit for worsening hypoxemia. He underwent an exploratory laparotomy for a perforated appendix $48$ hours ago. He was started on targeted antimicrobial therapy with piperacillin-tazobactam after peritoneal fluid cultures grew pan-sensitive *Escherichia coli* and *Bacteroides fragilis*. The patient is sedated and mechanically ventilated in assist-control volume mode. Current settings include a tidal volume of $400$ mL ( $6$ mL/kg of predicted body weight), respiratory rate of $28$ /min, positive end-expiratory pressure (PEEP) of $14$ cm H $_2$ O, and fraction of inspired oxygen (FiO $_2$ ) of $0.80$ .

Vital signs show a temperature of $38.2^\circ$ C ( $100.8^\circ$ F), pulse of $110$ /min, and blood pressure of $105/60$ mmHg while receiving a continuous norepinephrine infusion at $0.1$ $\mu$ g/kg/min. Arterial blood gas analysis reveals:
pH: $7.18$
PaCO $_2$ : $55$ mmHg
PaO $_2$ : $58$ mmHg
HCO $_3^-$ : $20$ mEq/L

Ventilator mechanics demonstrate a peak airway pressure of $40$ cm H $_2$ O and a plateau airway pressure of $36$ cm H $_2$ O. A bedside chest radiograph shows diffuse bilateral alveolar opacities.

Which of the following are the most appropriate next steps in the management of this patient? (Select exactly 2 options)

Decrease the tidal volume to $4$ mL/kg of predicted body weightAnswer
Initiate prone positioningAnswer
C
Increase the respiratory rate to $38$ /min to achieve appropriate respiratory compensation
D
Administer intravenous furosemide to reduce the alveolar fluid burden
E
Broaden the antimicrobial regimen to include an echinocandin

Answer

The most appropriate steps are to decrease the tidal volume to

4

mL/kg of predicted body weight and to initiate prone positioning.

This patient has severe acute respiratory distress syndrome (ARDS), defined by a PaO

_2

/FiO

_2

ratio of less than

100

mmHg (here,

58 / 0.80 = 72.5

mmHg) and bilateral alveolar opacities on imaging. Evidence-based management of ARDS centers on lung-protective ventilation to minimize ventilator-induced lung injury. The primary goal is to maintain a plateau airway pressure of less than

30

cm H

_2

O. Because this patient's plateau pressure is elevated at

36

cm H

_2

O, the tidal volume must be decreased from

6

mL/kg to

4

mL/kg of predicted body weight. While this intervention will likely exacerbate his hypercapnia and respiratory acidosis, this 'permissive hypercapnia' is a widely accepted trade-off to minimize alveolar overdistension. Furthermore, for patients with severe ARDS (PaO

_2

/FiO

_2

< 150

mmHg), prone positioning is strongly indicated. Prone ventilation reduces the ventral-dorsal transpulmonary pressure gradient, recruits atelectatic dorsal lung regions, improves ventilation-perfusion (V/Q) matching, and has demonstrated a significant mortality benefit.

Step-by-Step Solution

Calculate the PaO2/FiO2 ratio to determine the severity of respiratory failure.

The PaO2 is

58

mmHg and FiO2 is

0.80

. The ratio is

58 / 0.80 = 72.5

mmHg.

A ratio

< 100

mmHg with bilateral opacities indicates severe acute respiratory distress syndrome (ARDS).

Evaluate the ventilator mechanics against lung-protective goals.

The plateau pressure is

36

cm H2O, which exceeds the safe threshold of

< 30

cm H2O.

Elevated plateau pressures cause barotrauma and volutrauma (ventilator-induced lung injury).

Determine the necessary ventilator adjustments.

Tidal volume must be decreased from

6

mL/kg to

4-5

mL/kg of predicted body weight.

Lowering the tidal volume reduces the plateau pressure. The resulting respiratory acidosis (permissive hypercapnia) is tolerated to protect the lungs.

Identify adjunctive therapies for severe ARDS.

Prone positioning is indicated.

Prone positioning improves V/Q matching, recruits dorsal atelectatic lung units, and confers a mortality benefit in patients with a PaO2/FiO2 ratio

< 150

mmHg.

Key Concept

Management of severe ARDS requires strict adherence to lung-protective ventilation (targeting plateau pressure

< 30

cm H2O by reducing tidal volume) and early prone positioning for refractory hypoxemia.

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