A 45-year-old man is evaluated in the intensive care unit for persistent hypoxemia. He was… — USMLE Step 2 CK

A $45$ -year-old man is evaluated in the intensive care unit for persistent hypoxemia. He was admitted $2$ days ago for severe acute pancreatitis and was intubated yesterday due to acute respiratory distress syndrome (ARDS). He is a former smoker. His ideal body weight is $70$ kg ( $154$ lb). He is sedated and mechanically ventilated in volume-assist control mode.

Current ventilator settings are as follows:

Parameter	Value
Tidal volume	$420$ mL
Respiratory rate	$22$ /min
FiO₂	$0.70$
PEEP	$5$ cm H₂O

Vital signs are temperature $38.4^\circ$ C ( $101.1^\circ$ F), blood pressure $112/72$ mm Hg, and pulse $108$ /min. Central venous pressure is $8$ mm Hg. Auscultation of the lungs reveals diffuse bilateral crackles. A chest radiograph demonstrates diffuse, bilateral alveolar opacities without cardiomegaly.

Laboratory studies show:

Test	Result	Reference Range
Leukocyte count	$16,500$ /mm³	$4,500–11,000$ /mm³
TSH	$1.2$ µU/mL	$0.5–5.0$ µU/mL
Total T₄	$3.5$ µg/dL	$5.0–12.0$ µg/dL

Arterial blood gas analysis on the current ventilator settings reveals:

Parameter	Value
pH	$7.32$
PaCO₂	$46$ mm Hg
PaO₂	$48$ mm Hg
HCO₃⁻	$23$ mEq/L

Ventilator mechanics reveal a peak airway pressure of $36$ cm H₂O and a plateau airway pressure of $21$ cm H₂O.

Which of the following is the most appropriate next step in management?

Increase the positive end-expiratory pressureAnswer
B
Administer intravenous torsemide
C
Administer intravenous sodium bicarbonate
D
Administer intravenous cefepime and vancomycin
E
Administer intravenous levothyroxine

Answer

Increase the positive end-expiratory pressure

This patient has severe acute respiratory distress syndrome (ARDS) secondary to severe acute pancreatitis. His PaO₂ is

48

mm Hg on an FiO₂ of

0.70

(PaO₂/FiO₂ ratio

< 100

), indicating life-threatening hypoxemia. In ARDS, the primary goal of mechanical ventilation is to maintain adequate oxygenation (target PaO₂

55-80

mm Hg or SpO₂

88-95\%

) while preventing ventilator-induced lung injury through low tidal volumes (

6

mL/kg ideal body weight). This patient's tidal volume is correctly optimized. To improve severe hypoxemia when FiO₂ is already elevated, the most effective intervention is increasing Positive End-Expiratory Pressure (PEEP). PEEP recruits collapsed alveoli, increases functional residual capacity, and decreases intrapulmonary right-to-left shunting. His plateau pressure is

21

cm H₂O, providing a substantial safe margin to incrementally increase PEEP while keeping the plateau pressure below the recommended maximum of

30

cm H₂O.

Step-by-Step Solution

Identify the patient's primary life-threatening physiological derangement.

The patient has severe hypoxemia with a PaO₂ of 48 mm Hg on an FiO₂ of 0.70 (PaO₂/FiO₂ ratio < 100).

Recognizing the severity of the oxygenation failure dictates immediate ventilator adjustments rather than focusing on the mild hypercapnia or underlying SIRS.

Evaluate the current mechanical ventilation strategy against ARDS protocols.

The patient is correctly receiving lung-protective ventilation with a tidal volume of 6 mL/kg (420 mL for a 70 kg patient), but the PEEP is inappropriately low (5 cm H₂O) for the degree of hypoxemia.

Maintaining low tidal volumes prevents volutrauma, while adequate PEEP is required to prevent atelectrauma and improve oxygenation.

Assess the safety margin for increasing airway pressures.

The plateau airway pressure is 21 cm H₂O, which is well below the maximum recommended threshold of 30 cm H₂O.

Confirming a safe plateau pressure ensures that increasing PEEP to recruit alveoli will not inadvertently cause severe barotrauma.

Determine the most appropriate intervention based on the findings.

Increasing the positive end-expiratory pressure (PEEP) is the best next step.

Since FiO₂ is already high (0.70) and plateau pressure is low, up-titrating PEEP is the most effective and safe mechanism to improve alveolar recruitment and oxygenation.

Key Concept

In patients with severe ARDS and refractory hypoxemia on lung-protective ventilation, the most appropriate next step to improve oxygenation is to increase the Positive End-Expiratory Pressure (PEEP), provided the plateau airway pressure remains strictly below 30 cm H₂O.

Estimated Time:2m 0s

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