A 42-year-old woman is evaluated in the intensive care unit for worsening oxygenation. Two… — USMLE Step 2 CK

A $42$ -year-old woman is evaluated in the intensive care unit for worsening oxygenation. Two days ago, she was admitted to the hospital with severe acute pancreatitis. Over the past $12$ hours, she developed progressive hypoxemia and respiratory distress, requiring endotracheal intubation and mechanical ventilation. Her current ventilator settings are:

Parameter	Value
Mode	Assist-control volume ventilation
Tidal volume	$380$ mL ( $6$ mL/kg of predicted body weight)
Respiratory rate	$16$ breaths/min
Fraction of inspired oxygen ( $FiO_2$ )	$0.70$
Positive end-expiratory pressure (PEEP)	$5$ cm $H_2O$

Vital signs are temperature $37.8^\circ C$ ( $100.0^\circ F$ ), blood pressure $105/65$ mm Hg, and heart rate $112$ /min. Central venous pressure is $6$ mm Hg.
Chest radiograph reveals new-onset, bilateral, diffuse alveolar opacities without cardiomegaly or pleural effusions.
Arterial blood gas analysis on current settings shows:

Measurement	Value
pH	$7.28$
$PaCO_2$	$54$ mm Hg
$PaO_2$	$52$ mm Hg
$HCO_3^-$	$24$ mEq/L

Which of the following is the most appropriate next step in the management of this patient's respiratory failure?

Increase the positive end-expiratory pressureAnswer
B
Increase the tidal volume to 600 mL
C
Administer intravenous furosemide
D
Initiate intravenous meropenem
E
Administer intravenous sodium bicarbonate

Answer

The most appropriate next step is to increase the positive end-expiratory pressure (PEEP).

The correct answer is to increase the positive end-expiratory pressure (PEEP). This patient has developed acute respiratory distress syndrome (ARDS) as a complication of severe acute pancreatitis. ARDS is characterized by noncardiogenic pulmonary edema, alveolar collapse, and severe intrapulmonary shunting, leading to refractory hypoxemia. Management relies on lung-protective ventilation with low tidal volumes (

6

mL/kg of predicted body weight) to prevent volutrauma, which often results in permissive hypercapnia (a mild, tolerated respiratory acidosis with pH

> 7.15-7.20

). To manage her severe hypoxemia while minimizing oxygen toxicity from high

FiO_2

(ideally kept below

0.60

), the most appropriate intervention is to increase the PEEP. Higher PEEP recruits collapsed alveoli, decreases intrapulmonary shunting, and improves the

PaO_2/FiO_2

ratio.

Step-by-Step Solution

Identify the primary diagnosis based on the clinical presentation.

The patient has acute respiratory distress syndrome (ARDS) secondary to severe acute pancreatitis, evidenced by severe hypoxemia (

PaO_2/FiO_2

ratio

< 100

) and bilateral pulmonary opacities without left heart failure.

Recognizing ARDS is necessary to apply the appropriate lung-protective mechanical ventilation protocol.

Evaluate the current ventilator settings and arterial blood gas results.

The ABG shows severe life-threatening hypoxemia (

PaO_2

52

mm Hg) despite a high

FiO_2

(

0.70

), along with a mild respiratory acidosis (pH

7.28

) due to low tidal volume ventilation.

To determine the next ventilator adjustment, one must prioritize treating the life-threatening hypoxemia while maintaining lung-protective strategies (permissive hypercapnia).

Select the appropriate ventilator adjustment to improve oxygenation.

Increase the positive end-expiratory pressure (PEEP).

In ARDS, hypoxemia is primarily caused by intrapulmonary shunting from collapsed alveoli. Increasing PEEP recruits these alveoli and improves oxygenation without risking the volutrauma associated with higher tidal volumes or the oxygen toxicity associated with prolonged

FiO_2 > 0.60

Key Concept

In the management of ARDS, severe hypoxemia is addressed by increasing positive end-expiratory pressure (PEEP) to recruit collapsed alveoli, while a mild respiratory acidosis (permissive hypercapnia) is tolerated to maintain low tidal volumes and prevent ventilator-induced lung injury.

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