A 42-year-old woman is admitted to the intensive care unit with severe community-acquired … — USMLE Step 2 CK

A $42$ -year-old woman is admitted to the intensive care unit with severe community-acquired pneumonia. Twelve hours after admission, she requires endotracheal intubation and mechanical ventilation due to worsening hypoxemic respiratory failure. Her current medications include intravenous ceftriaxone and azithromycin.

Vital signs are temperature $38.9^\circ\text{C}$ ( $102^\circ\text{F}$ ), blood pressure $105/65\text{ mm Hg}$ , and heart rate $118/\text{min}$ .

Current mechanical ventilator settings (Volume Assist-Control mode):

Parameter	Value
Tidal volume	$380\text{ mL}$ ( $6\text{ mL/kg}$ of predicted body weight)
Respiratory rate	$24/\text{min}$
Fraction of inspired oxygen ( $FiO_2$ )	$0.90$ ( $90\%$ )
Positive end-expiratory pressure (PEEP)	$8\text{ cm H}_2\text{O}$

Arterial blood gas analysis on these settings reveals:

Parameter	Value
pH	$7.31$
$PaCO_2$	$48\text{ mm Hg}$
$PaO_2$	$55\text{ mm Hg}$
Bicarbonate	$24\text{ mEq/L}$

A chest radiograph demonstrates diffuse, bilateral alveolar opacities. A bedside echocardiogram reveals normal left and right ventricular function with an estimated ejection fraction of $65\%$ .

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

Increase the positive end-expiratory pressure (PEEP)Answer
Initiate prone positioningAnswer
C
Administer intravenous sodium bicarbonate to correct the acid-base derangement
D
Administer intravenous furosemide to manage the bilateral alveolar opacities
E
Escalate current antimicrobial therapy to piperacillin-tazobactam and vancomycin

Answer

The appropriate interventions are to increase the positive end-expiratory pressure (PEEP) and to initiate prone positioning.

This patient meets the Berlin criteria for severe acute respiratory distress syndrome (ARDS): acute onset, bilateral diffuse alveolar opacities, no evidence of left heart failure (normal echocardiogram), and a severe oxygenation defect with a

PaO_2/FiO_2

ratio of approximately

61\text{ mm Hg}

(

55 \div 0.90

). The fundamental tenets of mechanical ventilation in ARDS include low tidal volume ventilation to prevent volutrauma (already appropriately set at

6\text{ mL/kg}

) and adequate positive end-expiratory pressure (PEEP) to recruit collapsed alveoli. According to ARDS protocols, a PEEP of

8\text{ cm H}_2\text{O}

is insufficient for an

FiO_2

requirement of

90\%

; therefore, increasing the PEEP is a critical next step. Furthermore, prone positioning is strongly indicated for patients with moderate-to-severe ARDS (

PaO_2/FiO_2 < 150\text{ mm Hg}

). Proning improves ventilation-perfusion (

V/Q

) matching by relieving the weight of the heart and anterior lungs on the dependent posterior lung regions, thereby recruiting posterior alveoli and reducing mortality.

Step-by-Step Solution

Calculate the PaO2/FiO2 ratio to determine the severity of hypoxemia.

The ratio is 55 / 0.90 = 61.1 mm Hg.

Categorizing the degree of hypoxemia (ratio < 100 indicates severe ARDS) dictates subsequent management steps like proning.

Evaluate the current ventilator settings against ARDS management guidelines.

Tidal volume is appropriately low (6 mL/kg). However, a PEEP of 8 cm H2O is inadequate for an FiO2 of 90%.

ARDSNet protocols require high PEEP levels when high fractions of inspired oxygen are needed to prevent alveolar collapse.

Assess the indications for prone positioning.

The patient meets criteria for proning.

Prone positioning is strongly recommended in ARDS patients who remain severely hypoxemic (PaO2/FiO2 < 150 mm Hg) despite optimized ventilator settings.

Rule out concurrent conditions mimicking ARDS or requiring alternative treatments.

The normal echocardiogram rules out cardiogenic edema, and the ABG shows expected respiratory acidosis, not metabolic acidosis.

Ensures targeted therapies (like furosemide or bicarbonate) are not inappropriately administered.

Key Concept

Management of severe acute respiratory distress syndrome (ARDS) centers on treating the underlying cause, employing lung-protective mechanical ventilation (low tidal volumes, adequate PEEP), and initiating prone positioning for severe hypoxemia (PaO2/FiO2 < 150 mm Hg).

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