A critical care physician is reviewing the mechanical ventilator settings of a 55-year-old… — USMLE Step 2 CK

A critical care physician is reviewing the mechanical ventilator settings of a $55$ -year-old woman in the intensive care unit. She was admitted $2$ days ago for severe community-acquired pneumonia and required endotracheal intubation $12$ hours ago due to progressive hypoxemic respiratory failure. Her current temperature is $37.8^{\circ}C$ ( $100^{\circ}F$ ), blood pressure is $112/74$ mm Hg, and pulse is $98$ /min. She is heavily sedated and paralyzed. Chest radiography shows bilateral, diffuse alveolar opacities without cardiomegaly or pleural effusions. Echocardiography performed yesterday demonstrated a normal left ventricular ejection fraction. Current ventilator settings in volume assist-control mode are:

Fraction of inspired oxygen ( $FiO_2$ ): $0.80$
Tidal volume: $8$ mL/kg of predicted body weight
Respiratory rate: $14$ /min
Positive end-expiratory pressure (PEEP): $5$ $cm H_2O$

Arterial blood gas analysis reveals:
pH: $7.32$
$PaCO_2$ : $50$ mm Hg
$PaO_2$ : $54$ mm Hg

Which of the following interventions are indicated for the management of this patient's respiratory failure? (Select all that apply)

Decrease the tidal volume to $6$ mL/kg of predicted body weightAnswer
Increase the positive end-expiratory pressure (PEEP)Answer
C
Increase the respiratory rate to normalize the arterial pH and $PaCO_2$
D
Administer intravenous furosemide to manage the bilateral alveolar opacities
E
Broaden empiric antibiotic therapy for presumed ventilator-associated pneumonia

Answer

The correct interventions are to decrease the tidal volume to 6 mL/kg of predicted body weight and to increase the positive end-expiratory pressure (PEEP).

The correct interventions are decreasing the tidal volume and increasing the PEEP. This patient meets the Berlin criteria for severe Acute Respiratory Distress Syndrome (ARDS): acute onset, bilateral opacities not fully explained by heart failure (normal ejection fraction), and a

PaO_2/FiO_2

ratio of

67.5

(which is

< 100

). The cornerstone of mechanical ventilation in ARDS is the lung-protective ventilation strategy. A high tidal volume (

8

mL/kg) can lead to alveolar overdistension and volutrauma; therefore, reducing the tidal volume to

6

mL/kg (and occasionally lower, down to

4

mL/kg) is standard of care and has been proven to decrease mortality. Furthermore, the patient is profoundly hypoxemic (

PaO_2

54

mm Hg) despite a highly toxic

FiO_2

0.80

. Her PEEP is inadequately low at

5

cm H_2O

. Increasing the PEEP will recruit collapsed alveoli, decrease intrapulmonary shunting, and improve overall oxygenation, which will subsequently allow the physician to wean the

FiO_2

to non-toxic levels (

< 0.60

Step-by-Step Solution

Analyze the clinical scenario and diagnostic data.

The patient has a PaO2/FiO2 ratio of 67.5 (54 / 0.80), bilateral alveolar opacities, and a normal ejection fraction.

This establishes the diagnosis of severe Acute Respiratory Distress Syndrome (ARDS) secondary to pneumonia.

Evaluate the current mechanical ventilator settings.

Tidal volume is relatively high at 8 mL/kg, and PEEP is low at 5 cm H2O despite a very high FiO2 of 0.80.

Identifying suboptimal settings dictates the next steps in management.

Apply the ARDSnet protocol for lung-protective ventilation.

Tidal volume must be reduced to 6 mL/kg, and PEEP must be increased to improve oxygenation and allow FiO2 reduction.

High tidal volumes cause volutrauma and worsen mortality in ARDS, while adequate PEEP is required to prevent alveolar collapse (atelectrauma) and correct severe hypoxemia.

Key Concept

Management of Acute Respiratory Distress Syndrome (ARDS) requires a lung-protective ventilation strategy, prioritizing low tidal volumes to prevent volutrauma and adequate PEEP to recruit alveoli and improve oxygenation.

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