A 55-year-old woman is evaluated in the intensive care unit on postoperative day 3 followi… — USMLE Step 2 CK

A 55-year-old woman is evaluated in the intensive care unit on postoperative day 3 following an emergency laparotomy for a perforated diverticulum. She has been receiving intravenous piperacillin-tazobactam. Over the last 12 hours, she has developed worsening hypoxemia and increased work of breathing. Temperature is $38.2^\circ\text{C}$ ( $100.8^\circ\text{F}$ ), heart rate is $112/\text{min}$ , respiratory rate was $28/\text{min}$ prior to intubation, and blood pressure is $110/68 \text{ mm Hg}$ . Chest radiograph reveals new-onset bilateral alveolar opacities. Echocardiogram shows a normal left ventricular ejection fraction and no significant valvular disease. She is subsequently intubated and placed on mechanical ventilation.

Current ventilator settings are:
Mode: Assist-control volume
Tidal volume: $8 \text{ mL/kg}$ of actual body weight
PEEP: $5 \text{ cm H}_2\text{O}$
$\text{FiO}_2$ : $0.8$

Arterial blood gas analysis reveals:
pH: $7.31$
$\text{PaCO}_2$ : $48 \text{ mm Hg}$
$\text{PaO}_2$ : $58 \text{ mm Hg}$

Which of the following are appropriate management strategies for this patient's respiratory condition? (Select all that apply)

Decrease the tidal volume to $6 \text{ mL/kg}$ of predicted body weightAnswer
Increase the positive end-expiratory pressure (PEEP)Answer
C
Increase the tidal volume to $10 \text{ mL/kg}$ of actual body weight to normalize the arterial pH
D
Administer intravenous furosemide to rapidly clear pulmonary edema
E
Initiate empiric broad-spectrum antibiotics targeting hospital-acquired pulmonary pathogens

Answer

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

This patient has Acute Respiratory Distress Syndrome (ARDS) resulting from intra-abdominal sepsis. The cornerstone of ARDS management is lung-protective mechanical ventilation. This involves using low tidal volumes (

6 \text{ mL/kg}

of predicted body weight) to prevent alveolar overdistension (volutrauma) and applying adequate levels of positive end-expiratory pressure (PEEP) to prevent alveolar collapse at the end of expiration (atelectrauma) and to improve oxygenation. Her current tidal volume of

8 \text{ mL/kg}

based on actual body weight is too high, and her PEEP of

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

is insufficient given her severe hypoxemia on an

\text{FiO}_2

0.8

Step-by-Step Solution

Diagnose the cause of respiratory failure based on the clinical presentation and data.

The patient has Acute Respiratory Distress Syndrome (ARDS) secondary to intra-abdominal sepsis, characterized by acute onset of hypoxemia, bilateral alveolar opacities, and a normal echocardiogram (non-cardiogenic edema).

Establishing the correct diagnosis distinguishes the condition from cardiogenic pulmonary edema and guides specific ventilator management.

Evaluate the current mechanical ventilator settings and arterial blood gas values.

The current tidal volume (

8 \text{ mL/kg}

of actual body weight) is too high. The

\text{PaO}_2

is critically low (

58 \text{ mm Hg}

) despite a high

\text{FiO}_2

0.8

, indicating severe shunting. The PEEP (

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

) is too low for this degree of hypoxemia.

Identifying suboptimal ventilator settings is necessary to determine the appropriate therapeutic interventions.

Apply the principles of lung-protective mechanical ventilation.

Tidal volume should be reduced to

6 \text{ mL/kg}

of predicted body weight to prevent volutrauma. PEEP should be increased to recruit collapsed alveoli and improve oxygenation.

Lung-protective ventilation prevents ventilator-induced lung injury. Mild respiratory acidosis (permissive hypercapnia) is tolerated to achieve these low pressure and volume goals.

Key Concept

Lung-protective mechanical ventilation in Acute Respiratory Distress Syndrome (ARDS)

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