A 72-year-old female with a medical history of severe chronic obstructive pulmonary diseas… — USMLE Step 2 CK

A $72$ -year-old female with a medical history of severe chronic obstructive pulmonary disease and heart failure with reduced ejection fraction ( $35\%$ ) is evaluated in the intensive care unit for worsening hemodynamic instability. She was admitted to the hospital three days ago with a left lower lobe community-acquired pneumonia. Following progressive hypoxemia and increased work of breathing, she was intubated yesterday. She is currently sedated and paralyzed.

Over the past $4$ hours, her blood pressure has steadily declined despite receiving $30$ mL/kg of lactated Ringer solution.

Vital signs:
* Temperature: $38.9^\circ \text{C}$ ( $102.0^\circ \text{F}$ )
* Heart rate: $118$ /min
* Blood pressure: $76/40$ mmHg
* $\text{SpO}_2$ : $92\%$

Physical examination reveals a mottled appearance of the lower extremities. Jugular venous pressure is $8$ cm $\text{H}_2\text{O}$ . Auscultation demonstrates left basilar crackles and bilateral expiratory wheezing. Bedside echocardiography shows an ejection fraction of $35\%$ without right ventricular enlargement or pericardial effusion.

Sputum cultures obtained on admission are now growing *Streptococcus pneumoniae* susceptible to ceftriaxone, levofloxacin, and vancomycin. The patient is currently receiving empiric therapy with intravenous vancomycin, cefepime, and azithromycin.

Ventilator settings (Volume Control):
* Tidal volume: $380$ mL ( $6$ mL/kg predicted body weight)
* Respiratory rate: $12$ /min
* $\text{FiO}_2$ : $0.60$
* PEEP: $12$ cm $\text{H}_2\text{O}$

Arterial blood gas (drawn $15$ minutes ago):
* pH: $7.18$
* $\text{PaCO}_2$ : $44$ mmHg
* $\text{PaO}_2$ : $65$ mmHg
* $\text{HCO}_3^-$ : $16$ mEq/L

Which of the following are the most appropriate immediate steps in management? (Select all that apply)

Increase the set ventilator respiratory rateCevap
Initiate an intravenous norepinephrine infusionCevap
C
Administer intravenous furosemide
D
Maintain the current empiric broad-spectrum antibiotic regimen
E
Administer an intravenous sodium bicarbonate infusion

Cevap

The most appropriate immediate steps are to increase the set ventilator respiratory rate and initiate an intravenous norepinephrine infusion.

This patient presents with refractory septic shock and a mixed metabolic and respiratory acidosis. After failing to stabilize with

30

mL/kg of crystalloid fluids, the next critical step for her distributive shock is to initiate a first-line vasopressor, specifically a norepinephrine infusion. Concurrently, her arterial blood gas reveals a severe acidemia (pH

7.18

). Applying Winter's formula (

\text{Expected PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8

), her expected

\text{PaCO}_2

should be approximately

32

mmHg. Because her actual

\text{PaCO}_2

44

mmHg, she has an overlying acute respiratory acidosis. Since she is pharmacologically paralyzed and locked into a mandatory ventilator rate of

12

/min, she is incapable of providing the necessary respiratory compensation herself. The clinician must immediately increase the set ventilator respiratory rate to enhance minute ventilation and clear the excess carbon dioxide.

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Evaluate hemodynamic status and response to initial fluid resuscitation.

The patient remains profoundly hypotensive (blood pressure

76/40

mmHg) after receiving

30

mL/kg of crystalloids.

Refractory hypotension in the setting of infection indicates septic shock requiring immediate vasopressor support (norepinephrine) rather than diuresis, despite her history of heart failure.

Analyze the arterial blood gas using Winter's formula to determine the precise acid-base derangement.

Expected

\text{PaCO}_2 = 1.5 \times 16 + 8 = 32 \pm 2

mmHg. The actual

\text{PaCO}_2

44

mmHg.

Because the actual

\text{PaCO}_2

(

44

mmHg) is significantly higher than the expected range (

30

34

mmHg), there is a primary high-anion-gap metabolic acidosis complicated by a severe concurrent acute respiratory acidosis.

Determine the necessary mechanical ventilation adjustment.

The set ventilator respiratory rate must be increased.

The patient is pharmacologically paralyzed and cannot mount a compensatory tachypnea. The set rate of

12

/min is inadequate for her metabolic demand. Increasing the rate will increase minute ventilation and clear

\text{CO}_2

, resolving the respiratory component of the acidemia.

Review microbiological data to apply antimicrobial stewardship principles.

Sputum culture confirms *S. pneumoniae* susceptible to ceftriaxone.

Empiric broad-spectrum antibiotics (vancomycin, cefepime) should be narrowed to targeted monotherapy (e.g., ceftriaxone) to prevent resistance and toxicity.

Anahtar Kavram

Management of septic shock with mixed acid-base disorders requires vasopressor initiation after adequate fluid resuscitation and manual adjustment of minute ventilation in paralyzed patients to manage concurrent respiratory acidosis.

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Cevap

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