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Difficulty: HardPulmonary and Critical Care

A 52-year-old woman is evaluated in the emergency department for altered mental status and profound weakness. For the past week, she has experienced a worsening cough productive of rust-colored sputum, right-sided chest pain, and high fevers. Her husband notes she became progressively confused this morning.

Temperature is 39.4∘C39.4^\circ\text{C} (102.9∘F102.9^\circ\text{F}), pulse is 128/min128/\text{min}, respirations are 26/min26/\text{min}, and blood pressure is 82/50 mm Hg82/50\text{ mm Hg}. Pulse oximetry shows 88%88\% on 4 L/min4\text{ L/min} via nasal cannula. Physical examination reveals a lethargic patient who only opens her eyes to painful stimuli. Auscultation demonstrates bronchial breath sounds and crackles over the right lower lung field. Jugular venous pressure is 5 cm H2O5\text{ cm H}_2\text{O}.

Laboratory and arterial blood gas (ABG) studies are obtained:

Laboratory TestResult
Leukocytes19,500/mm319,500/\text{mm}^3
Serum sodium138 mEq/L138\text{ mEq/L}
Serum chloride102 mEq/L102\text{ mEq/L}
Serum bicarbonate12 mEq/L12\text{ mEq/L}
Blood urea nitrogen42 mg/dL42\text{ mg/dL}
Serum creatinine1.9 mg/dL1.9\text{ mg/dL}
Serum lactate6.5 mmol/L6.5\text{ mmol/L}
Arterial Blood Gas (on 4 L/min nasal cannula)Result
pH7.217.21
PaCO2\text{PaCO}_232 mm Hg32\text{ mm Hg}
PaO2\text{PaO}_258 mm Hg58\text{ mm Hg}

A rapid chest radiograph shows a dense consolidation in the right lower lobe. Intravenous fluids and appropriate narrow-spectrum antibiotics based on the local antibiogram for community-acquired pneumonia are immediately prescribed.

Which of the following is the most appropriate next step in the management of this patient?

  1. Endotracheal intubation and mechanical ventilationAnswer
  2. B
    Continue current management and closely monitor in the intensive care unit
  3. C
    Initiate noninvasive positive pressure ventilation and intravenous furosemide
  4. D
    Administer intravenous sodium bicarbonate
  5. E
    Discontinue current antibiotics and initiate intravenous vancomycin and cefepime

Answer

Endotracheal intubation and mechanical ventilation
This patient is presenting with septic shock secondary to severe community-acquired pneumonia, resulting in a primary high-anion-gap metabolic acidosis (elevated lactate). In any primary metabolic acidosis, the expected compensatory respiratory response (PaCO2\text{PaCO}_2) must be calculated using Winter's formula: Expected PaCO2=(1.5×HCO3−)+8±2\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. For this patient, the expected PaCO2\text{PaCO}_2 is (1.5×12)+8=26±2 mm Hg(1.5 \times 12) + 8 = 26 \pm 2\text{ mm Hg}. Her actual PaCO2\text{PaCO}_2 is 32 mm Hg32\text{ mm Hg}. Because her actual PaCO2\text{PaCO}_2 is significantly higher than the expected range, she has a superimposed primary respiratory acidosis. Even though 32 mm Hg32\text{ mm Hg} is below the normal physiological baseline of 40 mm Hg40\text{ mm Hg}, it is 'relative hypercapnia' for her severe state of metabolic acidosis. This finding indicates that her respiratory muscles are fatiguing and she is unable to maintain the necessary work of breathing. Given her impending respiratory failure, lethargy (inability to protect her airway), and hemodynamic instability, the most appropriate next step is endotracheal intubation and mechanical ventilation.

Step-by-Step Solution

1
Analyze the primary acid-base disorder.
The patient has acidemia (pH 7.217.21) with a low serum bicarbonate (12 mEq/L12\text{ mEq/L}), indicating a primary metabolic acidosis (lactic acidosis secondary to septic shock).
Identifying the primary disorder is required to determine the expected respiratory compensation.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×12)+8±2=26±2 mm Hg\text{PaCO}_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2\text{ mm Hg} (Range: 2424 to 28 mm Hg28\text{ mm Hg}).
Metabolic acidosis should trigger a compensatory hyperventilation to blow off CO2.
3
Compare expected PaCO2\text{PaCO}_2 to actual PaCO2\text{PaCO}_2 to identify any mixed disorders.
The actual PaCO2\text{PaCO}_2 is 32 mm Hg32\text{ mm Hg}, which is higher than the expected range of 24−28 mm Hg24-28\text{ mm Hg}. This indicates a concurrent, superimposed respiratory acidosis.
A higher than expected CO2 level means the patient is unable to sustain the necessary work of breathing.
4
Determine the appropriate clinical intervention based on the physiological state.
The patient has impending respiratory failure (fatiguing respiratory muscles) and altered mental status (lethargy). She requires definitive airway protection and mechanical ventilation.
Failure to secure the airway in a patient with mixed metabolic and respiratory acidosis and altered mental status will lead to respiratory arrest.

Key Concept

In patients with severe metabolic acidosis, the actual PaCO2\text{PaCO}_2 must be compared to the expected PaCO2\text{PaCO}_2 calculated by Winter's formula. A PaCO2\text{PaCO}_2 that is higher than expected indicates a superimposed respiratory acidosis, which in the setting of severe infection and altered mental status points to diaphragmatic fatigue and impending respiratory failure necessitating endotracheal intubation.
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Topics

Pulmonary and Critical Care

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