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Difficulty: MediumPulmonary and Critical Care

A 6565-year-old man is evaluated in the intensive care unit for worsening altered mental status and respiratory distress. He was admitted 22 days ago for community-acquired pneumonia and was started on appropriate intravenous antibiotics. His medical history includes chronic obstructive pulmonary disease (COPD) and hypertension. Temperature is 38.6∘C38.6^\circ\text{C} (101.5∘F101.5^\circ\text{F}), blood pressure is 88/54 mmHg88/54\text{ mmHg}, pulse is 112/min112/\text{min}, and respirations are 28/min28/\text{min}. Pulse oximetry shows 92%92\% on a 50%50\% Venturi mask. Physical examination reveals bilateral crackles and scattered wheezes. Laboratory results are as follows:

ParameterValue
Sodium142 mEq/L142\text{ mEq/L}
Potassium4.5 mEq/L4.5\text{ mEq/L}
Chloride104 mEq/L104\text{ mEq/L}
Bicarbonate14 mEq/L14\text{ mEq/L}
Blood urea nitrogen32 mg/dL32\text{ mg/dL}
Creatinine1.8 mg/dL1.8\text{ mg/dL}
Lactate4.2 mmol/L4.2\text{ mmol/L}

Arterial blood gas analysis on 50%50\% oxygen:

ParameterValue
pH7.227.22
PaCO2\text{PaCO}_235 mmHg35\text{ mmHg}
PaO2\text{PaO}_264 mmHg64\text{ mmHg}

Which of the following best characterizes this patient's current acid-base status?

  1. High anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis and concurrent respiratory alkalosis
  4. D
    Normal anion gap metabolic acidosis and concurrent respiratory acidosis
  5. E
    Primary respiratory acidosis with compensatory metabolic alkalosis

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
This patient has a mixed acid-base disorder consisting of a high anion gap metabolic acidosis and a concurrent respiratory acidosis. The acidemia (pH 7.227.22) is driven by a low bicarbonate (14 mEq/L14\text{ mEq/L}), indicating a primary metabolic acidosis. The anion gap is elevated at 24 mEq/L24\text{ mEq/L} (142−[104+14]142 - [104 + 14]), consistent with lactic acidosis secondary to septic shock from his severe pneumonia. To determine if respiratory compensation is adequate, Winter's formula must be applied: Expected PaCO2=(1.5×14)+8=29±2 mmHg\text{Expected PaCO}_2 = (1.5 \times 14) + 8 = 29 \pm 2\text{ mmHg}. Because the patient's measured PaCO2\text{PaCO}_2 (35 mmHg35\text{ mmHg}) is significantly higher than the expected maximal compensatory value of 31 mmHg31\text{ mmHg}, he has a concurrent respiratory acidosis. This indicates that his respiratory drive or mechanics (compromised by COPD and pneumonia) are failing to adequately clear CO2, signaling impending respiratory failure.

Step-by-Step Solution

1
Analyze the pH to determine the primary disorder.
The pH is 7.227.22, indicating acidemia.
Establishing acidemia or alkalemia is the first step in diagnosing an acid-base disorder.
2
Assess the primary driver of the acidemia.
The serum bicarbonate is low (14 mEq/L14\text{ mEq/L}), confirming a primary metabolic acidosis.
A low pH driven by a low bicarbonate defines metabolic acidosis.
3
Calculate the anion gap to classify the metabolic acidosis.
Anion Gap=Na+−(Cl−+HCO3−)=142−(104+14)=24 mEq/L\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 142 - (104 + 14) = 24\text{ mEq/L}.
An elevated anion gap (>12 mEq/L> 12\text{ mEq/L}) confirms a high anion gap metabolic acidosis, consistent with this patient's elevated lactate from septic shock.
4
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×HCO3−)+8±2=(1.5×14)+8±2=29±2 mmHg\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2\text{ mmHg} (Range: 2727-31 mmHg31\text{ mmHg}).
This determines if the lungs are adequately blowing off CO2 to compensate for the metabolic acidosis.
5
Compare the measured PaCO2 to the expected PaCO2.
The measured PaCO2\text{PaCO}_2 (35 mmHg35\text{ mmHg}) is greater than the expected range (2727-31 mmHg31\text{ mmHg}).
A PaCO2 higher than expected indicates inadequate ventilation (concurrent respiratory acidosis), which is a sign of impending respiratory failure in a patient with COPD.

Key Concept

In patients with metabolic acidosis, Winter's formula must be used to calculate expected respiratory compensation; a measured PaCO2 higher than the expected range indicates a concurrent respiratory acidosis.
Estimated Time:1m 30s
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Topics

Pulmonary and Critical Care

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