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Difficulty: EasyNephrology

Emergency medical services arrive with a 74-year-old woman who became acutely lethargic at her assisted living facility. According to the staff, she had been complaining of severe, diffuse abdominal pain that was out of proportion to her physical exam findings earlier in the morning. Her medical history includes atrial fibrillation, type 2 diabetes mellitus, and severe peripheral vascular disease.

On evaluation, her blood pressure is 78/46 mm Hg, pulse is 132/min and irregularly irregular, and respirations are deep and rapid at 28/min. Physical examination reveals mottled, cool extremities and a rigid, diffusely tender abdomen.

Point-of-care laboratory testing reveals:
Serum Na+Na^+: 141 mEq/L
Serum K+K^+: 4.9 mEq/L
Serum Cl−Cl^-: 105 mEq/L
Serum HCO3−HCO_3^-: 14 mEq/L
Blood urea nitrogen: 32 mg/dL
Creatinine: 1.6 mg/dL
Lactate: 9.4 mmol/L (normal: 0.5-1.0 mmol/L)

Arterial blood gas on room air:
pH: 7.23
PaO2PaO_2: 88 mm Hg

What is the expected arterial partial pressure of carbon dioxide (PaCO2PaCO_2) in mm Hg that indicates appropriate and isolated respiratory compensation for her primary acid-base disturbance?

  1. A
    21
  2. B
    24
  3. 29Answer
  4. D
    36
  5. E
    40

Answer

The expected target PaCO2PaCO_2 is 29 mm Hg.
The patient presents with severe abdominal pain out of proportion to exam and an irregularly irregular pulse (atrial fibrillation), highly suggestive of acute mesenteric ischemia. This has led to profound lactic acidosis, confirmed by her low pH (7.23), low bicarbonate (14 mEq/L), and elevated lactate (9.4 mmol/L). This represents a primary high anion gap metabolic acidosis. To determine if this is an isolated disorder, the expected physiological respiratory compensation must be calculated using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. For this patient, the calculation is (1.5×14)+8=21+8=29(1.5 \times 14) + 8 = 21 + 8 = 29 mm Hg. If her measured PaCO2PaCO_2 matches this target (or falls within the 27 to 31 mm Hg range), it indicates an appropriate and isolated respiratory compensation without a superimposed primary respiratory disorder.

Step-by-Step Solution

1
Identify the primary acid-base disorder from the provided laboratory values.
The patient has a low pH (7.23) and a low serum bicarbonate (14 mEq/L), which establishes the presence of a primary metabolic acidosis.
Identifying the primary disturbance is required to select the correct physiological compensation formula.
2
Calculate the expected respiratory compensation.
Using Winter's formula: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Plugging in the values: Expected PaCO2=(1.5×14)+8=21+8=29PaCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29 mm Hg.
Winter's formula determines if the patient's respiratory response (hyperventilation) is adequate for the degree of metabolic acidosis, helping to rule out mixed acid-base disorders.

Key Concept

In the setting of a primary metabolic acidosis, the appropriate respiratory compensation is calculated using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2.
Estimated Time:1m 0s
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Nephrology

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