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Difficulty: EasyNephrology

During morning rounds in the intensive care unit, the medical team evaluates a patient with septic shock secondary to a perforated viscus. The patient's arterial blood gas and basic metabolic panel are drawn to assess his acid-base status. The laboratory results reveal a severe primary metabolic acidosis with a serum bicarbonate level of 14 mEq/L14 \text{ mEq/L} and a significantly elevated serum lactate. To determine if a superimposed respiratory acid-base disorder is present, the physician calculates the expected respiratory response. Assuming appropriate respiratory compensation, which of the following is the expected partial pressure of carbon dioxide (PaCO2PaCO_2) in this patient?

  1. A
    14 mm Hg14 \text{ mm Hg}
  2. B
    21 mm Hg21 \text{ mm Hg}
  3. 29 mm Hg29 \text{ mm Hg}Answer
  4. D
    35 mm Hg35 \text{ mm Hg}
  5. E
    40 mm Hg40 \text{ mm Hg}

Answer

The expected PaCO2PaCO_2 is 29 mm Hg29 \text{ mm Hg} (with a normal physiological range of ±2\pm 2), calculated using Winter's formula.
The correct expected value is 29 mm Hg29 \text{ mm Hg}. In primary metabolic acidosis, peripheral chemoreceptors sense the low pH and stimulate the respiratory center to increase ventilation. This compensatory hyperventilation blows off CO2CO_2 in an attempt to normalize the pH. The expected PaCO2PaCO_2 can be reliably calculated using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. For a bicarbonate of 14 mEq/L14 \text{ mEq/L}, the calculation is (1.5×14)+8=21+8=29(1.5 \times 14) + 8 = 21 + 8 = 29. The expected PaCO2PaCO_2 target is therefore 29 mm Hg29 \text{ mm Hg} (range 2727 to 31 mm Hg31 \text{ mm Hg}). If the measured PaCO2PaCO_2 falls outside this range, a superimposed respiratory disorder is present.

Step-by-Step Solution

1
Identify the primary acid-base disturbance.
Primary metabolic acidosis.
The patient has a low serum bicarbonate level (14 mEq/L14 \text{ mEq/L}) driven by elevated lactate from septic shock.
2
Select the appropriate formula to calculate respiratory compensation.
Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2.
This formula is specifically validated to determine the expected degree of hyperventilation (and resulting PaCO2PaCO_2) in primary metabolic acidosis.
3
Calculate the expected value.
1.5×14+8=21+8=29 mm Hg1.5 \times 14 + 8 = 21 + 8 = 29 \text{ mm Hg}.
Plugging the patient's bicarbonate level into the formula yields the exact target central value.

Key Concept

Winter's formula (1.5×[HCO3−]+8±21.5 \times [HCO_3^-] + 8 \pm 2) is used to calculate the expected respiratory compensation (expected PaCO2PaCO_2) in a patient with a primary metabolic acidosis.
Estimated Time:45s
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Nephrology

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