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Zorluk: OrtaNephrology

A 32-year-old woman is brought to the emergency department by her partner due to 5 days of severe, voluminous diarrhea and abdominal cramping. She has a history of ulcerative colitis and panic disorder. In the waiting room, she experienced a severe panic attack and was noted to have visibly rapid, deep breathing. Physical examination reveals dry mucous membranes, decreased skin turgor, a heart rate of 115/min115/\text{min}, a respiratory rate of 28/min28/\text{min}, and a blood pressure of 105/65 mm Hg105/65\text{ mm Hg}.

Laboratory results are as follows:

Laboratory TestResultReference Range
Sodium136 mEq/L136 \text{ mEq/L}136−145 mEq/L136-145 \text{ mEq/L}
Potassium3.2 mEq/L3.2 \text{ mEq/L}3.5−5.0 mEq/L3.5-5.0 \text{ mEq/L}
Chloride115 mEq/L115 \text{ mEq/L}95−105 mEq/L95-105 \text{ mEq/L}
Bicarbonate12 mEq/L12 \text{ mEq/L}22−28 mEq/L22-28 \text{ mEq/L}
Blood urea nitrogen34 mg/dL34 \text{ mg/dL}7−18 mg/dL7-18 \text{ mg/dL}
Creatinine1.2 mg/dL1.2 \text{ mg/dL}0.6−1.2 mg/dL0.6-1.2 \text{ mg/dL}

Arterial blood gas (room air):

Arterial Blood GasResultReference Range
pH7.407.407.35−7.457.35-7.45
pCO2\text{pCO}_220 mm Hg20 \text{ mm Hg}35−45 mm Hg35-45 \text{ mm Hg}
pO2\text{pO}_298 mm Hg98 \text{ mm Hg}80−100 mm Hg80-100 \text{ mm Hg}

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

  1. Mixed normal anion gap metabolic acidosis and respiratory alkalosisCevap
  2. B
    Compensated normal anion gap metabolic acidosis
  3. C
    Mixed normal anion gap metabolic acidosis and respiratory acidosis
  4. D
    Mixed high anion gap metabolic acidosis and respiratory alkalosis
  5. E
    Compensated high anion gap metabolic acidosis

Cevap

Mixed normal anion gap metabolic acidosis and respiratory alkalosis
The patient's initial insult is severe diarrhea, which causes profound gastrointestinal loss of bicarbonate, resulting in a normal anion gap metabolic acidosis (NAGMA). This is confirmed by an anion gap of 9 mEq/L9 \text{ mEq/L} (136−115−12=9136 - 115 - 12 = 9). To evaluate if respiratory compensation is appropriate, Winter's formula is applied: Expected pCO2=(1.5×12)+8±2=26±2 mm Hg\text{Expected pCO}_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}. Because the patient's actual pCO2\text{pCO}_2 (20 mm Hg20 \text{ mm Hg}) is significantly lower than the expected range (24−28 mm Hg24-28 \text{ mm Hg}), she is 'blowing off' more CO2\text{CO}_2 than expected. This indicates a coexisting primary respiratory alkalosis, clinically correlating with her panic attack and hyperventilation in the waiting room. The two opposing disorders (acidosis and alkalosis) offset each other, resulting in a seemingly normal pH of 7.407.40.

Adım Adım Çözüm

1
Calculate the anion gap to identify the primary metabolic disturbance.
Anion Gap=Na+−(Cl−+HCO3−)=136−(115+12)=9 mEq/L\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 136 - (115 + 12) = 9 \text{ mEq/L}.
Since the anion gap is within the normal range (8−12 mEq/L8-12 \text{ mEq/L}), the patient has a normal anion gap metabolic acidosis (NAGMA), consistent with bicarbonate loss from severe diarrhea.
2
Apply Winter's formula to determine the expected respiratory compensation.
Expected pCO2=1.5×[HCO3−]+8±2=(1.5×12)+8±2=26±2 mm Hg\text{Expected pCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}. The expected range is 24−28 mm Hg24-28 \text{ mm Hg}.
Winter's formula determines if the lungs are appropriately compensating for a metabolic acidosis by decreasing pCO2\text{pCO}_2.
3
Compare the actual measured pCO2\text{pCO}_2 with the expected pCO2\text{pCO}_2 range.
The patient's actual pCO2\text{pCO}_2 is 20 mm Hg20 \text{ mm Hg}, which is lower than the expected range of 24−28 mm Hg24-28 \text{ mm Hg}.
Because the pCO2\text{pCO}_2 is lower than what pure metabolic compensation would dictate, there is an additional process driving CO2 down—a primary respiratory alkalosis (due to hyperventilation from her panic attack).

Anahtar Kavram

In patients with metabolic acidosis, Winter's formula (1.5×[HCO3−]+8±21.5 \times [\text{HCO}_3^-] + 8 \pm 2) must be used to calculate expected pCO2\text{pCO}_2. An actual pCO2\text{pCO}_2 lower than the expected range indicates a superimposed primary respiratory alkalosis.
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