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Difficulty: MediumNephrology

A rapid response team is called to evaluate a 45-year-old woman on the general medical ward due to acute-onset lethargy. The patient was admitted 24 hours ago for a severe exacerbation of ulcerative colitis, presenting with numerous large-volume, watery bowel movements per day. Over the past 12 hours, she has received three doses of intravenous hydromorphone to manage severe abdominal cramping. On physical examination, her temperature is 37.1°C (98.8°F), blood pressure is 105/65 mmHg, pulse is 112/min, and respiratory rate is 8/min with shallow breathing. She is difficult to arouse but withdraws appropriately to painful stimuli.

Stat laboratory studies reveal the following:
Serum sodium: 135 mEq/L
Serum potassium: 3.1 mEq/L
Serum chloride: 115 mEq/L
Serum bicarbonate: 10 mEq/L
Blood urea nitrogen: 42 mg/dL
Serum creatinine: 1.8 mg/dL

Arterial blood gas analysis on room air shows:
pH: 7.10
PaO2: 68 mmHg
PaCO2: 36 mmHg

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

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

Answer

Normal anion gap metabolic acidosis and concurrent respiratory acidosis
This patient's severe diarrhea has caused massive gastrointestinal bicarbonate loss, leading to a primary normal anion gap metabolic acidosis (NAGMA). The calculated anion gap is normal at 10 mEq/L (135−(115+10)135 - (115 + 10)). To determine if respiratory compensation is adequate, Winter's formula must be applied: Expected PaCO2=1.5×[HCO3−]+8±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. Given a bicarbonate of 10 mEq/L, the expected PaCO2PaCO_2 is 21 to 25 mmHg. The patient's actual PaCO2PaCO_2 is 36 mmHg. Because the measured PaCO2PaCO_2 is significantly higher than the expected range, the patient has a concurrent, superimposed respiratory acidosis. Clinically, this is explained by opiate-induced hypoventilation, as evidenced by her somnolence and a respiratory rate of 8/min following hydromorphone administration.

Step-by-Step Solution

1
Identify the primary acid-base disorder by looking at the pH and serum bicarbonate.
The pH is 7.10 (acidemia) and the HCO3−HCO_3^- is 10 mEq/L (low). This indicates a primary metabolic acidosis.
Establishing the primary disturbance is the first step in analyzing any mixed acid-base disorder.
2
Calculate the serum anion gap (AG).
AG = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 135−(115+10)=10135 - (115 + 10) = 10 mEq/L. This is within the normal range (8-12 mEq/L), confirming a normal anion gap metabolic acidosis (NAGMA).
Differentiating between a normal and high anion gap determines the etiology of the metabolic acidosis (in this case, gastrointestinal bicarbonate loss from diarrhea).
3
Use Winter's formula to calculate the expected PaCO2PaCO_2 for respiratory compensation.
Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient: (1.5×10)+8=23±2(1.5 \times 10) + 8 = 23 \pm 2 mmHg. The expected range is 21 to 25 mmHg.
Winter's formula determines if the lungs are appropriately compensating for the metabolic acidosis by hyperventilating and blowing off CO2CO_2.
4
Compare the expected PaCO2PaCO_2 to the patient's measured PaCO2PaCO_2.
The measured PaCO2PaCO_2 is 36 mmHg, which is significantly higher than the expected maximum of 25 mmHg.
Because the measured PaCO2PaCO_2 is higher than the expected compensated value, there is a concurrent primary respiratory acidosis. This is clinically consistent with the patient's low respiratory rate (8/min) following hydromorphone administration.

Key Concept

Diagnosis of mixed acid-base disorders using the anion gap and Winter's formula
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Topics

Nephrology

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