Examkin

Question

Difficulty: MediumNephrology

An 82-year-old man is brought to the emergency department due to sudden-onset, severe abdominal pain. He has a history of hypertension, type 2 diabetes mellitus, and atrial fibrillation for which he takes apixaban; however, he admits to missing several doses over the past week. During transport, paramedics administered intravenous morphine for pain control. On arrival, he appears drowsy but is arousable. Temperature is 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), blood pressure is 102/60 mmHg102/60\text{ mmHg}, pulse is 118/min118/\text{min} and irregular, and respiratory rate is 14/min14/\text{min}. The abdomen is slightly distended with absent bowel sounds, and there is minimal tenderness to palpation.

Laboratory results are as follows:

ParameterValue
Sodium140 mEq/L140\text{ mEq/L}
Potassium4.5 mEq/L4.5\text{ mEq/L}
Chloride104 mEq/L104\text{ mEq/L}
Bicarbonate12 mEq/L12\text{ mEq/L}
Blood urea nitrogen30 mg/dL30\text{ mg/dL}
Creatinine1.4 mg/dL1.4\text{ mg/dL}
Arterial blood gas (on room air):
ParameterValue
pH7.167.16
PaCO2\text{PaCO}_234 mmHg34\text{ mmHg}
PaO2\text{PaO}_272 mmHg72\text{ mmHg}

Which of the following best characterizes this patient's 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 appropriate metabolic compensation

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
This patient's clinical presentation (atrial fibrillation off anticoagulation with abdominal pain out of proportion to exam) is classic for acute mesenteric ischemia, leading to lactic acidosis. The pH is 7.167.16 (acidemia) and bicarbonate is 12 mEq/L12\text{ mEq/L}, defining a primary metabolic acidosis. The anion gap is elevated at 2424 (140−[104+12]140 - [104 + 12]), categorizing it as a high anion gap metabolic acidosis (HAGMA). To assess respiratory compensation, Winter's formula is utilized: Expected PaCO2=(1.5×HCO3−)+8±2\text{Expected PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. For this patient, the expected PaCO2=(1.5×12)+8=26±2 mmHg\text{PaCO}_2 = (1.5 \times 12) + 8 = 26 \pm 2\text{ mmHg}. Because the patient's actual PaCO2\text{PaCO}_2 is 34 mmHg34\text{ mmHg} (which is >28 mmHg>28\text{ mmHg}), he has an inadequate respiratory response. This relative hypoventilation, likely secondary to the recent administration of intravenous morphine, indicates a concurrent primary respiratory acidosis.

Step-by-Step Solution

1
Determine the primary acid-base disturbance.
The patient has an acidemia (pH 7.167.16). The HCO3−\text{HCO}_3^- is low (12 mEq/L12\text{ mEq/L}), confirming a primary metabolic acidosis.
pH < 7.40 indicates acidemia, and a matched decrease in bicarbonate establishes the primary metabolic nature of the disturbance.
2
Calculate the serum anion gap.
Anion Gap=Na+−(Cl−+HCO3−)=140−(104+12)=140−116=24\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (104 + 12) = 140 - 116 = 24.
Differentiating between normal (8-12) and high anion gap metabolic acidosis is the required next step in evaluating any metabolic acidosis.
3
Calculate expected respiratory compensation using Winter's formula.
Expected PaCO2=1.5×[HCO3−]+8±2=1.5×12+8±2=18+8±2=26±2 mmHg\text{Expected PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2 = 1.5 \times 12 + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2\text{ mmHg} (Range: 24-28 mmHg).
In primary metabolic acidosis, the lungs compensate by hyperventilating to blow off CO2\text{CO}_2. Winter's formula predicts the appropriate PaCO2\text{PaCO}_2 response.
4
Compare actual PaCO2\text{PaCO}_2 to expected PaCO2\text{PaCO}_2.
The actual PaCO2\text{PaCO}_2 (34 mmHg34\text{ mmHg}) is greater than the expected range (24−28 mmHg24-28\text{ mmHg}).
An actual PaCO2\text{PaCO}_2 higher than the expected compensated range indicates an inadequate respiratory drive, confirming a concurrent primary respiratory acidosis.

Key Concept

Calculation of Anion Gap and Winter's Formula to diagnose mixed acid-base disorders.
Rate this question

Topics

Nephrology

More questions from this topic