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Zorluk: OrtaNephrology

Emergency medical services bring a 72-year-old woman to the hospital after her daughter found her profoundly lethargic at home. According to the daughter, the patient has had severe, watery, non-bloody diarrhea for the past five days. Her past medical history is significant for severe chronic obstructive pulmonary disease requiring home oxygen therapy, hypertension, and osteoarthritis. Upon arrival, the patient is difficult to arouse.

Temperature is 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}), blood pressure is 88/5488/54 mmHg, pulse is 114114/min, and respiratory rate is 2626/min. Physical examination reveals dry mucous membranes, delayed capillary refill, and decreased breath sounds bilaterally with a prolonged expiratory phase.

Arterial blood gas (room air) and serum chemistries are obtained:

Laboratory ParameterResult
Arterial pH7.217.21
PaCO2\text{PaCO}_24242 mmHg
PaO2\text{PaO}_25656 mmHg
Sodium139139 mEq/L
Potassium3.13.1 mEq/L
Chloride114114 mEq/L
Bicarbonate1515 mEq/L
Blood urea nitrogen (BUN)4848 mg/dL
Creatinine2.12.1 mg/dL

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

  1. Mixed non-anion gap metabolic acidosis and respiratory acidosisCevap
  2. B
    Mixed high anion gap metabolic acidosis and respiratory acidosis
  3. C
    Non-anion gap metabolic acidosis with appropriate respiratory compensation
  4. D
    High anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Mixed non-anion gap metabolic acidosis and respiratory alkalosis

Cevap

Mixed non-anion gap metabolic acidosis and respiratory acidosis
The correct answer is a mixed non-anion gap metabolic acidosis and respiratory acidosis. The first step in evaluating this patient's acid-base status is identifying the primary disorder: the pHpH is 7.217.21 (acidemia) and the serum bicarbonate is 1515 mEq/L (decreased), signifying a primary metabolic acidosis. Next, the anion gap must be calculated: Na+−(Cl−+HCO3−)=139−(114+15)=10\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 139 - (114 + 15) = 10 mEq/L. Because this falls within the normal physiologic range (8−128-12 mEq/L), the patient has a normal anion gap (hyperchloremic) metabolic acidosis, which directly corresponds to her history of profound watery diarrhea causing massive gastrointestinal bicarbonate loss. The final step is assessing the adequacy of respiratory compensation using Winter's formula: Expected PaCO2=(1.5×15)+8±2=30.5±2\text{PaCO}_2 = (1.5 \times 15) + 8 \pm 2 = 30.5 \pm 2 mmHg. For appropriate respiratory compensation, her PaCO2\text{PaCO}_2 should fall between 28.528.5 and 32.532.5 mmHg. However, her measured PaCO2\text{PaCO}_2 is 4242 mmHg. Because her measured PaCO2\text{PaCO}_2 is substantially higher than expected, she is retaining CO2\text{CO}_2, representing a superimposed, concurrent primary respiratory acidosis. This occurs because her severe underlying chronic obstructive pulmonary disease and lethargy prevent her from achieving the necessary compensatory hyperventilation.

Adım Adım Çözüm

1
Analyze the primary acid-base disorder by looking at pH and bicarbonate.
The arterial pH is 7.217.21 (acidemia) and serum bicarbonate (HCO3−\text{HCO}_3^-) is 1515 mEq/L (low). This indicates a primary metabolic acidosis.
Establishing the direction of the primary disorder dictates which compensatory formula must be applied.
2
Calculate the anion gap (AG) to determine the etiology of the metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=139−(114+15)=139−129=10AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 139 - (114 + 15) = 139 - 129 = 10 mEq/L.
A normal AG (8−128-12 mEq/L) confirms a non-anion gap metabolic acidosis, which in this patient is due to profound gastrointestinal bicarbonate loss from diarrhea.
3
Determine expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×HCO3−)+8±2\text{PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. Here, (1.5×15)+8=22.5+8=30.5±2(1.5 \times 15) + 8 = 22.5 + 8 = 30.5 \pm 2 (range 28.528.5 to 32.532.5 mmHg).
Winter's formula determines the physiologic CO2\text{CO}_2 elimination expected in response to a specific degree of metabolic acidosis.
4
Compare the measured PaCO2\text{PaCO}_2 to the expected PaCO2\text{PaCO}_2.
The measured PaCO2\text{PaCO}_2 is 4242 mmHg, which is significantly higher than the maximum expected value of 32.532.5 mmHg.
An inappropriately elevated PaCO2\text{PaCO}_2 indicates a concurrent primary respiratory acidosis, secondary to hypoventilation from her underlying chronic obstructive pulmonary disease and severe fatigue.

Anahtar Kavram

Identifying mixed acid-base disorders using the anion gap and Winter's formula
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Nephrology

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