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Zorluk: OrtaNephrology

A 72-year-old man is evaluated in the intensive care unit for progressive lethargy and labored breathing. He was admitted 3 days ago for presumed urosepsis and is receiving broad-spectrum intravenous antibiotics. His temperature is 38.4∘C38.4^\circ\text{C} (101.1∘F101.1^\circ\text{F}), blood pressure is 82/50 mm Hg, pulse is 122/min, and respirations are 28/min. Physical examination shows a somnolent patient using accessory muscles of respiration.

Laboratory studies show the following:

ComponentValue
Serum sodium140 mEq/L
Serum potassium4.8 mEq/L
Serum chloride102 mEq/L
Serum bicarbonate14 mEq/L
Blood urea nitrogen52 mg/dL
Serum creatinine2.4 mg/dL
Serum lactate5.8 mmol/L
Arterial blood gas analysis on room air:
ComponentValue
pH7.21
PaCO2PaCO_236 mm Hg
PaO2PaO_260 mm Hg

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

  1. Elevated anion gap metabolic acidosis and concomitant respiratory acidosisCevap
  2. B
    Elevated anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Elevated anion gap metabolic acidosis and concomitant respiratory alkalosis
  4. D
    Normal anion gap metabolic acidosis and concomitant respiratory acidosis
  5. E
    Elevated anion gap metabolic acidosis and concomitant metabolic alkalosis

Cevap

Elevated anion gap metabolic acidosis and concomitant respiratory acidosis
The correct answer accurately identifies both the primary metabolic acidosis (elevated anion gap due to lactic acidosis from septic shock) and the superimposed respiratory acidosis. The respiratory acidosis is evident because the patient's measured PaCO2PaCO_2 of 36 mm Hg is significantly higher than the expected PaCO2PaCO_2 range of 27-31 mm Hg calculated by Winter's formula, reflecting respiratory muscle fatigue.

Adım Adım Çözüm

1
Determine the primary disturbance.
pH is 7.21 (acidemia) and HCO3−HCO_3^- is 14 mEq/L (low). This establishes a primary metabolic acidosis.
Identifying the primary derangement directs the choice of compensation formulas.
2
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=140−(102+14)=24AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (102 + 14) = 24. The normal AG is approximately 12. Thus, there is an elevated anion gap metabolic acidosis, consistent with lactic acidosis in sepsis.
Metabolic acidosis must be differentiated into normal vs. elevated anion gap etiologies.
3
Apply Winter's formula to determine expected respiratory compensation.
Expected PaCO2=(1.5×[HCO3−])+8±2=(1.5×14)+8±2=29±2Expected\ PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg. The expected range is 27 to 31 mm Hg.
Metabolic acidosis should trigger a predictable hyperventilatory response. Winter's formula defines this target.
4
Compare measured PaCO2PaCO_2 with expected PaCO2PaCO_2.
The measured PaCO2PaCO_2 is 36 mm Hg, which is higher than the maximum expected value of 31 mm Hg. This indicates a secondary respiratory acidosis.
If actual PaCO2PaCO_2 exceeds expected PaCO2PaCO_2, the lungs are hypoventilating relative to the systemic acid load (in this case, due to respiratory muscle fatigue).
5
Check the delta ratio for hidden metabolic disorders.
ΔRatio=AG−1224−HCO3−=24−1224−14=1210=1.2\Delta Ratio = \frac{AG - 12}{24 - HCO_3^-} = \frac{24 - 12}{24 - 14} = \frac{12}{10} = 1.2. This confirms a pure high anion gap metabolic acidosis.
To ensure no tertiary metabolic disorder (like concomitant metabolic alkalosis) is present.

Anahtar Kavram

In metabolic acidosis, adequate respiratory compensation must be verified using Winter's formula: Expected PaCO2=1.5×[HCO3−]+8±2Expected\ PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. If the measured PaCO2PaCO_2 is higher than expected, there is a concomitant respiratory acidosis, which often signals impending respiratory failure in a critically ill patient.
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