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Zorluk: OrtaPulmonary and Critical Care

A 62-year-old man is evaluated in the intensive care unit on day 2 of admission. He was admitted for septic shock secondary to a perforated gastric ulcer and underwent an emergent exploratory laparotomy. Postoperatively, he developed severe hypoxemia and bilateral pulmonary infiltrates consistent with acute respiratory distress syndrome (ARDS). He remains intubated and mechanically ventilated.

Current ventilator settings are as follows:

ParameterSetting
ModeVolume control, assist-control
Tidal volume400 mL (6 mL/kg predicted body weight)
Respiratory rate22/min
FiO20.70
PEEP12 cm H2O

Current vital signs include a temperature of 38.2°C (100.8°F), heart rate of 118/min, and blood pressure of 92/58 mm Hg while receiving a norepinephrine infusion. Pulse oximetry shows an oxygen saturation of 91%.

Arterial blood gas and serum chemistry results are as follows:

TestResult
Arterial Blood Gas
pH7.14
PaCO252 mm Hg
PaO265 mm Hg
HCO3-17 mEq/L
Serum Chemistry
Sodium141 mEq/L
Potassium4.5 mEq/L
Chloride100 mEq/L
Bicarbonate17 mEq/L
Blood urea nitrogen35 mg/dL
Creatinine2.1 mg/dL

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

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

Cevap

The correct characterization is a mixed respiratory acidosis and high anion gap metabolic acidosis.
This patient has a mixed respiratory acidosis and high anion gap metabolic acidosis (HAGMA). The pHpH of 7.147.14 indicates severe acidemia. The elevated PaCO2P_aCO_2 (5252 mm Hg) indicates a respiratory acidosis, and the low HCO3−HCO_3^- (1717 mEq/L) indicates a metabolic acidosis. Since both abnormalities independently pull the pH downward, they are both primary disorders. The anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-), which is 141−(100+17)=24141 - (100 + 17) = 24 mEq/L. Since this is greater than the normal range of 8-12 mEq/L, the metabolic component is a high anion gap metabolic acidosis, likely due to lactic acidosis from septic shock and acute kidney injury. The primary respiratory acidosis is expected in this patient due to 'permissive hypercapnia', a strategy used in ARDS management where low tidal volumes (66 mL/kg) are employed to prevent ventilator-induced lung injury (volutrauma). This deliberate hypoventilation inherently leads to CO2CO_2 retention. When applying Winter's formula (PaCO2=1.5×HCO3−+8±2P_aCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) to determine the expected physiological PaCO2P_aCO_2 for the patient's metabolic acidosis, the result is 31.531.5 to 35.535.5 mm Hg. The actual PaCO2P_aCO_2 of 5252 mm Hg is significantly higher, mathematically confirming the concurrent primary respiratory acidosis.

Adım Adım Çözüm

1
Analyze the pH to determine the primary disturbance direction.
pH=7.14pH = 7.14, which indicates severe acidemia.
Normal pH is 7.35-7.45. A value below 7.35 requires finding the primary acidic driver.
2
Evaluate PaCO2P_aCO_2 and HCO3−HCO_3^- to identify primary disorders.
PaCO2P_aCO_2 is elevated (5252 mm Hg) and HCO3−HCO_3^- is low (1717 mEq/L).
Because both the respiratory component (high CO2CO_2) and the metabolic component (low HCO3−HCO_3^-) independently lower the pH, both are primary disorders.
3
Calculate the anion gap (AG) to classify the metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=141−(100+17)=24AG = Na^+ - (Cl^- + HCO_3^-) = 141 - (100 + 17) = 24 mEq/L.
A normal AG is 8-12 mEq/L. An AG of 24 defines a High Anion Gap Metabolic Acidosis (HAGMA).
4
Apply Winter's formula to determine expected respiratory compensation for the HAGMA.
Expected PaCO2=1.5×17+8±2=33.5±2P_aCO_2 = 1.5 \times 17 + 8 \pm 2 = 33.5 \pm 2 mm Hg. The actual PaCO2P_aCO_2 is 5252 mm Hg.
Since the actual PaCO2P_aCO_2 (5252 mm Hg) is much higher than the expected range (31.5−35.531.5 - 35.5 mm Hg), it confirms an additional primary respiratory acidosis.

Anahtar Kavram

Diagnosis of mixed acid-base disorders using the anion gap and Winter's formula in the critical care setting.
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Konular

Pulmonary and Critical Care

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