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Zorluk: ZorNephrology

A 26-year-old woman is brought to the emergency department by her roommate due to severe anxiety, shortness of breath, and tingling around her mouth and hands. Her symptoms began 30 minutes ago following an argument. Her past medical history is significant for migraine headaches and a seizure disorder, which are managed with a daily medication she started 6 months ago. On physical examination, she appears anxious and is hyperventilating. Her lungs are clear to auscultation bilaterally. Inflation of a blood pressure cuff above her systolic pressure for 3 minutes elicits carpopedal spasm.

Vital signs are:
Temperature: 37.0°C (98.6°F)
Blood pressure: 115/75 mm Hg
Heart rate: 112/min
Respiratory rate: 32/min
Oxygen saturation: 99% on room air

Arterial blood gas (room air):
pH: 7.53
pCO2pCO_2: 18 mm Hg
pO2pO_2: 102 mm Hg

Serum chemistries:
Sodium: 139 mEq/L
Potassium: 3.7 mEq/L
Chloride: 114 mEq/L
Bicarbonate: 14 mEq/L

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

  1. Non-anion gap metabolic acidosis and respiratory alkalosisCevap
  2. B
    Non-anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Acute respiratory alkalosis with appropriate metabolic compensation
  4. D
    Chronic respiratory alkalosis with appropriate metabolic compensation
  5. E
    High anion gap metabolic acidosis and respiratory alkalosis

Cevap

Non-anion gap metabolic acidosis and respiratory alkalosis
This patient has a mixed acid-base disorder consisting of an acute respiratory alkalosis and a non-anion gap metabolic acidosis (NAGMA). The alkalemia (pH 7.53) and low pCO2pCO_2 (18 mm Hg) confirm a primary respiratory alkalosis due to acute hyperventilation from a panic attack (onset 30 minutes ago). The acute metabolic compensation for respiratory alkalosis dictates that HCO3−HCO_3^- should fall by 2 mEq/L for every 10 mm Hg drop in pCO2pCO_2. With a pCO2pCO_2 drop of 22 mm Hg, the expected HCO3−HCO_3^- is approximately 19.6 mEq/L. Because the patient's actual HCO3−HCO_3^- is much lower (14 mEq/L), a concomitant metabolic acidosis must be present. The anion gap is normal (139−(114+14)=11139 - (114 + 14) = 11), confirming a NAGMA. Clinically, this patient's underlying NAGMA is likely due to topiramate (used for migraines and seizures), which causes a Type 2 renal tubular acidosis via carbonic anhydrase inhibition.

Adım Adım Çözüm

1
Analyze the pH to determine the primary disorder.
The pH is 7.53, indicating alkalemia. The pCO2pCO_2 is 18 mm Hg (low), which is consistent with the alkalemia, identifying a primary respiratory alkalosis.
The direction of the pH deviation determines the primary acid-base disturbance.
2
Calculate the expected metabolic compensation for the acute respiratory alkalosis.
In acute respiratory alkalosis, HCO3−HCO_3^- decreases by 2 mEq/L for every 10 mm Hg decrease in pCO2pCO_2. The pCO2pCO_2 dropped by 22 (from 40 to 18). Expected HCO3−HCO_3^- drop = (22/10)×2=4.4(22/10) \times 2 = 4.4 mEq/L. Expected HCO3−HCO_3^- = 24−4.4=19.624 - 4.4 = 19.6 mEq/L.
Calculating expected compensation is necessary to determine if a secondary acid-base disorder is present.
3
Compare actual bicarbonate to expected bicarbonate.
The actual HCO3−HCO_3^- is 14 mEq/L, which is significantly lower than the expected 19.6 mEq/L. This indicates a superimposed metabolic acidosis.
A measured value lower than the expected compensated value confirms the presence of an independent acidotic process.
4
Calculate the anion gap to characterize the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)=139−(114+14)=11Na^+ - (Cl^- + HCO_3^-) = 139 - (114 + 14) = 11 mEq/L.
An anion gap of 11 is normal (reference 8-12 mEq/L), classifying the secondary disorder as a non-anion gap metabolic acidosis (NAGMA).

Anahtar Kavram

Mixed Acid-Base Disorders: Respiratory Alkalosis and Non-Anion Gap Metabolic Acidosis
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Konular

Nephrology

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