Examkin

Question

Difficulty: Very hardNephrology

A 68-year-old woman is brought to the emergency department by her son due to progressive lethargy and confusion over the past 24 hours. The son reports that she has experienced severe, watery diarrhea and intractable vomiting for the last 4 days following a suspected foodborne illness. Her past medical history is significant for severe chronic obstructive pulmonary disease (COPD) requiring home oxygen therapy. Upon arrival, her temperature is 37.1∘37.1^\circ C (98.8∘98.8^\circ F), blood pressure is 82/5082/50 mmHg, heart rate is 124124/min, and respiratory rate is 2828/min. Physical examination reveals a somnolent patient with profoundly dry mucous membranes, decreased skin turgor, and diffuse bilateral wheezing.

Laboratory studies reveal:

TestResult
Sodium140140 mEq/L
Potassium3.13.1 mEq/L
Chloride8686 mEq/L
Bicarbonate1414 mEq/L
Blood urea nitrogen6262 mg/dL
Serum creatinine2.82.8 mg/dL
Albumin4.04.0 g/dL
Lactate7.47.4 mmol/L

An arterial blood gas obtained on room air shows:

ParameterResult
pH7.117.11
PaCO2_24545 mmHg
PaO2_25858 mmHg

Which of the following combinations most accurately characterizes this patient's acid-base status?

  1. High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosisAnswer
  2. B
    High anion gap metabolic acidosis, normal anion gap metabolic acidosis, and respiratory acidosis
  3. C
    High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
  4. D
    High anion gap metabolic acidosis and respiratory acidosis only
  5. E
    Normal anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

Answer

The patient's acid-base status is best characterized by a high anion gap metabolic acidosis, a metabolic alkalosis, and a respiratory acidosis.
The correct combination characterizes a classic triple acid-base disorder. First, the anion gap is 4040 mEq/L (140−(86+14)140 - (86 + 14)), diagnosing a high anion gap metabolic acidosis (HAGMA) secondary to lactic acidosis from shock. Second, calculating the delta gap (40−12=2840 - 12 = 28) and adding it to the measured bicarbonate (14+28=4214 + 28 = 42) yields an expected bicarbonate greater than 2424 mEq/L, confirming a concurrent metabolic alkalosis (from intractable vomiting). Finally, Winter's formula predicts a PaCO 2_2 of 27−3127-31 mmHg. The measured PaCO 2_2 of 4545 mmHg is inappropriately high, confirming a superimposed respiratory acidosis due to COPD exacerbation and respiratory muscle fatigue.

Step-by-Step Solution

1
Analyze the primary disturbance.
The pH is 7.117.11 (acidemia) and the HCO 3−_3^- is 1414 mEq/L (low). This indicates a primary metabolic acidosis.
Identifying the primary direction of pH and matching it with the appropriate buffer system change establishes the baseline disorder.
2
Calculate the anion gap (AG).
AG = Na +^+ - (Cl −^- + HCO 3−_3^-) = 140−(86+14)=40140 - (86 + 14) = 40 mEq/L. Since 4040 > 1212, a high anion gap metabolic acidosis (HAGMA) is present.
An elevated anion gap confirms the presence of unmeasured anions, in this case, lactate due to tissue hypoperfusion.
3
Calculate the delta gap (Δ\Delta AG) and expected bicarbonate.
Δ\Delta AG = 40−12=2840 - 12 = 28 mEq/L. The patient's expected HCO 3−_3^- (if HAGMA were the only metabolic disorder) is: Measured HCO 3−_3^- + Δ\Delta AG = 14+28=4214 + 28 = 42 mEq/L. Since 4242 is significantly greater than a normal HCO 3−_3^- of 2424, a concurrent metabolic alkalosis is present.
The delta-delta ratio or expected bicarbonate calculation uncovers hidden metabolic disorders. In this patient, vomiting induced a severe metabolic alkalosis that partially masked the severity of the bicarbonate drop from the HAGMA.
4
Calculate expected respiratory compensation using Winter's formula.
Expected PaCO 2_2 = (1.5×14)+8=29±2(1.5 \times 14) + 8 = 29 \pm 2 mmHg. The measured PaCO 2_2 is 4545 mmHg.
Winter's formula determines if the respiratory response to metabolic acidosis is adequate. Since the measured PaCO 2_2 (4545 mmHg) is higher than the expected range (27−3127-31 mmHg), there is an uncompensated accumulation of CO 2_2, indicating a superimposed respiratory acidosis.

Key Concept

Diagnosis of a triple acid-base disorder requires sequential calculation of the anion gap, the delta-delta gap (or expected bicarbonate), and Winter's formula.
Estimated Time:3m 0s
Rate this question

Topics

Nephrology

More questions from this topic