Paramedics transport a -year-old male construction worker to the emergency department after he was trapped under a collapsed concrete wall for hours. En route to the hospital, he was administered mg of intravenous hydromorphone due to severe pain in his legs. Upon arrival, the patient appears lethargic and somnolent. Vital signs include a blood pressure of mm Hg, pulse of /min, and respiratory rate of /min. Physical examination reveals tense, swollen, and exquisitely tender bilateral lower extremities.
Laboratory results are as follows:
| Test | Value |
|---|---|
| Sodium | mEq/L |
| Potassium | mEq/L |
| Chloride | mEq/L |
| Bicarbonate | mEq/L |
| Blood urea nitrogen | mg/dL |
| Creatinine | mg/dL |
Arterial blood gas analysis on room air:
| Parameter | Value |
|---|---|
| mm Hg | |
| mm Hg |
Based on the laboratory findings, which of the following best describes this patient's acid-base status?
- APrimary high anion gap metabolic acidosis with appropriate respiratory compensation
- BPrimary high anion gap metabolic acidosis with superimposed respiratory alkalosis
- CPrimary normal anion gap metabolic acidosis with superimposed respiratory acidosis
- Primary high anion gap metabolic acidosis with superimposed respiratory acidosisAnswer
- EPrimary respiratory acidosis with appropriate metabolic compensation
Answer
Primary high anion gap metabolic acidosis with superimposed respiratory acidosis
This patient has acidemia () driven by a significantly reduced bicarbonate level ( mEq/L), defining a primary metabolic acidosis. The anion gap is elevated at (), indicating a high anion gap metabolic acidosis (HAGMA). In this vignette, the HAGMA is likely multifactorial, secondary to rhabdomyolysis-induced acute kidney injury (evidenced by elevated BUN, creatinine, and hyperkalemia from muscle breakdown) as well as potential lactic acidosis from local muscle ischemia.
To evaluate for secondary acid-base disorders, Winter's formula must be used: Expected . For this patient, the expected mm Hg. The measured ( mm Hg) is significantly higher than the expected range (- mm Hg). This means the patient is hypoventilating relative to his severe metabolic acidemia, which diagnoses a superimposed primary respiratory acidosis. This clinically aligns with opioid-induced respiratory depression stemming from the hydromorphone administered by EMS (supported by his bradypnea of /min and lethargy).
To evaluate for secondary acid-base disorders, Winter's formula must be used: Expected . For this patient, the expected mm Hg. The measured ( mm Hg) is significantly higher than the expected range (- mm Hg). This means the patient is hypoventilating relative to his severe metabolic acidemia, which diagnoses a superimposed primary respiratory acidosis. This clinically aligns with opioid-induced respiratory depression stemming from the hydromorphone administered by EMS (supported by his bradypnea of /min and lethargy).
Step-by-Step Solution
Key Concept
Patients with metabolic acidosis must be evaluated using Winter's formula to detect superimposed respiratory disorders. A measured higher than the expected calculated range indicates a concurrent respiratory acidosis.