A 65-year-old man is brought to the emergency department from his home. He is unresponsive. His son requested a wellness check because he had not heard from his father in 2 weeks. He reports that his father was sounding depressed during a telephone call. The paramedics found a suicide note and a half-empty bottle of antifreeze near the patient. The medical history includes hypertension and hyperlipidemia. The vital signs include: blood pressure 120/80 mm Hg, respiratory rate 25/min, heart rate 95/min, and temperature 37.0°C (98.5°F). He is admitted to the hospital. What do you expect the blood gas analysis to show?

Non-anion gap metabolic acidosis

A 72-year-old man being treated for benign prostatic hyperplasia (BPH) is admitted to the emergency department for 1 week of dysuria, nocturia, urge incontinence, and difficulty initiating micturition. His medical history is relevant for hypertension, active tobacco use, chronic obstructive pulmonary disease, and BPH with multiple urinary tract infections. Upon admission, he is found with a heart rate of 130/min, respiratory rate of 19/min, body temperature of 39.0°C (102.2°F), and blood pressure of 80/50 mm Hg. Additional findings during the physical examination include decreased breath sounds, wheezes, crackles at the lung bases, and intense right flank pain. A complete blood count shows leukocytosis and neutrophilia with a left shift. A sample for arterial blood gas analysis (ABG) was taken, which is shown below. Laboratory test Serum Na+ 140 mEq/L Serum Cl- 102 mEq/L Serum K+ 4.8 mEq/L Serum creatinine (SCr) 2.3 mg/dL Arterial blood gas pH 7.12 Po2 82 mm Hg Pco2 60 mm Hg SO2% 92% HCO3- 12.0 mEq/L Which of the following best explains the patient’s condition?

Metabolic acidosis complicated by respiratory acidosis

Metabolic acidosis complicated by respiratory alkalosis

Non-anion gap metabolic acidosis

Respiratory alkalosis complicated by metabolic acidosis

Respiratory acidosis complicated by metabolic alkalosis

A 14-year-old male presents to the emergency department with altered mental status. His friends who accompanied him said that he complained of abdominal pain while camping. They denied his consumption of anything unusual from the wilderness, or any vomiting or diarrhea. His temperature is 100.5°F (38.1°C), blood pressure is 95/55 mmHg, pulse is 130/min, and respirations are 30/min. His pupils are equal and reactive to light bilaterally. The remainder of the physical exam is unremarkable. His basic metabolic panel is displayed below: Serum: Na+: 116 mEq/L Cl-: 70 mEq/L K+: 4.0 mEq/L HCO3-: 2 mEq/L BUN: 50 mg/dL Glucose: 1010 mg/dL Creatinine: 1.2 mg/dL While the remainder of his labs are pending, the patient becomes bradypneic and is intubated. His ventilator is adjusted to volume control assist-control with a respiratory rate (RR) of 14/min, tidal volume (Vt) of 350 mL, positive end-expiratory pressure (PEEP) of 5 cm H2O, and fractional inspired oxygen (FiO2) of 40%. His height is 5 feet 5 inches. Intravenous fluids and additional medical therapy are administered. An arterial blood gas obtained after 30 minutes on these settings shows the following: pH: 7.05 pCO2 :40 mmHg pO2: 150 mmHg SaO2: 98% What is the best next step in management?

Increase respiratory rate and tidal volume

Increase tidal volume and positive end-expiratory pressure

Increase positive end-expiratory pressure

A 58-year-old man presents to the emergency department with a chief complaint of ringing in his ears that started several hours previously that has progressed to confusion. The patient denies any history of medical problems except for bilateral knee arthritis. He was recently seen by an orthopedic surgeon to evaluate his bilateral knee arthritis but has opted to not undergo knee replacement and prefers medical management. His wife noted that prior to them going on a hike today, he seemed confused and not himself. They decided to stay home, and roughly 14 hours later, he was no longer making any sense. Physical exam is notable for a confused man. The patient's vitals are being performed and his labs are being drawn. Which of the following is most likely to be seen on blood gas analysis?

pH: 7.30, PaCO2: 15 mmHg, HCO3-: 16 mEq/L

pH: 7.37, PaCO2: 41 mmHg, HCO3-: 12 mEq/L

pH: 7.41, PaCO2: 65 mmHg, HCO3-: 34 mEq/L

pH: 7.47, PaCO2: 11 mmHg, HCO3-: 24 mEq/L

pH: 7.31, PaCO2: 31 mmHg, HCO3-: 15 mEq/L

A person is exercising strenuously on a treadmill for 1 hour. An arterial blood gas measurement is then taken. Which of the following are the most likely values?

pH 7.57 PaO2 100, PCO2 23, HCO3 21

pH 7.56, PaO2 100, PCO2 44, HCO3 38

pH 7.32, PaO2 42, PCO2 50, HCO3 27

pH 7.38, PaO2 100, PCO2 69 HCO3 42

pH 7.36, PaO2 100, PCO2 40, HCO3 23

A 32-year-old woman is admitted to the emergency department for 36 hours of intense left-sided back pain that extends into her left groin. She reports that the pain started a day after a charitable 5 km (3.1 mi) marathon. The past medical history is relevant for multiple complaints of eye dryness and dry mouth. Physical examination is unremarkable, except for intense left-sided costovertebral pain. The results from laboratory tests are shown. Laboratory test Result Serum Na+ 137 Serum Cl- 110 Serum K+ 3.0 Serum creatinine (SCr) 0.82 Arterial blood gas Result pH 7.28 pO2 98 mm Hg pCO2 28.5 mm Hg SaO2% 98% HCO3- 15 mm Hg Which of the following explains this patient’s condition?

Decreased renal excretion of hydrogen ions (H+)

Decreased bicarbonate renal absorption

Decreased synthesis of ammonia (NH3)

Decreased excretion of nonvolatile acids

Metabolic acidosis mechanisms and compensation for USMLE Step 2 CK: High-Yield Physiology Lessons

Metabolic Acidosis - The Acid Test

Pathophysiology: Primary ↓ in serum bicarbonate ($HCO_3^−$), leading to ↓ blood pH.
Compensation: Hyperventilation (Kussmaul respirations) to ↓ $pCO_2$.
- Winter's Formula predicts response: Expected $pCO_2 = (1.5 \times [HCO_3^−]) + 8 \pm 2$.
Anion Gap (AG): Key diagnostic step. $AG = [Na^+] - ([Cl^−] + [HCO_3^−])$. Normal: 8-12 mEq/L.

⭐ Ingestion of ethylene glycol (antifreeze) or methanol are classic causes of high anion gap metabolic acidosis where prompt recognition and treatment are life-saving.

📌 HAGMA Mnemonic (MUDPILES):

Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.

Anion Gap - Mind the Gap

Calculated to differentiate causes of metabolic acidosis.
Formula: $AG = Na^+ - (Cl^- + HCO_3^-)$
Normal Range: 8-12 mEq/L. Represents unmeasured anions (mostly albumin).
High Anion Gap (HAGMA): Caused by accumulation of unmeasured organic acids.
- Glycols, Oxoproline, L-Lactate, D-Lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.
Normal Anion Gap (NAGMA): Hyperchloremic acidosis due to loss of bicarbonate.
- Hyperalimentation, Acetazolamide, Renal Tubular Acidosis, Diarrhea, Uretero-enteric fistula, Pancreatic fistula.

Approach to Elevated Anion Gap Metabolic Acidosis

⭐ Always correct the anion gap for albumin. For every 1 g/dL decrease in albumin (from 4.0), add 2.5 to the calculated anion gap.

Normal Anion Gap - No Gap, No Problem?

Hyperchloremic Metabolic Acidosis: Characterized by loss of bicarbonate ($HCO_3^−$) compensated by an equivalent increase in chloride ($Cl^−$), thus maintaining a normal anion gap.
Causes (📌 HARDUP):
- Hyperalimentation / Hyperchloremia
- Acetazolamide / Addison's disease
- Renal Tubular Acidosis (RTA)
- Diarrhea (most common cause)
- Ureteroenteric fistula
- Pancreatic fistula
Key Diagnostic Step: Calculate the Urine Anion Gap (UAG) to differentiate between renal and non-renal causes.
- $UAG = (U_{Na^+} + U_{K^+}) - U_{Cl^−}$

⭐ A negative UAG suggests a non-renal cause like diarrhea. The kidneys are functioning properly and are excreting excess acid as ammonium chloride ($NH_4Cl$), increasing urinary $Cl^−$ and making the UAG negative.

Compensation - The Body's Buffer Zone

Primary Response: Lungs rapidly expel CO₂ to counteract acidosis.
Respiratory Compensation: Hyperventilation (Kussmaul respirations) creates a compensatory respiratory alkalosis.
- Predict the expected PaCO₂ with Winter's Formula:
  - $Expected PaCO₂ = (1.5 × [HCO₃⁻]) + 8 ± 2$
- If measured PaCO₂ is higher, suspect a concurrent respiratory acidosis.

⭐ Respiratory compensation is fast, initiating within minutes and peaking at 12-24 hours. It is the most important compensatory mechanism in metabolic acidosis.

Kussmaul Breathing Patterns in Metabolic Acidosis

Renal Compensation: Slower (days); kidneys increase H⁺ excretion and HCO₃⁻ reabsorption.

High-Yield Points - ⚡ Biggest Takeaways

Metabolic acidosis is characterized by a primary decrease in serum HCO₃⁻.

The main compensatory mechanism is hyperventilation to decrease PaCO₂.

Always calculate the anion gap to narrow the differential diagnosis between HAGMA and NAGMA.

Use Winter's formula (Expected PaCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2) to determine if respiratory compensation is adequate.

HAGMA involves the addition of an unmeasured acid (e.g., MUDPILES).

NAGMA involves the loss of bicarbonate (e.g., HARDASS).

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