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 24-year-old woman presents to the emergency department after she was found agitated and screaming for help in the middle of the street. She says she also has dizziness and tingling in the lips and hands. Her past medical history is relevant for general anxiety disorder, managed medically with paroxetine. At admission, her pulse is 125/min, respiratory rate is 25/min, and body temperature is 36.5°C (97.7°F). Physical examination is unremarkable. An arterial blood gas sample is taken. Which of the following results would you most likely expect to see in this patient?

pH: increased, HCO3-: decreased, Pco2: decreased

pH: increased, HCO3-: increased, Pco2: increased

pH: decreased, HCO3-: decreased, Pco2: decreased

pH: decreased, HCO3-: increased, Pco2: increased

pH: normal, HCO3-: increased, Pco2: increased

A 27-year-old man presents to the emergency department with his family because of abdominal pain, excessive urination, and drowsiness since the day before. He has had type 1 diabetes mellitus for 2 years. He ran out of insulin 2 days ago. The vital signs at admission include: temperature 36.8°C (98.2°F), blood pressure 102/69 mm Hg, and pulse 121/min. On physical examination, he is lethargic and his breathing is rapid and deep. There is a mild generalized abdominal tenderness without rebound tenderness or guarding. His serum glucose is 480 mg/dL. Arterial blood gas of this patient will most likely show which of the following?

↓ pH, ↓ bicarbonate and ↑ anion gap

↑ pH, ↑ bicarbonate, and normal pCO2

↑ pH, normal bicarbonate and ↓ pCO2

↓ pH, ↓ bicarbonate and normal anion gap

↓ pH, normal bicarbonate and ↑ pCO2

What is the primary mechanism for maintaining acid-base balance during prolonged vomiting?

Increased bicarbonate excretion

Increased chloride reabsorption

Three days after undergoing an open cholecystectomy, a 73-year-old man has fever and abdominal pain. He has hypertension, type 2 diabetes mellitus, chronic obstructive pulmonary disease, and benign prostatic hyperplasia. He had smoked one pack of cigarettes daily for 40 years but quit 1 year ago. He does not drink alcohol. Prior to admission to the hospital, his medications included lisinopril, metformin, ipratropium, and tamsulosin. He appears acutely ill and lethargic. His temperature is 39.5°C (103.1°F), pulse is 108/min, respirations are 18/min, and blood pressure is 110/84 mm Hg. He is oriented only to person. Examination shows a 10-cm subcostal incision that appears dry and non-erythematous. Scattered expiratory wheezing is heard throughout both lung fields. His abdomen is distended with tenderness to palpation over the lower quadrants. Laboratory studies show: Hemoglobin 10.1 g/dl Leukocyte count 19,000/mm3 Serum Glucose 180 mg/dl Urea Nitrogen 25 mg/dl Creatinine 1.2 mg/dl Lactic acid 2.5 mEq/L (N = 0.5 - 2.2 mEq/L) Urine Protein 1+ RBC 1–2/hpf WBC 32–38/hpf Which of the following is the most likely underlying mechanism of this patient's current condition?

Intraabdominal abscess formation

A 58-year-old cirrhotic man with ascites undergoes large volume paracentesis (6 liters removed). Four hours later, he becomes hypotensive (BP 80/50 mmHg) and tachycardic (HR 115/min). Labs show: Cr 2.1 mg/dL (baseline 1.0), Na+ 128 mEq/L, Hct 38%. What is the most appropriate immediate management?

5% albumin 6-8 grams per liter of ascites removed

Octreotide and midodrine for hepatorenal syndrome

Vasopressors to maintain blood pressure

Re-infusion of filtered ascitic fluid

A 42-year-old woman with prolonged vomiting from gastroparesis is admitted with weakness. Labs show: K+ 2.1 mEq/L, pH 7.51, HCO3- 42 mEq/L, Mg2+ 1.4 mg/dL. She receives 80 mEq of IV potassium chloride over 24 hours, but repeat K+ is 2.3 mEq/L. What explains the refractory hypokalemia?

Hypomagnesemia preventing potassium retention

Secondary hyperaldosteronism from volume depletion

Insufficient potassium replacement dose

Ongoing losses from continued vomiting

Metabolic alkalosis promoting intracellular potassium shift

A 65-year-old diabetic man with TURP syndrome presents with confusion, nausea, and seizures 2 hours post-operatively. Labs show: Na+ 115 mEq/L, serum osmolality 240 mOsm/kg. He weighs 70 kg. What is the target sodium correction rate and fluid management strategy?

3% saline to increase sodium by 4-6 mEq/L in first 2-4 hours, then slower correction

Conivaptan infusion for water diuresis

Normal saline infusion with fluid restriction

Rapid correction to 135 mEq/L with 3% hypertonic saline over 4 hours

Increase sodium by 6-8 mEq/L in first 24 hours with 3% saline boluses

Acid-base disorders in surgical patients for USMLE Step 2 CK: High-Yield Surgery Lessons

Acid-Base Basics - The Body's pH Tango

pH: Critical measure of acidity. Normal range: 7.35-7.45.
- < 7.35 = Acidosis
- 7.45 = Alkalosis
Regulation: A delicate balance maintained by two main organs:
- Lungs (Respiratory): Rapidly adjust $PaCO_2$ (acid). Normal: 35-45 mmHg.
- Kidneys (Metabolic): Slowly regulate $HCO_3^-$ (base). Normal: 22-26 mEq/L.
Governing Principle: The Henderson-Hasselbalch equation: $pH \propto \frac{[HCO_3^-]}{PaCO_2}$.

⭐ Exam Favourite: Acidosis causes extracellular potassium ($K^+$) excess (hyperkalemia) as $H^+$ ions move into cells, forcing $K^+$ out. Alkalosis does the opposite, causing hypokalemia.

Metabolic Acidosis - Sour Grapes

Primary Disturbance: Low pH from a primary decrease in serum bicarbonate ($HCO_3^-$). Compensatory response is hyperventilation to decrease $pCO_2$.
Key Diagnostic Step: Calculate the anion gap (AG) to narrow the differential diagnosis.
- Formula: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$
- Normal AG is < 12 mEq/L.

High AG (HAGMA) Causes: 📌 MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.
Normal AG (NAGMA) Causes: 📌 HARDUPS: Hyperalimentation, Acetazolamide, RTA, Diarrhea, Uretero-enteric fistula, Pancreatic fistula, Saline.

⭐ In surgical patients, NAGMA is commonly caused by loss of bicarbonate-rich fluids. Think diarrhea, a high-output ileostomy, or a pancreatic/biliary fistula.

Metabolic Alkalosis - Basic Instinct

Etiology: Most common acid-base disorder in surgical patients, from loss of H⁺ or gain of bicarbonate ($HCO_3^-$).

Key Causes: Vomiting, nasogastric (NG) suction, diuretic therapy, massive blood transfusion (citrate metabolism).

Diagnosis: ABG shows pH > 7.45, ↑ PaCO₂, ↑ $HCO_3^-$. Urine chloride (UCl) is key to management.

⭐ Paradoxical Aciduria: With severe hypokalemia, kidneys excrete H⁺ instead of K⁺ to conserve potassium, causing acidic urine despite systemic alkalosis.

Respiratory Rollercoaster - Huffing & Puffing

📌 ROME: Respiratory is Opposite (↑$pCO_2$ leads to ↓pH; ↓$pCO_2$ leads to ↑pH).

Respiratory Acidosis (Hypoventilation)
- Primary Problem: Inadequate alveolar ventilation causing $CO_2$ retention.
- Surgical Causes: Post-op atelectasis, CNS depression (opioids, anesthesia), airway obstruction, severe pneumonia.
- Labs: pH < 7.35, $pCO_2$ > 45 mmHg.
- Compensation: Slow renal $HCO_3^-$ retention.
Respiratory Alkalosis (Hyperventilation)
- Primary Problem: Excessive $CO_2$ elimination.
- Surgical Causes: Pain/anxiety (most common), sepsis, iatrogenic over-ventilation.
- Labs: pH > 7.45, $pCO_2$ < 35 mmHg.
- Compensation: Slow renal $HCO_3^-$ excretion.

Davenport Diagram: Uncompensated Acid-Base Disorders

⭐ In acute respiratory acidosis, expect $HCO_3^-$ to increase by 1 mEq/L for every 10 mmHg rise in $pCO_2$. In chronic states (>48h), it compensates much more, rising by 3-4 mEq/L.

High-Yield Points - ⚡ Biggest Takeaways

Metabolic acidosis with an elevated anion gap suggests lactic acidosis from shock or sepsis.

Metabolic alkalosis, the most common disorder, is typically from NG tube suction or vomiting and is saline-responsive.

Respiratory acidosis stems from hypoventilation (e.g., opioids, airway obstruction).

Respiratory alkalosis is caused by hyperventilation (e.g., pain, anxiety, mechanical over-ventilation).

Always treat the underlying cause revealed by the ABG, not just the numbers.

Lungs compensate rapidly (minutes-hours); kidneys compensate slowly (days).

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play