A 62-year-old man presents to the emergency department with confusion. The patient’s wife states that her husband has become more somnolent over the past several days and now is very confused. The patient has no complaints himself, but is answering questions inappropriately. The patient has a past medical history of diabetes and hypertension. His temperature is 98.3°F (36.8°C), blood pressure is 127/85 mmHg, pulse is 138/min, respirations are 14/min, and oxygen saturation is 99% on room air. Physical exam is notable for a confused man with dry mucous membranes. Initial laboratory studies are ordered as seen below. Serum: Na+: 135 mEq/L Cl-: 100 mEq/L K+: 3.0 mEq/L HCO3-: 23 mEq/L BUN: 30 mg/dL Glucose: 1,299 mg/dL Creatinine: 1.5 mg/dL Ca2+: 10.2 mg/dL Which of the following is the most appropriate initial treatment for this patient?

Insulin, normal saline, and potassium

A 28-year-old research assistant is brought to the emergency department for severe chemical burns 30 minutes after accidentally spilling hydrochloric acid on himself. The burns cover both hands and forearms. His temperature is 37°C (98.6°F), pulse is 112/min, respirations are 20/min, and blood pressure is 108/82 mm Hg. Initial stabilization and resuscitation is begun, including respiratory support, fluid resuscitation, and cardiovascular stabilization. The burned skin is irrigated with saline water to remove the chemical agent. Which of the following is the most appropriate method to verify adequate fluid infusion in this patient?

Pulmonary capillary wedge pressure

A 48-year-old woman is transferred from her primary care physician's office to the emergency department for further evaluation of hypokalemia to 2.5 mEq/L. She was recently diagnosed with hypertension 2 weeks ago and started on medical therapy. The patient said that she enjoys all kinds of food and exercises regularly, but has not been able to complete her workouts as she usually does. Her temperature is 97.7°F (36.5°C), blood pressure is 107/74 mmHg, pulse is 80/min, respirations are 15/min, and SpO2 is 94% on room air. Her physical exam is unremarkable. Peripheral intravenous (IV) access is obtained. Her basic metabolic panel is obtained below. Serum: Na+: 135 mEq/L Cl-: 89 mEq/L K+: 2.2 mEq/L HCO3-: 33 mEq/L BUN: 44 mg/dL Glucose: 147 mg/dL Creatinine: 2.3 mg/dL Magnesium: 2.0 mEq/L What is the next best step in management?

Obtain urine sodium and creatinine

Administer isotonic saline 1 liter via peripheral IV

Administer potassium chloride 40mEq via peripheral IV

Administer potassium bicarbonate 50mEq per oral

Three days after being admitted to the hospital because of a fall from the roof of a two-story building, a 27-year-old man is being monitored in the intensive care unit. On arrival, the patient was somnolent and not oriented to person, place, or time. A CT scan of the head showed an epidural hemorrhage that was 45 cm3 in size and a midline shift of 7 mm. Emergency surgery was performed with craniotomy and hematoma evacuation on the day of admission. Perioperatively, a bleeding vessel was identified and ligated. Postoperatively, the patient was transferred to the intensive care unit and placed on a ventilator. His temperature is 37°C (98.6°F), pulse is 67/min, and blood pressure is 117/78 mm Hg. The ventilator is set at a FiO2 of 55%, tidal volume of 520 mL, and positive end-expiratory pressure of 5.0 cm H2O. In addition to intravenous administration of fluids, which of the following is the most appropriate next step in managing this patient's nutrition?

Enteral feeding via nasogastric tube

Enteral feeding using a percutaneous endoscopic gastrostomy (PEG) tube

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

Replacement of ongoing losses for USMLE Step 1: High-Yield Surgery Lessons

Ongoing Losses - The Body's Leaks

Ongoing losses are abnormal fluid and electrolyte deficits occurring in real-time, distinct from maintenance or pre-existing deficit corrections. Replacement therapy must match the volume and composition of the fluid being lost.

Gastrointestinal (GI):
- Nasogastric tube (NGT) output, emesis
- Fistula / Stoma output
- Diarrhea
Renal:
- Polyuria (e.g., diabetes insipidus)
Insensible / Third Space:
- Fever, tachypnea
- Burns, pancreatitis, sepsis

Daily GI fluid and electrolyte balance

⭐ Gastric fluid is rich in H⁺, Cl⁻, and K⁺. Large volume losses via NGT suction or vomiting classically cause hypochloremic, hypokalemic metabolic alkalosis.

Fluid Composition - What's in the Goo?

Knowing the electrolyte profile of lost fluids is key to selecting appropriate replacement therapy. Values are typical mEq/L ranges.

Fluid Source	$Na^+$	$K^+$	$Cl^-$	$HCO_3^-$
Saliva	10	26	10	30
Gastric (Stomach)	60	10	130	0
Pancreatic	140	5	75	115
Bile	145	5	100	35
Small Bowel	110	5	105	30
Diarrhea/Ileostomy	130	20-30	110	45

Pancreatic/Bile/Small Bowel: High in $HCO_3^-$. Losses from fistulas or drains can cause metabolic acidosis.

⭐ Exam Favorite: Diarrheal fluid is rich in potassium and bicarbonate. Significant losses, especially in children, can rapidly lead to hypokalemia and non-anion gap metabolic acidosis.

Replacement Strategy - The Right Stuff

Principle: Replace ongoing losses volume-for-volume (e.g., mL-for-mL) in a set time frame, typically every 4-6 hours.
Fluid Choice Algorithm: The replacement fluid depends on the source of the loss. Match the effluent with the appropriate solution.

⭐ For high-volume nasogastric tube (NGT) output (>1.5 L/day), check gastric fluid electrolytes every 12h and tailor replacement fluids accordingly to prevent metabolic alkalosis.

Monitoring - Checking the Gauges

Key Indicators & Targets:
- Urine Output: Goal is >0.5 mL/kg/hr.
- Vitals: Normalizing HR, BP.
- Daily Weights: Sensitive indicator of net fluid balance.
- Labs: Serial electrolytes, BUN/Cr.
- Exam: Moist mucous membranes, normal skin turgor, no edema.

Clinical assessment of fluid status

⭐ In elderly patients, skin turgor is less reliable due to decreased elasticity; check mucous membranes for a more accurate assessment of hydration status.

High-Yield Points - ⚡ Biggest Takeaways

Gastric losses (e.g., NG tube) cause hypokalemic, hypochloremic metabolic alkalosis; replace with Normal Saline + KCl.

Pancreatic, biliary, or high-output ileostomy losses are rich in bicarbonate, causing normal anion gap metabolic acidosis.

Replace bicarbonate-rich losses with an isotonic fluid like Lactated Ringer's to prevent acidosis.

Guide replacement by monitoring serum electrolytes, urine output (>0.5 mL/kg/hr), and vital signs.

Account for third-space losses post-op or in sepsis with isotonic crystalloids.

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