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 7-year-old boy is brought to the emergency room because of severe, acute diarrhea. He is drowsy with a dull, lethargic appearance. He has sunken eyes, poor skin turgor, and dry oral mucous membranes and tongue. He has a rapid, thready pulse with a systolic blood pressure of 60 mm Hg and his respirations are 33/min. His capillary refill time is 6 sec. He has had no urine output for the past 24 hours. Which of the following is the most appropriate next step in treatment?

Start IV fluid resuscitation with normal saline or Ringer’s lactate, along with monitoring of vitals and urine output

Start IV fluid resuscitation by administering colloid solutions

Provide oral rehydration therapy to correct dehydration

Give initial IV bolus of 2 L of Ringer’s lactate, followed by packed red cells, fresh frozen plasma, and platelets in a ratio of 1:1:1

A 35-year-old man is brought to the emergency department from a kitchen fire. The patient was cooking when boiling oil splashed on his exposed skin. His temperature is 99.7°F (37.6°C), blood pressure is 127/82 mmHg, pulse is 120/min, respirations are 12/min, and oxygen saturation is 98% on room air. He has dry, nontender, and circumferential burns over his arms bilaterally, burns over the anterior portion of his chest and abdomen, and tender spot burns with blisters on his shins. A 1L bolus of normal saline is administered and the patient is given morphine and his pulse is subsequently 80/min. A Foley catheter is placed which drains 10 mL of urine. What is the best next step in management?

Additional fluids and escharotomy

Additional fluids and admission to the ICU

A 29-year-old man is brought to the emergency room 6 hours after the onset of severe epigastric pain and vomiting. His heart rate is 110/min and blood pressure is 98/72 mm Hg. He is diagnosed with acute pancreatitis, and fluid resuscitation with normal saline is initiated. Which of the following is the most likely immediate effect of fluid resuscitation in this patient?

Increase in myocardial oxygen demand

Increase in plasma oncotic pressure

Increase in glomerular filtration fraction

Increase in volume of distribution

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

Three hours after undergoing open proctocolectomy for ulcerative colitis, a 42-year-old male complains of abdominal pain. The pain is localized to the periumbilical and hypogastric regions. A total of 20 mL of urine has drained from his urinary catheter since the end of the procedure. Temperature is 37.2°C (98.9°F), pulse is 92/min, respirations are 12/min, and blood pressure is 110/72 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 99%. Physical examination shows a 20 cm vertical midline incision and an ileostomy in the right lower quadrant. There is no fluid drainage from the surgical wounds. The urinary catheter flushes easily and is without obstruction. Cardiopulmonary examination shows no abnormalities. Serum studies show a blood urea nitrogen of 30 mg/dL and a creatinine of 1.3 mg/dL. Which of the following is the most appropriate next step in management?

Administer intravenous furosemide

A 45-year-old man is brought to the emergency department following a house fire. Following initial stabilization, the patient is transferred to the ICU for management of his third-degree burn injuries. On the second day of hospitalization, a routine laboratory panel is obtained, and the results are demonstrated below. Per the nurse, he remains stable compared to the day prior. His temperature is 99°F (37°C), blood pressure is 92/64 mmHg, pulse is 98/min, respirations are 14/min, and SpO2 is 98%. A physical examination demonstrates an unresponsive patient with extensive burn injuries throughout his torso and lower extremities. Hemoglobin: 13 g/dL Hematocrit: 36% Leukocyte count: 10,670/mm^3 with normal differential Platelet count: 180,000/mm^3 Serum: Na+: 135 mEq/L Cl-: 98 mEq/L K+: 4.7 mEq/L HCO3-: 25 mEq/L BUN: 10 mg/dL Glucose: 123 mg/dL Creatinine: 1.8 mg/dL Thyroid-stimulating hormone: 4.3 µU/mL Triiodothyronine: 48 ng/dL Thyroxine: 10 ug/dL Ca2+: 8.7 mg/dL AST: 89 U/L ALT: 135 U/L What is the best course of management for this patient?

Continued management of his burn wounds

Start patient on intravenous ceftriaxone and vancomycin

Regular levothyroxine sodium injections

Immediate administration of propranolol

Crystalloid vs. colloid solutions for USMLE Step 2 CK: High-Yield Surgery Lessons

Body Fluid Compartments - The Battlefield

Total Body Water (TBW) constitutes ~60% of lean body weight.
📌 Rule of 60-40-20 (% of body weight):
- 60% Total Body Water
- 40% Intracellular Fluid (ICF) (~2/3 of TBW)
- 20% Extracellular Fluid (ECF) (~1/3 of TBW)
ECF is further divided:
- Interstitial Fluid: 3/4 of ECF
- Plasma Volume: 1/4 of ECF

⭐ Third-spacing is the sequestration of ECF into serosal cavities (e.g., ascites) or injured tissue, rendering it non-contributory to circulation.

Crystalloids - Salty Water Squad

Aqueous solutions of mineral salts or other water-soluble molecules. The go-to initial fluid for resuscitation.
Mechanism: ↑ intravascular volume, but fluid rapidly redistributes to the entire extracellular space. Only ~25% remains intravascular after 1 hour.

Major Types

Normal Saline (NS - 0.9% NaCl)
- Composition: 154 mEq/L Na⁺, 154 mEq/L Cl⁻.
- Use: General fluid resuscitation, especially in metabolic alkalosis or hyponatremia.
- ⚠️ Warning: Large volumes can cause non-anion gap hyperchloremic metabolic acidosis.
Lactated Ringer’s (LR)
- Composition: More "balanced" or physiologic. Contains Na⁺, Cl⁻, K⁺, Ca²⁺, and lactate (buffer).
- Use: Trauma, burns, sepsis, GI fluid loss. Preferred for most resuscitations.
- 💡 Traditionally avoided in hyperkalemia, but K⁺ content is low (4 mEq/L).

⭐ In patients with diabetic ketoacidosis (DKA), using balanced crystalloids like Lactated Ringer's instead of Normal Saline is associated with a faster resolution of the acidosis.

Colloids - The Plasma Protectors

Mechanism: Solutions with large, osmotically active molecules (e.g., proteins, starches) that are retained in the intravascular space, increasing plasma oncotic pressure.
Primary Function: Potent volume expanders; maintain circulatory volume for a longer duration than crystalloids.
Types & Characteristics:
- Albumin (5%, 25%): Natural plasma protein. Used in hypoalbuminemia, spontaneous bacterial peritonitis (SBP), and hepatorenal syndrome. Less immunogenic.
- Dextrans: Glucose polymers. Risk of anaphylaxis and interference with blood cross-matching.
- Hydroxyethyl Starches (HES): Synthetic. ⚠️ Warning: Associated with ↑ risk of acute kidney injury and coagulopathy; use is now restricted.
Clinical Use: Reserved for specific indications where rapid, sustained volume expansion is critical and crystalloids are insufficient.

⭐ Exam Favorite: The SAFE trial (Saline versus Albumin Fluid Evaluation) showed that in a general ICU population, albumin was as safe as saline for resuscitation but offered no survival benefit. However, a subgroup analysis suggested potential benefit in patients with severe sepsis.

The Showdown - Clash of the Solutions

Feature	Crystalloids (e.g., Normal Saline, LR)	Colloids (e.g., Albumin, Starches)
Composition	Water, electrolytes, small solutes	Large, osmotically active molecules
Distribution	Distributes throughout ECF	Stays intravascularly (↑ oncotic pressure)
Volume	Requires larger volumes (approx. 3:1 ratio)	Smaller volume for same plasma expansion
Use/Risks	Initial resuscitation; risk of edema	Specific uses (burns); expensive, anaphylaxis risk

High‑Yield Points - ⚡ Biggest Takeaways

Crystalloids (e.g., Normal Saline, Lactated Ringer's) are the first-line choice for most initial fluid resuscitation.

They rapidly distribute throughout the entire extracellular fluid (ECF), requiring larger volumes for intravascular expansion.

Colloids (e.g., Albumin) contain large molecules that remain in the intravascular space, making them more potent volume expanders.

Colloid use is generally reserved for specific conditions like severe burns or hypoalbuminemia.

Primary risk of crystalloids is edema; colloids carry risks of anaphylaxis and coagulopathy.

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