ABSITE 2026 Review: Fluids and Electrolytes for Surgical Exams

Understanding Body Water Distribution

• 🎯 Total body water in adults is approximately 60% in males and 50% in females by weight (in kg).
• 💧 This water is divided into intracellular (2/3) and extracellular (1/3) compartments.
• 🏥 Of the extracellular fluid, 1/4 is intravascular and 3/4 is extravascular.
• 👶 Average blood volume in pediatric patients is calculated at 80 cc/kg.

Maintenance and Resuscitative Fluids

• 🏥 Maintenance fluids for adults are typically D5 half normal saline with potassium; for pediatric patients, D5 normal saline with potassium; and for neonates, D5 quarter normal saline with potassium.
• ⚡ Resuscitative fluids commonly include isotonic crystalloids like Lactated Ringer's (LR) and normal saline.
• ⚖️ Normal saline contains 154 mEq/L of sodium and chloride, while LR has lower sodium and chloride but includes potassium, calcium, and bicarbonate, making it more physiologic.

Fluid Rate Calculation and AKI Assessment

• 🧮 The 4-2-1 rule is used for calculating hourly maintenance fluid rates: 4 cc/kg for the first 10 kg, 2 cc/kg for the second 10 kg, and 1 cc/kg for every kg over 20 kg.
• 🔍 The FINA formula (Urine Na / Serum Na) / (Urine Cr / Serum Cr) x 100 helps differentiate causes of acute kidney injury (AKI): <1 for pre-renal, >2 for renal, >4 for post-renal.
• 🚫 The FeNa formula is used if diuretics have been administered.

Hyponatremia and Hypernatremia Management

• 💡 Hyponatremia evaluation involves measuring serum and urine osmolality.
• 💧 Low serum osmolality with high urine osmolality suggests causes like SIADH, renal losses, or adrenal insufficiency.
• 📉 Treatment for hyponatremia may include fluid restriction, sodium supplementation, or vasopressin receptor antagonists.
• 🌡️ Hypernatremia can be hypovolemic (water loss), euvolemic (diabetes insipidus), or hypervolemic (excessive hypertonic fluids).
• 🧠 Neurogenic diabetes insipidus (DI) due to head injury is treated with vasopressin (DDAVP), while nephrogenic DI is managed with free water supplementation and thiazide diuretics.

Potassium and Calcium Abnormalities

• ⚡ Hyperkalemia presents with peaked T-waves on EKG and is managed by stabilizing the myocardium with calcium, shifting potassium intracellularly with insulin/D5/albuterol, and increasing excretion with diuretics or binding with Kexalate.
• 💔 Hypokalemia can lead to prolonged QT intervals, weakness, paresthesias, and arrhythmias.
• 🦴 Hypercalcemia symptoms include kidney stones, bone pain, GI distress, polyuria, and psychiatric changes, often caused by hyperparathyroidism or malignancy.
• 💧 Acute symptomatic hypercalcemia is managed with normal saline, loop diuretics, and bisphosphonates.

Magnesium and Acid-Base Balance

• ⚡ Hypomagnesemia can cause prolonged QT intervals and is associated with diuretics, certain antibiotics, chemotherapy, and PPIs.
• 🫁 Acid-base abnormalities are assessed using pH, PaCO2, and bicarbonate levels.
• 🧮 Anion gap metabolic acidosis is characterized by elevated anion gap and causes like DKA, uremia, and lactic acidosis (MUD PILES mnemonic).
• 💧 Non-anion gap metabolic acidosis can result from diarrhea or renal tubular acidosis.
• 🤮 Metabolic alkalosis is often caused by gastric losses (vomiting, NG suction) or contraction alkalosis from diuretics.

Quick Hits and Clinical Pearls

• ⚡ Sodium primarily determines serum osmolality, while potassium is the main intracellular cation.
• 👶 Pediatric fluid boluses are 20 cc/kg for crystalloids and 10 cc/kg for blood products.
• 🍽️ Refeeding syndrome commonly involves hypophosphatemia, hypokalemia, and hypomagnesemia.
• 🩺 Pyloric stenosis in infants causes hypochloremic, hypokalemic metabolic alkalosis due to paradoxical aciduria.
• 💨 Acidosis causes a right shift in the oxygen-hemoglobin dissociation curve, promoting oxygen unloading.