Biochemical investigation of fluid and electrolyte balance Flashcards
A patient with severe diarrhea is found to have hypokalemia. What is the main mechanism?
Excessive fluid loss (vomiting, diarrhea) leads to potassium depletion through the kidneys and gut.
A dehydrated elderly patient presents with confusion and dry mucous membranes. What biochemical test is crucial?
Serum sodium (Na⁺) to assess for hypernatremia, which can occur due to inadequate fluid intake.
What are the primary cations in intracellular and extracellular compartments?
- Intracellular: K⁺, Mg²⁺
- Extracellular: Na⁺, Ca²⁺
A diabetic ketoacidosis (DKA) patient has high anion gap metabolic acidosis. What causes the widened anion gap?
Accumulation of ketoacids (β-hydroxybutyrate, acetoacetate) increases unmeasured anions.
What is the most important determinant of extracellular fluid volume?
Sodium (Na⁺), because water follows sodium due to osmotic effects.
A hypertensive patient is on spironolactone and develops hyperkalemia. What is the mechanism?
Spironolactone is an aldosterone antagonist, reducing renal K⁺ excretion, leading to K⁺ retention.
How does ADH regulate water balance?
ADH increases aquaporin expression in renal tubules, promoting water retention and urine concentration.
A patient with severe burns has hyponatremia despite receiving fluids. What is the cause?
Loss of sodium-rich plasma due to increased vascular permeability and fluid shifts.
A marathon runner drinks excessive water during the race and collapses. What electrolyte disorder is likely?
Hyponatremia due to dilutional effect, causing cerebral edema.
Why is plasma osmolality measured in hyperglycemic patients?
To assess for hyperosmolar hyperglycemic state (HHS), where high glucose increases osmolality, leading to dehydration.
A patient with adrenal insufficiency (Addison’s disease) has hyponatremia and hyperkalemia. Why?
Lack of aldosterone leads to Na⁺ loss and K⁺ retention in the kidneys.
A Cushing’s syndrome patient has hypokalemia and hypertension. Explain the mechanism.
Excess cortisol mimics aldosterone, leading to K⁺ loss and Na⁺ retention
A patient with excessive vomiting has hypochloremic metabolic alkalosis. Why?
Loss of HCl from gastric secretions reduces chloride and hydrogen ions, increasing bicarbonate (HCO₃⁻).
A heart failure patient on loop diuretics develops hypokalemia. What is the cause?
Increased renal K⁺ excretion due to diuretic-induced Na⁺ loss, leading to compensatory aldosterone activation.
What condition is suspected in a tuberculosis patient with low CSF chloride?
Tuberculous meningitis, which reduces CSF Cl⁻ due to increased permeability and protein influx.
A cystic fibrosis (CF) patient has elevated sweat chloride (>60 mmol/L). What is the underlying defect?
Mutation in CFTR gene, impairing chloride transport in sweat glands.
A patient with renal failure develops hyperphosphatemia and hypocalcemia. What is the mechanism?
Reduced renal excretion of phosphate increases serum phosphate, which binds calcium, lowering serum Ca²⁺.
A malnourished alcoholic patient develops tetany and arrhythmias. What electrolyte should be checked?
Magnesium (Mg²⁺)—deficiency can cause hypocalcemia and cardiac instability.
A patient on IV fluids develops fluid overload and pulmonary edema. What mistake might have been made?
Excess isotonic fluid administration causing volume expansion beyond capacity.
A patient with diabetic nephropathy has persistent hyponatremia despite normal sodium intake. What is likely?
SIADH (Syndrome of Inappropriate ADH Secretion)—excess ADH retains free water, diluting Na⁺.
What is the best initial test for suspected hypernatremia?
Serum sodium (Na⁺) and plasma osmolality to assess dehydration or free water loss.
What is the formula for plasma osmolality?
2[Na⁺] + glucose/18 + BUN/2.8 (Normal: 275-295 mOsm/kg).
A patient with head trauma develops high urine osmolality and low serum sodium. What is the diagnosis?
SIADH, causing inappropriate water retention and dilutional hyponatremia.
What is the role of Hematocrit (Hct) in fluid status evaluation?
Increased in dehydration, decreased in fluid overload or anemia.
What is the preferred blood collection tube for electrolyte measurement?
Dry tube (serum) or Lithium heparin (plasma). Avoid EDTA (falsely lowers Ca²⁺ & K⁺).
A patient with chronic diarrhea has metabolic acidosis with normal anion gap. Why?
of HCO₃⁻ in stool leads to non-anion gap acidosis (renal compensation maintains Cl⁻).
What does a urinary Na⁺/K⁺ ratio <1 suggest?
Hyperaldosteronism—aldosterone increases Na⁺ reabsorption and K⁺ excretion.
A young athlete collapses after using diuretics for weight loss. What is the concern?
Severe hypokalemia, leading to cardiac arrhythmias.
What renal hormone system is activated in hypovolemia?
Renin-Angiotensin-Aldosterone System (RAAS) to retain Na⁺ & water, increasing BP.
A patient in ICU with sepsis develops high lactate & metabolic acidosis. What is happening?
Lactic acidosis due to tissue hypoxia & anaerobic metabolism.
