renal

• Fluid compartments


  • 40% non-water, 60% water
  • 1/3 extracellular, 2/3 intracellular
  • 1/4 plasma, 3/4 interstitial

  
• Normal oral fluid intake is ~2 L/day
• Fluid output


  • Urine: 1.0 L/day
  • Stool: 0.25 L/day
  • Insensible losses: 0.75 L/day
• Insensible fluid losses increase in pathologic states like fever and burns
• Intravenous fluids


  • Normal saline
  • Lactated ringers
  • D5 ½ normal saline
  • D5W
  • Hypertonic saline

  
• Normal saline


  • Approximately same osmolarity as plasma
  • Results in influx of chloride ions and acidosis

  
• Lactated ringers


  • Isotonic, contains sodium, chloride, potassium, calcium, and lactate
  • Lactate metabolized to bicarbonate, acts as buffer in acidosis
• The SMART trial showed improved outcomes with lactated ringers compared to normal saline in critically ill patients

  
• Half normal saline


  • Hypotonic, used to replace daily losses of sodium and water

  
• D5W


  • Hypertonic, draws fluid out of tissues into vascular space, used for elevated intracranial pressure or severe hyponatremia
• Hypertonic saline


  • 3% saline, used in severe, symptomatic hyponatremia
• Crystalloid and colloid solutions

  • Crystalloids: water and salts
  • Colloids: water and large molecules like albumin
• Hypovolemia


  Decreased fluid volume, causes include vomiting, diarrhea, poor oral intake, fluid leakage
• Hypervolemia


  Excess fluid volume, causes include heart failure, cirrhosis, nephrotic syndrome
• Sodium


  Normal range 135-145 mEq/L, affects brain when low or high
• Plasma osmolality


  Amount of solutes in plasma, key solute is sodium
• Causes of high plasma osmolality with hyponatremia

  • Hyperglycemia
  • Mannitol
• Pseudohyponatremia can occur with hyperlipidemia or hyperproteinemia
• Antidiuretic hormone (ADH)


  Controls plasma sodium concentration, any cause of high ADH can cause hyponatremia
• Causes of high ADH

  • Perceived hypovolemia
  • Hypervolemia
  • True hypovolemia
  • Adrenal insufficiency
  • Hypothyroidism
• SIADH


  Syndrome of inappropriate ADH secretion, leads to hyponatremia without volume depletion
• Causes of SIADH


  • Drug-induced
  • Paraneoplastic
  • CNS disorders
  • Pulmonary disease
  • Renal failure
  • Psychogenic polydipsia
  • Special diets
• Renal failure hyponatremia


  Kidneys cannot excrete free water normally, urine osmolality is high
• Psychogenic polydipsia and special diets

  Hyponatremia with low urine osmolality, indicates ADH is low
• Causes of hyponatremia by volume status

  • Hypervolemic: heart failure, cirrhosis
  • Euvolemic: SIADH, polydipsia, special diets
  • Hypovolemic: volume depletion, diuretics, Addison's
• Hyponatremia workup


  1. History and physical exam

  2. Serum tests: glucose, BUN, creatinine

  3. Determine volume status and urinary osmolality
• Hyponatremia treatments


  • Identify and treat underlying cause
  • Free water restriction
  • Sodium chloride tablets
  • Hypertonic saline
  • Vaptans

  
• Normal saline worsens hyponatremia in SIADH due to excess ADH
• Central pontine myelinolysis


  Osmotic demyelination syndrome associated with overly rapid correction of hyponatremia
• Hypernatremia


  Serum sodium > 145 mEq/L, caused by lack of access to free water

  
• Causes of hypernatremia


  • Diabetes insipidus
  • Hypercalcemia
  • Hypokalemia
  • Drugs like lithium and amphotericin B
• Diabetes insipidus


  Loss of ADH effects leading to excessive free water loss, causes polyuria and polydipsia
• Diagnosing diabetes insipidus


  1. Water deprivation test

  2. Desmopressin administration
• Hypernatremia treatment


  Provide water, IV fluids like D5W, caution with overly rapid correction to avoid cerebral edema
• Central diabetes insipidus treatment

  Desmopressin, an ADH analog
• Nephrogenic diabetes insipidus treatment

  Treat underlying causes like hypercalcemia or hypokalemia, use thiazide diuretics or NSAIDs
• Potassium


  Normal range 3.5-5.0 mEq/L, needed for heart and skeletal muscle function
• Causes of hypokalemia


  • Gastrointestinal losses
  • Renal losses
  • Hypomagnesemia
  • Redistribution into cells
• Hypokalemia signs and symptoms

  Muscle weakness, paralysis, arrhythmias, EKG changes
• Hypokalemia treatments


  • Oral or IV potassium
  • Potassium-sparing diuretics
  • Treat hypomagnesemia