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

  
• Normal saline


  Approximately same osmolarity as plasma, 154 mEq/L sodium chloride vs. 140 mEq/L plasma, 308 mOsmol/liter vs. 285 mOsm/L plasma, 25% remains in intravascular space, used for volume replacement in hypovolemic and septic shock

  
• Normal saline


  • Results in influx of chloride ions, causes shift of bicarbonate ions into cells, causes acidosis, acidosis leads to potassium shift out of cells and increased serum potassium
• Lactated ringers


  Balanced fluid, isotonic, lactate metabolized to bicarbonate, acts as buffer in acidotic states, most common use is trauma resuscitation, does not cause hyperkalemia
• The SMART trial showed improved outcomes with lactated ringers compared to normal saline in critically ill patients
• Half normal saline


  Hypotonic solution, does not remain intravascular, used as maintenance fluids to replace daily losses of sodium and water

  
• D5W


  Dextrose metabolized leaving only free water, used to correct hypernatremia
• Hypertonic saline


  Hypertonic solution, draws fluid out of tissues into vascular space, used for elevated intracranial pressure and severe hyponatremia
• Types of intravenous fluids


  • Crystalloids
  • Colloids
• Crystalloids


  Contain water and salts, e.g. normal saline, half normal saline
• Colloids


  Contain water and large molecules, e.g. albumin, more expensive with no proven benefit over crystalloids
• Hypovolemia


  Decreased fluid volume, causes include vomiting, diarrhea, poor oral intake, third spacing/fluid leak, clinical features include decreased urine output, dry mucous membranes, poor skin turgor, hypotension, treated with oral intake and IV fluids
• Hypervolemia


  Excess fluid volume, causes include heart failure, cirrhosis, nephrotic syndrome, clinical features include weight gain, pitting edema, elevated jugular venous pressure, pulmonary edema, treated with diuretics
• Sodium


  Normal range 135-145 mEq/L, hypo and hypernatremia affect the brain, low sodium leads to fluid into cells and brain swelling, high sodium leads to fluid out of cells and brain shrinkage
• Plasma osmolality


  Amount of solutes present in plasma, key solute is sodium, should be low in hyponatremia

  
• Serum osmolality = 2 * [Na] + Glucose + BUN, normal range 275-295 mOsm/L
• Causes of hyponatremia with high osmolality


  • Hyperglycemia
  • Mannitol
• Causes of hyponatremia with normal osmolality


  • Artifact in serum Na measurement
  • Hyperlipidemia
  • Hyperproteinemia (multiple myeloma)
• In hyponatremia, the urine should be diluted with more free water than solutes, normal osmolality is 50-1200 mOsm/kg, low urine osmolality (<100 mOsm/kg) and low urinary sodium (<30 mEq/L) indicates appropriate ADH response

  
• Antidiuretic hormone (ADH)


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


  • Perceived hypovolemia
  • Hypervolemia: heart failure, cirrhosis
  • True hypovolemia: diuretics, GI losses, sweating/exercise
• Other rare causes of high ADH


  • Adrenal insufficiency
  • Hypothyroidism
• SIADH


  Syndrome of Inappropriate Antidiuretic Hormone Secretion, hyponatremia due to inappropriate ADH release, absence of other causes for high ADH, euvolemic with high urinary osmolality (>100 mOsm/kg)
• Causes of SIADH


  • Drug-induced
  • Paraneoplastic
  • CNS disorders
  • Pulmonary disease
• Non-ADH causes of hyponatremia


  • Renal failure
  • Psychogenic polydipsia
  • Special diets
• Renal failure hyponatremia


  Kidneys cannot excrete free water normally, urine osmolality greater than 200-250 mOsm/kg
• Psychogenic polydipsia and special diets hyponatremia


  Urine osmolality less than 100 mOsm/kg, indicates ADH is low and kidneys are working to eliminate free water
• Causes of hyponatremia by volume status


  • Hypervolemic: heart failure, cirrhosis
  • Euvolemic: SIADH, polydipsia, special diets, hypothyroidism
  • Hypovolemic: volume depletion, diuretics, Addison's disease
• Workup of hyponatremia includes history, physical exam, serum glucose, BUN and creatinine
• Treatments for hyponatremia


  • Identify and treat underlying cause
  • Free water restriction
  • Sodium chloride tablets (for euvolemic)
  • Hypertonic saline (for severe, symptomatic)
  • Vaptan drugs (for severe hyponatremia of heart failure)

  
• Normal saline worsens hyponatremia in SIADH due to excess ADH causing free water retention
• Demeclocycline


  Tetracycline antibiotic used as an ADH antagonist in chronic SIADH
• Central pontine myelinolysis


  Osmotic demyelination syndrome associated with overly rapid correction of hyponatremia, usually >10 meq per 24 hours, results in demyelination of central pontine axons and neurological deficits
• Hypernatremia


  Sodium >145 mEq/L, caused by lack of access to free water leading to free water loss exceeding sodium loss

  
• Causes of hypernatremia


  • Febrile illness
  • Burns
  • Diarrhea
  • Diuretics
• Diabetes insipidus


  Loss of ADH effects leading to excessive free water loss, can be central (trauma, tumors) or nephrogenic (many causes), results in polyuria and polydipsia