Pathophysiology of Water Handling Flashcards Preview

CVPR: Renal > Pathophysiology of Water Handling > Flashcards

Flashcards in Pathophysiology of Water Handling Deck (22)
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1

Plasma sodium definition

  • concentration term; not a measure of total body sodium content
  • [ECF Na] = ECF Na / ECF H2O
  • low plasma sodium ==> deficit of sodium and/or relative excess of water

2

Serum osmolality definition/relationship to sodium

  • Sodium = most abundant cation in ECF ==> concentration is major determimnant of tonicity/osmolality
  • Sosm (mosm/kg) = 2xNa + (BUN/2.8) + (Glucose/18)
  • tonicity of ECF reflects the tonicity of cells bc water flows freely accross cellular membranes

3

Normal urine diluting system

  • normal fxn of the diluting segment: dilution @ water-imp. ascending LOH + distal convoluted tubule via reabsorption of NaCl
  • normal delivery to diluting segment: normal GFR + proximal reabsorption
  • absence of vasopressin ==> collecting duct remains impermeable to water ==> water remains and dilutes urine

4

Normal maximal ability to dilute urine

  • maximal free water excretion + ~20% of GFR

    • normal renal fxn ==> 24L/day

    • impaired renal fxn (e.g. 20% of normal GFR) ==> 4 L/day

  • **excessive water intake alone does not cause hyponatremia unless it exceeds 1L/hr (in patients w/normal renal fxn)


     

5

Normal renal concentrating mechanism

  • allows for excretion of urine concetration = 4 x plasma ==> 1200 mOsm/kg H2O
  • ability to generate hypertonic interstitium: LOH active transport of chloride
  • secretion of ADH: renders collecting duct permeable to water
  • collecting duct response to ADH

6

ADH: osmoregulation vs. volume regulation

  • increases collecting duct permeability ==> water reabsorption
  • is released in response to hyperosmolality and volume depletion
  • after about 6-7% volume depletion, the "volume response" dominates the "osmolality response."  

  • ADH is normally osmoregulatory, but during stress becomes a volume regulatory hormone.

7

Characteristics of Hypertonic Hyponatremia

  • shift of water from cells in response to a non-sodium solute ==> e.g. glucose
  • hyperglycemia, mannitol or glycerol
  • per 100 mg/dL serum glucose increase, serum sodium falls 1.6 mEq/L

8

Characteristics of Isotonic Hyponatremia

  • =hyponatremia w/normal plasma osmolality
  • caused by [lab artifiact] in association with hyperlipidemia or hyperproteinemia (i.e. multiple myeloma)

9

Type of hypotonic hyponatremia

  • Hypervolemic: TB Na+; TB water ++
  • Euvolumemic: TB Na; TB water+
  • Hypovolemic: TB Na - -; TB water - 

10

Characteristics of hypovolemic hypotonic hyponatremia

  • =low total body sodium (+clinical low ECF volume) 
  • renal loss (UNa>20)
    • diuretic overuse
    • salt-losing nephritis
    • osmotic diuresis
  • extrarenal loss (UNa<20)
    • GI loss

11

Characteristics of Hypervolemic Hypotonic Hyponatremia

  • =increased total body sodium + clinically increased ECF volume
  • CHF, cirrhosis, nephrotic syndrome, advanced chronic or acute renal failure
    • decreased in effective vascular volume ==> release of ADH ==> excess volume
  • characterised by edema

12

Characteristics of Euvolemic Hypotonic Hyponatremia

  • =normal total body sodium + clinically normal ECF volume
  • ADH secretion is increased despite absence of stimuli
  • Hypothyroidism
  • Drugs: Nicotine, SSRI, antipsychotics
  • adrenal insufficiency
  • primary polydipsia
  • SIADH (syndrome of inappropriate ADH)

13

Diseases that may cause SIADH

  • carcinomas
    • lung (small cell)
    • duodenum
    • pancreas
  • Pulmonary diseases
    • pneumonia
    • abcess
    • TB
    • ventilation
  • CNS disorders
    • meningitis
    • enchephalitis
    • psychosis
    • trauma

14

Signs of hyponatremia

  • GI complaints: anorexia, nausea, vomiting
  • altered sensorium
  • seizures

15

Treatment of hyponatremia

  • dietary water restriction
  • correct causative disorder
  • hypertonic NaCl w/ or w/out furosemide (not too quickly)

16

Main root causes of hypernatremia

  • =elevated sodium plasma concentration
  • ADH is decreased/ineffective
  • Daily water intake is less than required to compensate for normal losses

17

Types of hypernatremia

  • Decreased TB Na
  • Normal TB Na
  • Increased TB Na

18

Hypernatremia w/decreased total body Na

  • total body water loss >> TB salt loss
  • GI loss (diarrhea), skin loss (burns), diuretics w/out sufficient water intake

19

Hypernatremia w/increased total body Na

  • rare
  • usually due to hypertonic fluid e.g. sodium bicarbonate or hypertonic saline

20

Hypernatremia w/normal total body Na

  • Central diabetes insipidus = ADH deficiency 
    • most cases = idiopathic
    • other causes: head trauma, surgery, neoplasm
    • urine volume = 3-15L/day
    • kidneys respond to exogenous ADH
    • complete ==> [urine] < 200 mOsm/kg
    • partial can concentrate but not maximally
  • Nephrogenic DI = collecting duct does not respond to ADH
    • will not respond to exogenous ADH
    • congenital or acquired
    • causes: chronic renal failure, hypercalcemia, hypokalemia, drugs

21

Clinical presentation of hypernatremia

  • cellular dehydration = water out of cells
  • neuromuscular irritability = twitches, hyperreflexia, seizures
  • ==> coma, death

22

Tx of hypernatremia

  • aim = restore serum tonicity to normal + correct sodium imbalance
  • water needed (L) = 0.6 x weight (kg) x [(actual Na/desired Na) - 1]
  • restore water slowly to avoid cerebral edema