P year‎ > ‎Medicine‎ > ‎

12.11.26 cPBL

Potassium handling
  • Beta agonists => K+ uptake into cells
  • Beta blockers => K+ release
  • Insulin => Effect on Na/K ATPase
  • Bicarbonate => Exchange of cellular H+ for Na+ => Stimulation of the sodium-potassium ATPase
  • Risonium binds K+ in the gut
    • One of the only things that actually gets rid of it
  • Think about INPUT
    • e.g. Fruit juices
  • Aldosterone
    • Required for K+ secretion
    • Also for H+ secretion (compete)
  • Hyponatraemia can raise K+ and H+
    • As there's less available for exchange in DCT

Addison's
  • Described tuberculous loss of adrenals
    • => Lose everything (cortisol and aldosterone)
    • Contrast with ACTH deficit which specifically drops cortisol
  • ACTH rises to desperately try to increase cortisol
    • Cross-reacts with melanocyte stimulating hormone receptors
    • => Dark skin

SIADH
  • Diagnosis of EXCLUSION
    • Don't just fluid-restrict sick patients!
  • Exclude:
    • Thyroid and cortisol dysfunction
      • Required for water excretion
    • Kidney injury
    • Adrenal dysfunction
    • Medications
      • Thiazide diuretics
      • SSRIs
      • Sulphonylureas
      • etc..
  • If you think it's SIADH, try fluid restricting and CHECK IT'S WORKING!
  • Check urinary sodium
    • Will be >60 mM
    • As the body is desperately trying to offload volume


Volume status
  • Not the same as hydration status
  • Assess with lying + standing BP
  • Urine
    • Max osmolality = 1000 mOsM
    • Min osmolality = 100 mOsM
    • [Na] varies but can be undetectable

Compositions of common fluids
  • Hartmann's
    • Na -131 mM 
    • Cl - 111 mM
    • Lactate - 29 mM
    • K - 5 mM
    • Ca - 2 mM
  • Normal saline (300 mOsm/L) (cf hyperchloraemic acidosis)
    • Na - 154 mM
    • Cl - 154 mM
  • Lactated Ringers (273 mOsm/L)
    • Na - 130 mM
    • Cl - 109 mM
    • Lactate - 28 mM
    • K - 4 mM
    • Ca - 1.5 mM
  • 5% dextrose
    • 50 g/L dextrose
  • Gelofusine
    • Na - 154 mM
    • Cl - 125 mM
    • Gelatin 40 g/L

Hyperkalaemia
  • Excessive intake
  • Excessive release
    • Rhabdomyolysis, burns, tumor lysis syndrome
    • Massive blood transfusion or hemolysis
    • Shifts/transport out of cells (acidosis, low insulin, beta-blockers, digoxin, succinylcholine)
  • Ineffective elimination
    • Renal insufficiency
      • Medication
      • ACE inhibitors and ARBs
      • Potassium-sparing diuretics
      • NSAIDs
      • Calcineurin inhibitors
      • Trimethoprim
      • Pentamidine
    • Mineralocorticoid deficiency or resistance
      • Addison's disease
      • Aldosterone deficiency
      • Congenital adrenal hyperplasia
      • Type IV renal tubular acidosis (resistance of renal tubules to aldosterone)
    • Gordon's syndrome 

Hypokalaemia
  • Inadequate intake
  • Gastrointestinal/integument loss
    • Diarrhoea, perspiration
    • Vomiting 
  • Urinary loss
    • Medications 
      • Thiazide diuretics
      • Loop diuretics
      • Amphotericin B
      • Cisplatin
    • DKA
      • Obligate loss of potassium from kidney tubules as a cationic partner to the negatively charged ketone, β-hydroxybutyrate
    • Hypomagnesemia
      • Mg is required for adequate processing of potassium
    • Alkalosis 1
      • Causes a shift of potassium from the plasma and interstitial fluids into cells
    • Alkalosis 2
      • Acute rise of plasma HCO3- concentration will exceed the capacity of the renal proximal tubule to reabsorb this anion
      • Potassium will be excreted as an obligate cation partner to the bicarbonate
    • Disease states that lead to abnormally high aldosterone levels
      • Renal artery stenosis
      • Primary hyperaldosteronism
    • Hereditary defects of renal salt transporters
      • Bartter syndrome, Gitelman syndrome
  • Distribution away from ECF
    • Insulin, epinephrine, beta agonists, xanthines

Notes
  • Na/K ATPase 
    • Dies faster in the fridge => Don't refrigerate cold samples!
    • Genetic trait of fast-dying
  • Cortisol is normally around 300
    • But should be >1000 when stressed (e.g. ITU)
    • Don't forget the context!
  • Volume status trumps serum osmolality in homeostasis
  • Normal Na intake = 100 mmol per day
  • Only about 50% of total calcium is ionised
  • Glucocorticoids are required to excrete water

Comments