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