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