Chemical Pathology - Potassium Flashcards
what is the normal potassium level?`
3.5-5.3mmol/L
how is potassium regulated?
- loss through GI tract
- renal regulation and secretion (Ang II, aldosterone)
- movement from intracellular and extracellular
what does renin do?
convert angiotensinogen to Ang-1
where is renin released from?
Liver
JGA cells
when is renin released?
- low BP in renal artery
- low Na+ in macula densa by JGA
- SNS beta-1 receptor activation
what then happens to Ang-1?
Ang-1 to Ang-2 in the lungs via ACE
Ang-2 acts on adrenals to release aldosterone
what does aldosterone do?
excretes K+ and increases Na+ retention
what are the triggers for aldosterone release?
- Ang 2
- high K
describe the action of aldosterone
- binds to MR steroid receptor
- aldosterone inc number of open Na channels in luminal membrane
- inc Na resorption
- lumen becomes electronegative –> creates electrical gradient
- causes K to secreted into lumen
what are the causes of hyperkalaemia?
- dec GFR (renal failure)
- dec renin (T4 RTA e.g. diabetic nephropathy, NSAIDs)
- ACEi (dec. Ang 1 to Ang 2)
- ARBs
- Addison’s disease (adrenal damage = dec aldosterone)
- aldosterone antagonists
- K release from cells (rhabdo, acidosis)
how does acidosis cause hyperkalaemia?
- H/K transporter is disrupted with H+ cells taken into cells to stabilise the disturbance
- K+ is excreted in response (to maintain electronegativity)
what are the main causes of hyperkalaemia?
- renal impairment (reduced renal excretion)
- drugs (ACEi, ARBs, spironolactone)
- low aldosterone (Addison’s, T4 RTA)
- release from cells (rhabdo, acidosis)
what are the ECG changes with hyperkalaemiaq?
- peaked T waves (early)
- broad QRS
- flat P wave
- prolonged PR (and bradycardia)
- sine wave
what is the management of hyperkalaemia?
- 10mL 10% calcium gluconate (stabilise)
- 100ml 20% dextrose (drive K into cells)
- 10U insulin
- nebulised salbutamol
- Tx underlying cause
overall what are the causes of hypokalaemia?
GRRR
- GI losses
- renal losses
- redistribution into cells
- Rare causes
how do GI losses cause hypokalaemia?
- diarrhoea
- vomiting
- fistulas
how does renal losses lead to hypokalaemia?
- MR excess (hyperaldosteronism/Conn’s, Cushing’s)
- more Na delivery to distal tubule because hasn’t been reabsorbed earlier on
- in distal nephron, Na in and K out
- more Na in, more K loss
= osmotic diuresis
which conditions can cause hypokalaemia through redistribution of cells?
- insulin/inulinomas
- beta-agonists
- alkalosis
how does alkalosis cause hypokalaemia?
- alkalosis = low H+
- shift of K+ into cells in exchange for H+ in H/K transporter
= hypokalaemia
what are the rare causes of hypokalaemia?
- RTA T1, T2
- hypomagnesaemia
which drugs can cause renal potassium loss?
- loop diuretics (furosemide)
- thiazides (bendroflumethiazide)
(more Na reaches and is absorbed in DCT, more electronegative, loss of K+ down electrochemical gradient)
what are the clinical features of hypokalaemia?
- muscle weakness
- cardiac arrhythmias
- polyuria and polydipsia (nephrogenic DI from low K+ or high Ca)
ECG changes in hypokalaemia?
ST depression
flat T waves
U waves
what do you think of with a low K+ and HTN?
Conn’s
how would you screen for Conn’s?
aldosterone: renin ratio
Conn’s = high aldosterone: renin ratio (aldosterone suppresses renin)
how do you manage a K level of 3.0-3.5?
- oral KCl (2 SandoK tablets, TDS, 48 hours)
- recheck K
- Tx underlying cause (e.g. with spironolactone)
how do you manage a K level of <3.0?
- IV KCl
- Max rate of 10mmol/hour (can irritate peripheral veins)
- Tx underlying cause (e.g. with spironolactone)
what does RTA T1 cause?
- classic distal RTA (normally distal nephron secretes H+)
- hypokalaemia
- less H+ excretion
what does RTA T2 cause?
- proximal distal RTA (normally proximal tubule resorbs filtered bicarbonate)
- hypokalaemia
- less HCO3- reabsorption
what does RTA T4 cause?
- hypoaldosteronism (normally aldosterone facilitate excretion of H+ bound to ammonia)
- hyperkalaemia
- hypoaldosteronism
