Potassium Flashcards
What is potassium important?
strict regulation is crucial for vital physiologic processes:
- resting membrane potenital
- propagation of AP
- hormone secretion and action
- vascular tone
- systemic blood pressure control
- acid-base homeostasis
- gastrointestinal motility
- glucose and insulin metabolism
- mineralocorticoid action
- renal concentrating ability
- fluid and electrolyte balance
Why is maintaining potassium levels at the correct level important?
hypokalaemia or hyperkalaemia patients have an increased rate of death from any cause
- deranged potassium levels are associated with progression of cardiac and kidney disease and interstitial fibrosis
In a healthy individual how is potassium lost?
via kidneys 90%
via gut and skin 10%
What is the reference serum potassium concentration?
3.5-5.3 mmol/L
Why is potassium relevant in terms of food?
approximate levels of extracellular K in an adult are 60-80 mmol total extracellular potassium and 20-25mmol of total plasma potassium
meals may contain more potassium than the total plasma potassium content
rapid clearance by renal and extra-renal mechanisms reduce variation in plasma potassium to no more than 10%
What are examples of high potassium foods?
medium banana (110) 1/2 papaya (100) 1/2 cup prune juice (95) 1/4 cup of raisins (69) medium mango (84) or kiwi (62) 1/2 cup of cubed cantaloupe (55) or diced honeydew melon (5) medium pear (50)
What does potassium stimulate in the serum and in the urine?
serum
- K stimulates ATPase which increases hormone release (insulin, catecholamines, aldosterone)
urine
- K stimulates renal cell uptake and secretion of K
What is the main regulator of K?
kidney = main regulator of total body K
When K is taken in how it redistributed between ECF and ICF?
through hormones, acid base status, plasma osmolarity and potassium sequestration into liver and muscle
Within the kidney which regions filter K?
PCT - 60-70%
Ascending loop = 20-30%
Dct - 10% = Fine tuning by aldosterone K+/H+ for Na
of 600mmol/day filter, about 100 is excreted
When K levels are too high how are they returned to normal levels?
insulin and catecholamines act to increase K uptake into cells
Aldosterone acts on the kidney to increase its excretion
What is the effect of aldosterone ?
exchange of Na for H+ or K+
net loss of K
net gain of sodium
What are the causes of an overactive RAS system and what does it lead to ?
Hypokalaemia
- Conn’s = aldosterone
- Cushings= cortisol
- Renal artery stenosis = renin
What are the causes of an underactive RAS system and what does it lead to/
hyperkalaemia
- adrenal insufficiency
- ACE inhibitors
- Spironolactone
How are insulin and catecholamines involved in regulating K?
Drive cellular uptake of K via activation of Na/K ATPase
occurs in the liver, skeletal muscle (catecholamines), adipose tissue (insulin)
What does metabolic acidosis cause in relation to K?
increases plasma K
- inhibition of renal tubular K secretion
- shift of K from ICF to ECF
MOA:
- inter-relationship between ECF H+ and K+
- ECF H+ affects K+ entry into cells
therefore if H enters cells (ICF) instead of K the K in the ECF rises => hyperkalaemia
What does metabolic alkalosis cause in relation to K?
reduced plasma K
In terms of acidosis what happens at the kidneys in relation to K?
H+ load blocks K excretion therefore you get hyperkalaemia
- with increased H+ (low pH more H+ is exchanged for Na
- consequence of this is loss of H+ rather than K+
What happens to K in terms of an acidosis being caused by hyper-cholaemia?
hypercholaemia due to bicarbonate loss e.g renal tubular acidosis
- the Cl- cannot pass the plasma membrane to maintain neutrality therefore K is lost from cells and loss in urine
What happens in the serum and in the urine during alkalosis?
serum
- H+ moves from cells to ECF as buffer
- to preserve electrochemical neutrality K and Na enter the cell therefore ECF K falls
Urine
- alkalosis increases renal K loss
What happens to K during insulin deficiency e.g. DKA?
movement of water and K from ICF to ECF
then in the urine the increased fluid delivery to the lumen causes increased K excretion
=> hyperglycaemia, diuretics, poorly absorbed anions
What is defined as hyperkalaemia?
K>5mmol/L
if>6.5- 7 =altered electrical excitability
medical emergency at >7.5
=> arrhythmias, cardiac arrests, ECG changes (tall T waves, wide QRS, wide PR)
What are the main causes of hyperkalaemia?
increased intake (oral or IV) Reduced excretion Altered distribution Factitious Drugs
What are the causes of reduced excretion of K?
renal failure mineralocorticoid deficiency (Addison's) Tubular defects
What are the causes of altered distribution of K?
acidosis
insulin deficiency
crush injury, haemolysis, tumour lysis
hyperkalaemic periodic paralysis
What are the factitious causes of hyperkalaemia?
improper collection
haematological disorders
What drugs can cause hyperkalaemia?
ACEI, ARBs
potassium sparing diuretics
potassium supplements
What are the commonest causes of hyperkalaemia?
chronic kidney disease
redistribution secondary to acidosis
What are the consequences of hyperkalaemia?
neuromuscular - weakness, parasthesia, paralysis
Gastrointestinal - nausea, vomiting, pain, ileus
Cardiovascular - conduction defects, arrhythmias, cardiac arrest
How do you treat acute hyperkalaemia >6.5?
Cardioprotection
- calcium gluconate to increase threshold potentials
- ECG monitor
- care with pts on digoxin
Redistribution
- glucose + insulin
- beta agonist
- bicarbonate
removal
- loop diuretic - furosemide, bumetanide
- ion exchange resins
- dialysis /haemofiltration
How is chronic hyperkalaemia treated?
commonest cause is chronic kidney disease so its important to treat the underlying cause
low potassium diet
correction of metabolic acidosis
What are some common causes of artefactual high k?
overnight storage prior to sample separation
taken from a vein above an IV infusion of potassium/dextrose
blood taken into EDTA anticoagulant
also haematological malignancies with very high WBC or plt
What are the main causes of hypokalaemia?
Losses
- GI= fistula, diarrhoea, purgative abuse, villous adenoma
- Renal = loop diuretics, renal tubular acidosis, mineralocorticoid excess
Inadequate intake
- usually only appears if superimposed on other losses - diarrhoea, diuretics
Redistribution
- insulin, alkalosis, salbutamol, hypokalaemia periodic paralysis, familial periodic paralysis
What are the consequences of hypokalaemia?
skeletal muscle - weakness, paralysis
GI - paralytic ileus
Kidney - impaired concentrating ability, tubular defects
Cardiac - conduction defects, arrhythmias, digoxin toxicity
What ECG changes are apparent in hypokalaemia?
flattened T wave becoming T wave inversion
ST depression
prominent P wave and prolonged PR interval
U waves - V2-4
torsades de point
What is the treatment for hypokalaemia?
oral intake - normal intake + 60mmol/day
equivalent - 8 slow K tablets, 10 effervescent K tablets
IV treatment
- 10mmol/hr max
- dilute potassium ampoule - NEVER straight from ampoule >40mmol/L
