Electrolyte abnormalities (sodium and potassium)

What is pH?

amount of H+ ion in the blood normally between 7.35-7.45

What is PaO2?

partial pressure of oxygen

refers to amount of oxygen dissolved in the blood normally between 11-13kPa

What is PaCO2?

amount of CO2 dissolved in the blood usually between 4.7-6kPa

What is HCO3?

amount of bicarbonate in the blood

usually between 22-26mmol/L

it is excreted and reabsorbed by the kidneys

What is lactate?

end product of anaerobic respiration, usually <2mmol/L

caused by poor oxygenation and tissue perfusion (sepsis, shock, heart failure), or increased oxygen demand (liver/kidney failure)

What does a low blood pH mean?

pH <7.35 indicates acidaemia either due to CO2 retention (as this is acidic), or Hco3 loss (as this is a base)

What does a high blood pH mean?

pH >7.45 indicates alkalaemia due to HCo3 retention, or CO2 loss

How does the acid base system work?

both renal and respiratory systems work together to maintain acid-base balance so when one system fails another may compensate

What is hypernatremia?

high sodium concentration >146mmol/L

What are causes of hypovolemic hypernatremia with high concentrated urine?

severe dehydration, eg: poor oral intake, diarrhoea, vomiting, diuretics

What are causes of euvolemic hypernatremia with normal urine?

mild dehydration. eg: poor oral intake, diarrhoea, vomiting, diuretics

What are causes of hypovolemic hypernatremia with dilute urine?

diabetes insipidus

What are causes of hypervolemic hypernatremia with dilute urine?

sodium gain, eg: excess IV fluids

hypervolemia can be recognised through peripheral oedema

What are risk factors for hypernatremia?

water loss

diabetes insipidus

sodium gain

How does hypernatremia present?

fatigue

weakness

confusion

seizures

thirst

How are sodium levels controlled?

through the action of aldosterone on the DCT and collecting duct to increase sodium reabsorption from the urine

natriuretic peptides: ANP, BNP, CNP contribute to sodium homeostasis by reducing sodium reabsorption from the DCT and inhibiting renin

What investigations are done for hypernatremia?

• U&Es showing high NA+, high urea, high albumin

• urine osmolarity

How is hypernatremia managed?

• treat underlying cause

• give water orally if possible

• IV fluids using 5% dextrose, o.18% NaCL

• use 0.9% if hypovolaemic as it causes less marked fluid shifts and is hypotonic in a hypertonic patient

What is hyponatremia?

low serum sodium <133mmol/L

plasma Na+ depends on both sodium and water, so hyponatremia doesn’t necessarily mean sodium depletion

What are risk factors for hyponatremia?

increasing age, hospitalisation, diuretics, SSRIs, anti-psychotics, carbamazepine, heart failure, kidney/liver disease, endocrine disorders

How does hyponatremia present?

nausea, vomiting, lethargy, headache, muscle cramps, weakness, confusion, seizures, reduced Glasgow Coma Scale score

What investigations can be done for hyponatremia?

• U&Es

• TFTs, cortisol levels, LFTs, nt-proBNP

• plasma osmolarity

• fluid status, urine osmolarity, urine sodium

How is true hyponatremia determined?

measure serum osmolarity

• low plasma osmolarity <275mOsmol/kg is true hyponatremia

• normal plasma osmolarity may indicate pseudohyponatremia caused by high serum lipids or proteins

• high plasma osmolarity >295mOsmol’kg may be caused by hyperglycaemia

How is hyponatremia managed?

avoid rapid correction to prevent osmotic demyelination syndrome (central pontine demyelination)

correct underlying cause

• hypovolaemic hyponatremia: IV fluids with 0.9% may be given under supervision

• hypervolaemic hyponatremia: fluid restriction

• vasopressor receptor antagonists promote water excretion without loss of electrolytes

• emergency- hypertonic (1.8%) saline can be given undersupervisiom

What causes hypovolemic hyponatremia with high urine sodium?

occurs when there is fluid and sodium loss due to renal causes:

• primary adrenal insufficiency

• renal/cerebral salt wasting

What causes hypovolemic hyponatremia with low urine sodium?

occurs when there is fluid and sodium loss due to extrarenal causes:#

• vomiting and diarrhoea

• third spacing

What causes euvolemic hyponatremia with high urine sodium?

occurs when there is normal sodium in the body but increased fluid which dilutes the sodium

• SIADH

• secondary adrenal insufficiency

• hypothyroidism

What causes euvolemic hyponatremia with low urine sodium?

primary polydipsia

What causes hypervolemic hyponatremia with low urine sodium?

occurs when there is increased fluid and sodium retention, but more fluid than sodium

• heart failure

• liver cirrhosis

• nephrotic syndrome

What is hyperkalaemia?

high serum potassium >5.5mmol/L

potassium >6.5mmol/L is a potential emergency and needs urgent assessment

What are renal causes of hyperkalaemia?

AKE, drugs, CKS, renal tubular acidosis

What are tissue breakdown causes of hyperkalaemia?

rhabdomyolysis

tumour lysis

haemolysis

What are endocrine causes of hyperkalaemia?

DKA

Addison’s

metabolic acidosis

What is pseudo-hyperkalaemia?

raised potassium due to lysis of RBCs (most commonly due to prolonged transit time to the lab or a difficult venepuncture)

What are symptoms of hyperkalaemia?

• fast irregular pulse

• chest pain

• weakness

• light headedness

• palpitations

What are ECG changes in hyperkalaemia?

• PR prolongation

• flattened or absent P waves

• tall, tented T waves

• wide QRS >0.12s

• ventricular tachycardia

• bradycardia

• sine wave appearance

• cardiac arrest

Why does hyperkalaemia affect the heart?

Potassium is the most abundant intracellular cation (positively charged ion) in the body. The intracellular concentration is around 20 times greater than in the extracellular fluid, resulting in a large concentration gradient. This maintains the excitability of nerve and muscle cells.

Hyperkalaemia results in progressive conduction problems, as it lessons the concentration gradient

What drugs can cause hyperkalaemia?

• Potassium-sparing diuretics (e.g. amiloride)

• Aldosterone antagonists (e.g. spironolactone)

• Angiontensin converting enzyme inhibitors (e.g. ramipril)

• Angiotensin-II receptor antagonists (e.g. losartan)

• Non-steroidal anti-inflammatory drugs (e.g. ibuprofen)

• Heparin

How is mild hyperkalaemia (5.5-5.9) managed?

1. treat cause

2. remove potassium from the body (calcium resonium or sodium polystyrene sulfonate)

What is calcium resonium?

binds to K+ in the gut preventing absorption and bringing K+ levels down over a few days

How is moderate (6-6.4) hyperkalaemia managed?

1. cardiac monitoring

2. shift potassium into cells with glucose/insulin

3. remove potassium from the body (calcium resonium or sodium polystyrene sulfonate)

How is severe hyperkalaemia (>6.5) managed?

1. seek expert help and set up cardiac monitoring

2. cardioprotection: 10ml of 10% calcium chloride IV over 5 minutes

3. shift potassium into cells with glucose/insulin: 10units short acting insulin with 25g glucose IV over 30 minutes

4. shift potassium into cells with salbutamol 10-20mg

5. remove potassium from the body, renal replacement therapy may be indicated if underlying pathology cannot be corrected

What is hypokalaemia?

low serum potassium <3.5mmol/L

<2.5mmol/L requires urgent treatment

What are causes of hypokalaemia?

decreased intake: poor diet, malabsorption

increased loss: GI (diarrhoea), renal (tubular disorders)

endocrine: Cushing’s, hyperaldosteronism

drugs

What medications can cause hypokalaemia?

diuretics

laxatives

steroids

insulin

beta-2 agonists

aminoglycosides (gentamicin)

How does hypokalaemia present?

• fatigue

• weakness, hypotonia, hyporeflexia

• muscle pain

• constipation

• ascending paralysis

• palpitations

• tetany (involuntary muscle contractions)

What ECG changes are present in hypokalaemia?

• u waves

• t wave flattening

• ST depression

• long PR interval

• arrhythmias

What may blood investigations show for hypokalaemia?

hypokalaemia

hypomagnesemia

How is mild hypokalaemia (3-3.5) managed?

oral K+ supplement

treat underlying cause

monitor K+

How is severe hypokalaemia (<2.5) managed?

1. seek expert advice

2. cardiac monitoring

3. IV potassium chloride cautiously (max rate 20mmol/hour, max concentration 40mmol/L) NEVER GIVE A BOLUS

4. treat underlying cause

5. monitor K+ every 3 hours