Electrolytes

Question 1

How do you classify hyponatraemia

Answer 1

By the volume:

Hypovolaemic, euvolaemic or hypervolaemic

The concentration of sodium depends on the level of both sodium and water in the body.

Question 2

How much of the body is water, and what proportion of this is extracellular vs intacellular

Answer 2

60%- 2/3 is intracellular, 1/3 is extra

Question 3

Distinguish the osmolarity and sodium concentration between intracellular and extracellular fluid

What is the most abundant anion in the extracellular vs intracellular space

Answer 3

Osmolarity is equal between the two

Sodium concentration is much higher extracellularly than intracellularly

But potassium and magnesium concentrations are the most common cations intracellularly

In EC space, chloride most abundant, but IC this is Po43- and negative plasma proteins

Question 4

Broad two principles for hyponatraemia mechanisms, and the 3 classifications that follow

Answer 4

Sodium is more a measure of fluid status than salt concentration

Losing more sodium than water

Gaining more water than sodium

• Hypervolaemic hyponatreamia (gaining lots of water but small amount of sodium)
• Euvolaemic hyponatreamia (increasing H20 with a normal Na+, i.e. contrary to the name!)
• Hypovolaemic hyponatraemia (losing both sodium and water, but more sodium than water)

Question 5

Causes of hypervolaemic hyponatraemia

Answer 5

Congestive heart failure
Cirrhosis
Nephrotic syndrome

Question 6

Outline pathophysiology of hypervolaemic hyponatraemia

Answer 6

Leakage of fluid out into vessels leading to oedema and low circulating volume.

This causes ADH to be released which just reabsorbs water via collecting ducts

It also leads to aldosterone release, which retains Na+ but also water.

Overall, more water is gained than sodium

Question 7

Causes of hypovolaemic hyponatraemia

Answer 7

Diarrhoea and vomiting (where enterocytes put Na+ into gut lumen, and these are not reabsorbed)

Diuretics (when Na+ remains in the tubule and goes into the urine)

Cerebral salt wasting

Question 8

What type of electrolyte imbalance can cerebral salt wasting lead to and why

Answer 8

HYPOVOLAEMIC HYPONATRAEMIA

This condition disrupts the SNS activation of sodium reabsorption in the kidneys.

Can follow meningitis

Question 9

Definition recap and causes of euvolaemic hyponatraemia

Answer 9

It actually means INCREASED H2O and normal Na+

The reason it’s called this is because you DON’T have fluid pouring into the interstitial space so no oedema

There are some causes with DILUTE urine (think about this as when the body has lots of water):

• Adrenal insufficiency
• Drinking too much (water, or beer!)

Some with CONCENTRATED URINE:

• Syndrome of inappropriate ADH secretion leads to H2O retention
• Hypothyroidism (mechansim not understood)

Question 10

Why is there euvolaemic hyponataemia in the case of adrenocortical failure

Answer 10

Remember that euvolaemic hyponatraemia actually means the total body sodium is normal, but that there is just too much water (but NOT leading to oedema)

Hyponatremia is mediated by increased release of antidiuretic hormone (ADH) which results in water retention and a reduction in the plasma sodium concentration (even though there isn’t actually less Na+, because BNP is released which reabsorbs this in the PCT)

Question 11

What is false hyponatraemia

Answer 11

Body sodium and water are normal

But excess lipids (e.g. hypertriglyveridaemia) or proteins (e.g. multiple myeloma) which affect the lab machinery to give a false hyponatraemia

Question 12

Symptoms of hyponatreamia

Answer 12

Nausea and vomiting, muscle cramps

Sever (<120mEq/L):

1) cerebral oedema leading to confusion, coma and death
2) raised ICP, which can squash the blood vessels leading to ischaemia/herniation

Question 13

Why is there cerebral oedema in hyponatramiea

Answer 13

Water moves from the EC to the IC compartment (cells in CNS swell up and die)

Question 14

What investigations for hypotraemia

Answer 14

-Low serum osmolarity shows true hyponatramia

-Urine osmolality to differentiate the two types of euvolaemic hyponatramia:
(if urine really concentrated >100mOsm/kg), could be SIADH, whereas if it’s dilute it could be (<100mOsm/kg) could be due to taking in too much fluid/adrenocortical failure

-Urine Na+:
20-40mEq/l- suggests SIADH and cerebral salt wasting
-<20mEq- suggests hypovolaemia

Question 15

What type of hyponatramiea do the following signs suggest:

• Oedema
• Dehydration

Answer 15

Odema- hypervolaemic hyponatramia

Dehydration- Hypovolaemic hyponatramia

Question 16

How to manage severe hyponatraemia, why must you be careful?

Answer 16

HYPERTONIC SALINE

Careful to avoid cerebral pontine myelinolysis

Question 17

How do you manage each of the following types of hypontraemia:

hypovolaemic
euvolaemic
hypercolaemic

Answer 17

Hypovolaemic: hypertonic saline infusion

Euvolaemic:

Question 18

…

Answer 18

…

Question 19

…

Answer 19

…

Question 20

…

Answer 20

…

Question 21

What usually defines hyperkalaemia

Answer 21

Serum K+>5.5mMol/L

Question 22

Categorise the causes of hyperkalaemia

Answer 22

1. Drugs: ACEi/ARBs/Potassium sparing diurectics (spironolactone/amiloride)/NSAIDs/digoxin (in toxicity)/b blockers/trimethoprim/heparin
2. Renal failure (inability to excrete K+)
3. Iatrogenic:
   - drugs,
   - over replacement of K+ with IV fluids,
   - blood transfusions gives a load of K+
4. Excessive release from cells:
   - Tissue necrosis (burns, rhabdo, trauma)
   - Massive haemolysis (e.g. ABO incompatibility)
   - Low insulin (DKA)
   - Acidosis
5. Addison’s disease:
   - Lack of aldosterone

Question 23

Why does heparin/LMW

H cause hyperkalaemia

Answer 23

It causes hypoaldosteronism. Reduces synthesis of aldosterone and blocks ATII receptors on zona glomerulosa

Question 24

Why does acidosis lead to hyperkalaemia

Answer 24

When the plasma becomes acidotic the body attempts to correct this by promoting muscle cells to absorb hydrogen ions out of the plasma.
This exchange mechanism involves the transfer of potassium ions into the serum, resulting in plasma K+ levels rising.

Question 25

Symptoms from hyperkalaemia

Answer 25

1. Palpitations
2. Chest pain
3. ARRHYTHMIAS (risk increases with values above 6.5mMol/L

TENTED T WAVES

Question 26

Investigations for hyperkalaemia

Answer 26

U&Es (renal failure can cause)

FBCs (sudden drop in Hb in acute haemolysis)

ECG (can cause arrhythmia)

Insulin (DKA can cause it)

Serum cortisol (Addison’s disease can cause)

ABG (acidosis can cause)

Question 27

ECG changes associated with hyperkalaemia

Answer 27

Tall tented T waves

Flattening of P waves

Broad QRS complexes

Question 28

Acute management of hyperkalaemia

Answer 28

1. IN THE EVIDENCE OF ECG HYPERKALAEMIA: Protect heart:
   -10mL, 10% calcium gluconate
   To stabiolise the myocardium (onset action 1-3 minutes, DOA 30-60mins)
2. Shift K+ into cells
   -Insulin and glucose infusion (Rapidly acting insulin combined with glucose as an infusion helps drive potassium into the cells and out of the serum. The glucose is used to prevent hypoglycaemia whilst the potassium is being shifted intracellularly)
   (Onset action 10-20 minutes, DOA 2-6hrs)

AND
-Salbutamol (adjuvant therapy) promites movement of potassium into cells. Onset of action 15-30 minutes. DOA- 4-6hrs

CHECK SERUM POTASSIUM AT 1 / 2 / 6 / 12 hours after treatment

Question 29

Long term correction of hyperkalaemia

Answer 29

Consider the underlying cause

Think about common cuases:

1. Have they recently had IV fluids or blood transfusion
2. Are they taking any causative drugs
3. Renal impairment
4. Acidotic (ABG)

Question 30

What happens to the total body calcium following acute hyperkalaemia management

Answer 30

Acute management provides only temporary reduction of K+ levels via shifting K+ out of the plasma and into cells, meaning total body K+ levels remain unchanged. As a result, the patient will likely become hyperkalemic once again if the underlying cause is not treated.

Thus you need to assess underlying cause

Question 31

Any long term hyperkalaemia management medication (other than treating cause)

Answer 31

Calcium resonum

not 1st line.

Proivdes slow method of reducing K+ levels, by binding K+ in the bowel and preventing its reabsorption

Used in patients with resistant hyperkalaemia who require long term management

Most commonly used in those with chronic renal failure

As mentioned above, senior input should be sought before using this drug

As with all drugs that lower K+, there is the risk of causing hypokalaemia

Question 32

Intermittent confusion, non-specific abdominal pain and constipation is highly suggestive of what electrolyte imbalance

Answer 32

Hypercalcaemic crisis

Question 33

Why can hyponatraemia be fatal if severe

Answer 33

Severe hyponatraemia is defined as serum sodium < 120 mmol/L and is associated
with neurological symptoms such as seizures, hallucinations, confusion and memory
loss. Hyponatraemia can be fatal if the serum sodium drops acutely over 24-28
hours as this can lead to cerebral oedema, coning and respiratory arrest.

Question 34

What might lead to an erroneously low sodium reading

Answer 34

The presence of high levels of lipids
or proteins in the sample, which dilutes the aqueous component of the extracellular
compartment thereby decreasing the apparent sodium concentration.

Question 35

What are the signs of central pontine myelinolysis

Answer 35

Paralysis, dysarthria and dysphagia