Potassium Disorders (background lecture) Flashcards
What is the primary cation in the intracellular space?
Potassium
Describe the distribution of potassium in the plasma
Only a small fraction of total body potassium is found in the plasma
What are 3 important functions of potassium?
- Nerve transmission
- Cardiac contractility
- Muscle contractility
Describe the relationship between serum potassium levels and signs of toxicity
Signs of toxicity (especially cardiac arrhythmias) are directly related to plasma potassium
Describe the role of potassium in intracellular volume
It may play important roles in the control of intracellular volume, similar to the role of sodium in controlling extracellular volume
Describe the applicability of serum potassium to total body potassium
Serum concentration doesn’t reflect total body potassium very well, but it is still a relatively useful marker
What is a risk of giving high volumes of intravenous potassium?
This slows the heart
Where is potassium mainly found?
In muscle
What is the main mechanism of potassium clearance from the body?
It is 95% renally excreted
Why may a haemolysed sample lead to a false hyperkalaemia?
As there is potassium in red blood cells (erythrocytes)
What are 3 factors that increase the movement of potassium from the extracellular fluid to the intracellular fluid?
ß2-adrenergic stimulation
alkalosis
Insulin
What are 4 factors that increase the movement of potassium from the intracellular fluid to the extracellular fluid?
- Acidosis
- Plasma hyperosmolality
- ⍺-adrenergic stimulation
- Exercise
What are 5 factors which increase the urinary excretion of potassium?
- Plasma potassium levels
- Urine flow
- Kidney injury
- Aldosterone
- Sodium delivery
What are 5 factors which increase the urinary excretion of potassium?
- Plasma potassium levels
- Urine flow
- Kidney injury
- Aldosterone
- Sodium delivery
Describe the role of the kidneys in hyperkalaemia
It is very unlikely to have any degree of hyperkalaemia without at least some degree of renal impairment (even if mild)
Describe the relationship between serum potassium and body pH
Serum potassium will increase by 0.6 mmol/L for every 0.1 reduction in pH from 7.4, and decrease by 0.6 mmol/L for every 0.1 increase in pH from 7.4
Explain the activity of potassium in acidosis
Hydrogen is buffered intracellularly (pushed into cells) as a compensatory response, pushing potassium out of the cells, resulting in hyperkalaemia
Explain the activity of potassium in alkalosis
Hydrogen is buffered extracellularly (pushed out of cells) as a compensatory response, drawing potassium into the cells, resulting in hypokalaemia
Describe the activity of potassium in the kidneys
It is freely filtered in the glomerulus, but it is mostly reabsorbed prior to the distal tubule
Describe the activity of potassium in the kidneys
It is freely filtered in the glomerulus, but it is mostly reabsorbed prior to the distal tubule
How does hyperkalaemia affect aldosterone?
Hyperkalaemia causes and increase in aldosterone
Describe what determines the concentration of potassium in the urine
Immediate-relate is mostly a factor of potassium secretion in the distal tubule, rather than just glomerular filtration
What are the 2 aetiologies of hypokalaemia?
- Apparent deficit
- True deficit
What is an apparent potassium deficit?
Intracellular shifting of potassium
Decreased are 3 causes of a true potassium deficit?
- Decreased intake
- Increased output (extrarenal)
- Renal loss
What are 3 causes of intracellular shifting of potassium, leading to an apparent potassium deficit?
- Alkalosis
- ß-2 adrenergic stimulation
- Insulin (more common with intravenous boluses or infusions)
What are 5 causes of decreased intake of potassium?
- Alcoholism
- Potassium-free diets
- Intravenous fluids
- Anorexia nervosa
- Bulimia
What are 4 causes of increased extrarenal output of potassium?
- Vomiting
- Diabetes
- Laxative abuse
- Intestinal fistulas
What are 6 causes of increased renal potassium loss?
- Corticosteroids (especially fludrocortisone and hydrocortisone)
- Amphotericin B
- Diuretics (loop and thiazide)
- Hyperaldosteronism
- Cushing’s syndrome
- Liquorice ingestion
If a patient with hypokalaemia has hypertension, what may this indicate?
This could be due to a mineralocorticoid excess
How can you check for mineralocorticoid excess?
Check the plasma aldosterone:renin
What are 3 causes of transient hypokalaemia?
- Adrenergic hypokalaemia (stress)
- Post-carbohydrate meals
- Post exercise
If a patient with hypokalaemia has normal/increased plasma bicarbonate, and their urine potassium is <20 mmol/L, what does this suggest?
Either inadequate intake or extrarenal loss
If a patient with hypokalaemia has normal/increased plasma bicarbonate, and their urine potassium is >20 mmol/L, what does this suggest?
Renal loss
If a patient with hypokalaemia has decreased plasma bicarbonate, and their urine potassium is <20 mmol/L, what does this suggest?
Acute diarrhoea
If a patient with hypokalaemia has decreased plasma bicarbonate, and their urine potassium is >20 mmol/L, what does this suggest?
Renal loss
What is the main indicator of renal loss of potassium?
Urinary potassium > 20 mmol/L
What is the main cause of urinary loss of potassium?
Renal impairment/poisoning
What effect does hypokalaemia have on the T-wave?
Decreased T-wave amplitude
What effect does hypokalaemia have on the T-wave?
Decreased T-wave amplitude
How does hypokalaemia affect blood pressure?
It causes hypotension
How does hypokalaemia affect digoxin?
Hypokalaemia increases the risk of digoxin toxicity
How does hypokalaemia affect the PR interval?
Severe hypokalaemia causes PR prolongation
How does hypokalaemia affect heart rhythm?
Hypokalaemia causes rhythm disturbances
How does hypokalaemia affect the ST segment?
Hypokalaemia causes ST segment depression
How does hypokalaemia affect the QRS complex?
Hypokalaemia causes QRS widening
How does hypokalaemia affect the QRS complex?
Hypokalaemia causes QRS widening
Describe the metabolic and endocrine effects of hypokalaemia
Most serve as compensatory mechanisms
How does hypokalaemia affect aldosterone release?
Hypokalaemia causes decreased aldosterone release
How does hypokalaemia affect insulin release?
Hypokalaemia causes decreased insulin release
How does hypokalaemia affect renal responsiveness to antidiuretic hormone?
Hypokalaemia reduces renal responsiveness to antidiuretic hormone
How does hypokalaemia affect reflexes?
Severe hypokalaemia causes areflexia
What are two common muscular symptoms of hypokalaemia?
Cramps and weakness
How does hypokalaemia affect smooth muscle?
Hypokalaemia causes loss of smooth muscle function (ileus and urinary retention with severe hypokalaemia)
How does hypokalaemia affect urine concentration?
Hypokalaemia results in inability to concentrate urine
How does hypokalaemia affect the kidneys?
Hypokalaemia can cause nephropathy
What does the P wave represent?
Atrial contraction