4. Hypokalaemia and hyperkalaemia

Question 1

What is the normal range of plasma [K+]?

Answer 1

3.5 - 5.5 mmol/L-1

Question 2

What are hypo- and hyperkalaemia?

Answer 2

Hypokalaemia = plasma [K+] is too low ( <3.5 mmol/L-1 )
Hyperkalaemia = plasma [K+] is too high ( >5.5 mmol/L-1)

Question 3

Why are cardiac myocytes so sensitive to changes in [K+]?

Answer 3

1. K+ permeability dominates the resting membrane potential (is close to Ek)
2. Myocytes have many different kinds of K+ channels - some behave in a peculiar way

Question 4

What are the effects of hyperkalaemia on ventricular myocyte APs? What is the effect of this on AP conduction?

Answer 4

1. Increased plasma [K+]… so decreased membrane potential (more positive) - slight membrane depolarisation.
2. Inactivates some of the voltage-gated Na+ channels.
3. Slower Na+ influx… so slower upstroke of AP.
4. Slower AP conduction.

Question 5

What are the effects of hyperkalaemia on pacemaker cell AP generation?

Answer 5

• Pacemaker potential requires MPs more negative than -50 mV. The more negative the MP, the greater AP generation, because HCN channels are activated by hyperpolarisation.
• Decreased MP due to hyperkalaemia… decreases HCN channel activation… fewer pacemaker cells generate AP.

Question 6

What are the effects of hyperkalaemia on heart rate?

Answer 6

• May initially get an increase in excitability (decreased MP so easier to reach threshold and fire AP).
• But if enough Na channels are inactivated and conductions slows enough, can cause ASYSTOLE.
• Effect depends on extent of hyperkalaemia and how quickly it develops.
  ~ mild: 5.5-5.9 mmol/L
  ~ moderate: 6.0-6.4 mmol/L
  ~ severe: >6.5 mmol/L

Question 7

What are the possible treatments for hyperkalaemia?

Answer 7

• Calcium gluconate/chloride (stabilises cell membrane against undesirable depolarisation).
• Insulin + glucose (insulin promotes K+ mov into cells)

These won’t work if heart has already stopped.

Question 8

Describe the changes to the AP curve caused by hyperkalaemia.

Answer 8

1. Increased resting MP
2. Slower curve upstroke
3. Narrower curve

Question 9

Describe the changes to the AP curve caused by hypokalaemia.

Answer 9

Shifts downstroke of curve to right - wider curve (i.e. lengthens AP)

Question 10

What are the effects of hypokalaemia on ventricular myocyte APs? What is the effect of this on AP conduction?

Answer 10

1. Delays repolarisation… so lengthens AP.

2. Longer AP leads to early after depolarisations (EADs)… not enough time for proper APs so oscillations in MP.

Question 11

What are the effects of hypokalaemia on heart function?

Answer 11

Incorrect pattern of excitability due to early after depolarisations means less time for diastole due to lengthened systole.
Can result in VENTRICULAR FIBRILLATION… so no CO.