Describe the determinants of serum potassium. Outline the consequences of acute hyperkalaemia.
Most candidates did not appreciate that serum potassium is a function of two variables:
1. Total body potassium
2. Distribution between the extracellular and intracellular fluid compartments.
Approximately 98% of total body potassium is intracellular due to the action ofNa+/K+
ATPase. Potassium is important in the electrophysiology of excitable cells and changes in
serum potassium can affect their function. Hence the importance of keeping the serum
potassium within a narrow normal range.
Again most candidates did not provide the overview that serum potassium levels reflect a
balance between intake, output and transcellular distribution. Normal dietary intake is highly
variable. Transcellular distribution by the mechanisms of insulin and glucagon,
catecholamine's and P2 activity and acid base changes all work to rapidly restore changes in
serum potassium levels back towards the normal range. Many candidates did not provide any
details on the long term renal regulation of serum potassium involving distal tubule
potassium secretion and aldosterone and also the effect of distal tubular flow and sodium
The effects of hyperkalaemia were better described than the first part of the question. Most
candidates concentrated on the cardiac effects where most marks were awarded. The effects
of an increased potassium on the cardiac action potential earned extra marks. The correlation
between actual serum potassium level and ECG changes is variable and depends on many
factors including how acute or chronic the hyperkalaemia is.
Treatment of hyperkalaemia was mentioned by a few candidates but attracted no extra