Hypokalaemia is a plasma [K+] of < 3.5 mmol/l.
The main concerns are cardiac arrythmias, respiratory failure and hepatic encephalopathy.
The main effect is on the resting membrane potential of cells, which becomes hyperpolarized as the ratio between the ICF and ECF [K+] increases. The ECF potassium comprises only 2% of the total potassium in the body and this is reflected in the relative [K+] of the ICF and ECF. If the ECF [K+] falls from 4 to 3 a comparable fall in ICF [K+] would take it from 150 to 112.5 mmol/l. The actual change is less than half this and as a result the ratio of ICF to ECF [K+] must rise. The resting membrane potential therefore rises as more K+ diffuses out of cells down an exaggerated concentration gradient.