What is the risk of low potassium. how low potassium needs to be to cause irregular heart beat?

Usually 3.3 or lower: This depends on other health issues. Other electrolyte disorders that are present might make a person more sensitive to low potassium. Certain meds, most critically digoxin, increase sensitivity to low potassium as well. Perhaps most important is the reason why postassium is low; if the abnormality is caused by a systemic illness then problems might occur even at a level of 3.4 to 3.6 or so.