The accumulation of potassium ions within the extracellular space begins within seconds of a coronary artery becoming totally occluded. Within 5 minutes, The extracellular potassium concentration within the ischemic zone has risen from its normal value of approximately 4.5mM to levels of 10-12 mM. It plateaus at this level for 10-15 minutes and then begins to rise again, albeit at a slower rate, reaching levels of approximately 20 mM after 40 minutes of ischemia. As discussed in the previous chapter (see page 5.2.11), changes in extracellular potassium are associated with changes in resting membrane potential (RMP) and an increase in extracellular potassium concentration within the ischemic zone from 4.5 to 10mM is associated with a change in the RMP of the cells within the ischemic zone from -85 to approximately -60 mV. The subsequent increase in extracellular potassium to 20 mM results in a further change in the RMP to the range of -45 mV.