Sometimes, the AV junction and the ventricles become irritable and exceed their corresponding inherent rates. For example, if the SA node fires at a rate of 72 bpm, and the AV junction suddenly discharges at 98 bpm, the latter becomes the heart’s pacemaker. This pacemaker “takeover” by the AV junction is known as irritability. 

In some cases, the SA node rate drops below its lower limit to the point at which another governor exceeds its rate. In this case, the governor with the next highest inherent rate—the AV junction—takes over. This phenomenon is known as escape. It is one of the heart’s protective mechanisms.