1. An apparatus comprising a computer readable medium storing instructions for causing a computing system to perform operations comprising: receiving, from two or more ECG leads, an ECG signal of a patient; generating at least two orthogonal ECG vectors based on the ECG signal; processing the at least two generally orthogonal ECG vectors to determine a loop trajectory of at least a portion of the ECG signal; identifying a trajectory bifurcation by comparing the loop trajectory to a control loop trajectory for a plurality of cardiac cycles; and determining an indicator of a cardiac event based on the trajectory bifurcation.