(A) Very early PAC (DI -4 ms) followed by a pause then AF. (B) Steep Restitution May Explain PAC-Induced AF (in red). S1, S2, F1 and F2 show marked APD oscillations, because of steep APD restitution (slope = 1.2; slope > 3 by monoexponential fit), then AF onset. AF cycles continue to track restitution, even though wavelets meander (altered activation sequence after F4).