Figure 2.

A schematic of AV interval dynamics without (a) and with (b) control. In both panels the schematic is superimposed over five series of AV intervals (annotated with different gray symbols) that correspond to consecutive control attempts at the same nominal VA interval in one alternans pacing and control trial. These AV intervals immediately followed control termination, and therefore obeyed f(AV_n, ) as they drifted away from the unstable steady state AV* and back into alternans. Thus, f(AV_n, ) was approximated by a quadratic curve fit to the AV intervals. The intersection of f(AV_n, ) with the line of identity (the diagonal line AV_n+1 = AV_n) is the unstable steady state AV*. Without control (a), the AV intervals alternate indefinitely between points 1 and 2 via the dynamic route depicted by the dotted lines, and never explore the unstable interior region of f(AV_n, ). b shows how the VA control perturbation of Eq. 3 shifts the function along the line of identity to the location of the dash-dot curve f(AV_n, + δVA_n). By doing so, point 1 becomes point 1′ (i.e., AV_n+1 is increased). When the function is returned to f(AV_n, ) at the next beat (i.e., δVA_n+1 = 0), control succeeds as the AV interval progresses to point 2, which is at the unstable steady state AV*.