A three-dimensional approach to visualize dynamical properties of cardiac ventricular action potential repolarization

The action potential (AP) of excitable cells consists in a rapid depolarization (and a transient inversion of polarity) of their membrane potential (Vm) which then, more slowly, recovers its resting value. AP is therefore graphically represented as a function of a single variable Vm (t). The morphology of AP waveform assumes a particular significance for the cardiac working cells, where it triggers and modulates excitation-contraction coupling. I describe here a novel compact three-dimensional representation of cardiac membrane excitation, measured by means of a combined current-voltage clamp protocol, where the third dimension is given by instantaneous membrane current (Im)-Vm profiles, taken as repolarization proceeds. These three coordinates (t, Vm, Im) describe a surface which intersects the Im = 0 plane along the AP trajectory. I have previously performed measurements of this type in vivo, and adopt them here iteratively at a very high t, Vm, Im-resolution on different cardiac ventricular AP mathematical models. These 3D t-Vm-Im surfaces provide a direct and compact representation of several key features of cardiac membrane repolarization, which include: refractory period, supernormal excitability, all-or-nothing repolarization window. They also provide a unique tool to visualize and measure membrane resistance during repolarization, as well as the amount of repolarization reserve available for a given cell type. A further, and perhaps the more important, feature of this novel computational/graphical method is the possibility of discriminating between very similar or even identical AP waveforms, endowed though with quite different dynamical properties.