Simultaneous Optical and Electrical Recordings

Development of antiarrhythmic electrical therapies requires knowledge of the characteristics of cardiac arrhythmias and effects of electrical stimulation on the heart. Much of the available knowledge is obtained from measurements of extracellular potentials or transmembrane potentials in the heart using electrical or optical methods. With an extracellular electrode in contact with the heart, activation of the cells is detected by observing the intrinsic negative deflection of the extracellular potential. (In this chapter, the occurrence of a rapid increase in inwardly directed membrane current during the phase zero depolarization is termed activation, whereas the transition of a dye molecule to the excited state by light absorption is termed excitation.) Maps constructed from the times of these deflections at several locations in the heart indicate the spatiotemporal distributions of activation. Also maps of the distribution of extracellular potentials in the heart during a shock reveal characteristics of the electric field, which can be correlated with effects of shocks on the activation. Extracellular potential measurements can also indicate repolarization using t-wave analysis, and action potential contour using the monophasic action potential produced by pressure or suction near the electrode. Most electrical measurements do not detect activation during a shock pulse because of interference by the shock's electric field.

Another way to examine arrhythmic activation and effects of shocks is by optical action potential measurements. An isolated heart or heart tissue specimen is stained with a transmembrane voltage-dependent fluorescent dye and illuminated with excitation light. Absorption of light by dye molecules induces an excited molecular state. The molecules emit fluorescence photons as part of the process of decay to the ground state. The spectrum of emitted fluorescence from di-4-ANEPPS shifts toward shorter wavelengths when the heart cells depolarize. This shift reverses when cells repolarize. Therefore, intensity of the long wavelength portion of the fluorescence spectrum (e.g., red light containing wavelengths >570 nm in rabbit heart containing di-4-ANEPPS excited with blue light at 488 nm) decreases when cells depolarize, and then increases during repolarization. The resulting signal contains an inverted optical action potential that is linearly related to the transmembrane potential. However, there is not an absolute calibration between the optical signal and membrane voltage.