The ECG and the single channel

Our ability to understand and use the ECG has increased with the availability of tools to study the heart's electrical system. Great advances were achieved with direct electrical recordings from intact animal and human hearts. The microelectrode opened the way to recording of the cellular action potentials and their underlying currents. We now have two new and powerful methods to study cardiac electrophysiology--the patch clamp and molecular biology. We have begun to characterize the behavior of the elementary unit of membrane current, the single channel. The single channel can best be visualized as existing in a finite set of states related sequentially as in a Markov chain. The primary structures of several channels have now been determined by cloning, and the structural determinants of channel function can be explored. We have entered the molecular age of electrophysiology and can expect a greater understanding of the basic physiological and pathophysiological processes underlying the ECG. We can also expect powerful drugs to be designed based on the structures of their channel targets. The ECG remains a rich area for study and an ever better clinical tool.