1. Introduction artifacts. Last but not least, important differences between unipolar and bipolar recordings of both MAPs and conven-Monophasic action potentials (MAPs) are extracellularly tional electrograms will be discussed, as they pertain to the recorded wave forms that, under optimal conditions, can fidelity and spatial resolution of these recordings. reproduce the repolarization time course of transmembrane action potentials (TAPs) with high fidelity [1–3]. While TAP recordings require the impalement of an individual 2. Brief history of MAP recording techniques cardiac cell by a glass-microelectrode and therefore generally are limited to in vitro preparations, MAPs can be 2.1. The ‘injury’method recorded from the endocardium and epicardium of the in situ beating heart, including that of human subjects. MAP The first MAP was recorded in 1882 by Burdon-Sanderrecordings therefore are suitable for studying the charac- son and Page [7] who placed one electrode on the intact teristics of local myocardial electrophysiology, especially epicardial surface and the other on an injured site of a frog of repolarization, in the clinical setting. This has made heart, and thereby captured the phasic electrical changes of MAP recordings an important bridge between basic and the cardiac cycle on a charcoal-covered recording drum. clinical electrophysiology in multiple areas of arrhythmia Burdon-Sanderson and Page produced this injury by research [4]. cutting into the myocardium, and termed these recordings Despite the growing use of the MAP recording method, monophasic action currents (later to be replaced by there still is surprisingly little hard data on the exact monophasic action potentials). Until then, electrodes mechanism by which MAPs are created and recorded. New placed on intact heart tissue had been recording mul-methods for recording MAPs recently have been proposed, tiphasic deflections, similar to those recorded today in the and new theories and models have been suggested to human electrophysiology laboratory by conventional elecexplain the mechanisms that underlie the genesis of the trode catheters; so the term monophasic was a logical MAP signal. There have been attempts to record MAPs choice. It was believed for several decades that such from sites within the myocardial wall, an effort spurred on monophasic action potentials could only be produced by by the discovery that mid-myocardial cells (M-cells) have traumatic tissue injury or cellular disruption. Methods such different repolarization characteristics than either epicar- as cutting, stabbing or burning of a myocardial site were dial or endocardial cells [5], and these different charac- invented to produce monophasic ‘injury’currents [8]. In teristics have been found to be important for the develop- 1934, Schutz [9] introduced the suction electrode for ment of torsade de pointes arrhythmias [6]. This article experimental MAP recording. In 1969, Korsgren et al.[10] attempts to review the available information on the MAP used a suction electrode catheter in a patient and recorded, genesis and its particular recording modes. It also high- for the first time, MAPs from the in situ ventricular lights some important MAP quality criteria and guidelines endocardium. This pioneering step subsequently was amfor correct interpretation of MAPs and how to avoid plified by the work of Olsson and coworkers [11–13] who