The Photodiode Array: A Critical Cornerstone in Cardiac Optical Mapping

The human heart pumps oxygenated blood to the organs and extremities in order to maintain normal physiologic function, while simultaneously pumping deoxygenated blood to the lungs for reoxygenation. Coordinated contraction of individual cardiac myocytes provides the mechanical force necessary to produce sufficient pressure and ensure that distant organs and extremities remain oxygenated. Before cardiac myocytes may contract, they must undergo excitation in order to begin the sequence of events which results in an intracellular calcium (Cai) rise, which in turn precipitates actin-myosin binding and ultimately results in contraction. The electrical signature of this series of events is reflected in the cardiac action potential (AP), a segment of a transmembrane voltage (Vm) recording which indicates electrical excitation (depolarization) and relaxation (repolarization) of the myocardium. The duration, amplitude, upstroke velocity (dVm/dt), and overall morphology of the cardiac AP are important markers of the electrical status of the heart. Studies of the cardiac AP have provided important insights into the mechanisms which drive the transition from a normal, healthy heartbeat toward a deadly cardiac arrhythmia. Early recordings of the cardiac AP were obtained using microelectrodes (Coraboeuf & Weidmann, 1949a; Coraboeuf & Weidmann, 1949b; Draper & Weidmann, 1951; Sano et al., 1959; Sano et al., 1960; Weidmann, 1951). Although this method was highly effective in tracking temporal changes in the Vm of individual cells, the method could not be easily applied to the problem of tracking excitation over a region of tissue. Extracellular electrode mapping offered a partial solution to this problem and was sufficient to determine activation times in regions of tissue, but with this method the details of repolarization were lost and had to be estimated using indirect indicators. Further, this method required that the electrodes be in direct contact with the tissue. This made defibrillation studies difficult, since large amplitude defibrillation shocks typically obscure the details of activation during electrical recordings. Monophasic action potential (MAP) recordings were capable of elucidating the details of repolarization without damaging tissue, and have even been recorded in the beating human heart using a cardiac catheter (Shabetai et al., 1968). However they too were restricted by having little or no spatial resolution and could not be