Use of micromachined probes for the recording of cardiac electrograms in isolated heart tissues

Micromachined probes, with iridium (Ir) microelectrodes on silicon shanks, were evaluated to assess their suitability for cardiac electrogram recording. The electrochemical activation (anodic oxidation) procedure for the circular Ir microelectrode was investigated using the square wave potential according to the electrode size, number of cycles, and cathodic-anodic potential level of the square wave. Increase in the charge storage capacity was pronounced either in smaller electrodes or with higher potential level of the square wave. The electrode impedance reduced in a similar manner with increasing number of cycle irrespective of the electrode size. With either lower potential level (-0.70/+0.60 V) or smaller number of cycle (200 cycles) than those for the activation of stimulating electrode, the likelihood of overactivation of the recording microelectrode can be minimized. These anodic IrOx film (AIROF) microelectrodes were used for the recording of extracellular electrograms in two different ex vivo cardiac tissue preparations. A single-shank microprobe was applied to the left ventricle of a mouse heart. Both the spontaneous and paced transmural responses propagating between epicardium and endocardium were obtained. Longitudinal cardiac wavefronts propagating along the rabbit papillary muscle were also recorded with a unique multiple-shank design. The measured mean amplitude and the propagation velocity of the extracellular voltage were 12.2 +/- 1.8 mV and 58.9 +/- 2.2 cm/s, respectively (n = 27). These microprobes with precisely defined electrode spacing make a useful tool for the spatial and temporal mapping of electrical properties in isolated heart tissues ex vivo.