Abstract
Optical pacing has been demonstrated to be a viable alternative to electrical pacing in embryonic hearts. In this study, the feasibility of optically pacing an adult rabbit heart was explored. Hearts from adult New Zealand White rabbits (n = 9) were excised, cannulated and perfused on a modified Langendorff apparatus. Pulsed laser light (λ = 1851 nm) was directed to either the left or right atrium through a multimode optical fiber. An ECG signal from the left ventricle and a trigger pulse from the laser were recorded simultaneously to determine when capture was achieved. Successful optical pacing was demonstrated by obtaining pacing capture, stopping, then recapturing as well as by varying the pacing frequency. Stimulation thresholds measured at various pulse durations suggested that longer pulses (8 ms) had a lower energy capture threshold. To determine whether optical pacing caused damage, two hearts were perfused with 30 µM of propidium iodide and analyzed histologically. A small number of cells near the stimulation site had compromised cell membranes, which probably limited the time duration over which pacing was maintained. Here, short-term optical pacing (few minutes duration) is demonstrated in the adult rabbit heart for the first time. Future studies will be directed to optimize optical pacing parameters to decrease stimulation thresholds and may enable longer-term pacing.
Highlights
Cardiac electrophysiology and pacing are routinely used to treat and study arrhythmias, electrical pacing suffers from several drawbacks including low spatial precision [1], stimulation artifacts in electrode recordings [2], interference from magnetic fields [3] and the requirement of tissue contact [4]
The timing between the trigger and QRS complex was 84.2 ± 26.3 ms (N = 4). This was similar to the AV delay of 81 ± 19 ms previously reported in the rabbit, indicating that conduction of the optically paced action potentials propagated through the AV node [15]
We have shown that when combining electrical and optical stimulation in the buccal nerve of the Aplysia californica, both excitation and inhibition of action potentials could be elicited depending on the energy of the light pulses [32]
Summary
Cardiac electrophysiology and pacing are routinely used to treat and study arrhythmias, electrical pacing suffers from several drawbacks including low spatial precision [1], stimulation artifacts in electrode recordings [2], interference from magnetic fields [3] and the requirement of tissue contact [4]. We have recently demonstrated that infrared optical pulses can noninvasively pace early-stage quail embryo hearts without the use of any exogenous agents or genetic modifications [4]. Pacing near the threshold level produced no evidence of ultrastructural or functional damage to the tissue. This technique is enabling new methods to study the developing heart. Because the study was limited to early-stage embryonic quail hearts that were still tubular, semi-translucent and less than a millimeter in length, it did not make clear whether adult hearts, which are much larger, denser and more structurally complex, can be effectively paced with an infrared pulsed laser
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