| Summary: | <i>Elsholtzia ciliata</i> essential oil (<i>E. ciliata</i>) has been developed in Lithuania and internationally patented as exerting antiarrhythmic properties. Here we demonstrate the pharmacological effects of this herbal preparation on cardiac electrical activity. We used cardiac surface ECG and a combination of microelectrode and optical mapping techniques to track the action potentials (APs) in the Langendorff-perfused rabbit heart model during atrial/endo-/epi-cardial pacing. Activation time, conduction velocity and AP duration (APD) maps were constructed. <i>E. ciliata</i> increased the QRS duration and shortened QT interval of ECG at concentrations of 0.01–0.1 μL/mL, whereas 0.3 μL/mL (0.03%) concentration resulted in marked strengthening of changes. In addition, the <i>E. ciliata</i> in a concentration dependent manner reduced the AP upstroke dV/dt<sub>max</sub> and AP amplitude as well as APD. A marked attenuation of the AP dV/dt<sub>max </sub>and a slowing spread of electrical signals suggest the impaired functioning of Na<sup>+</sup>channels, and the effect was usedependent. Importantly, all these changes were at least partially reversible. Our results indicate that <i>E. ciliata</i> modulates cardiac electrical activity preferentially inhibiting Na<sup>+</sup> conductance, which may contribute to its effects as a natural antiarrhythmic medicine.
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