Einthoven’s triangle and Cabrera circle. (A) Definitions of durations for waves (P, Q, R, S, and T), intervals (QRS, QT, and ST), and segments (ST) are illustrated in two magnified and stretched normal cardiac cycles from human lead II (manual tracing over actual recording) and from zebrafish lead reverse II (rII). The normal physiological Q wave is typically absent or barely visible in adult human lead II and adult zebrafish lead rII. (B) To construct human Einthoven’s triangle, three pairs of electrodes for three bipolar Einthoven leads are placed on both arms and the left leg to represent the three sides of an imaginary inverted equilateral triangle enclosing the human heart. To construct zebrafish Einthoven’s triangle, we inserted two pairs of electrodes for the two bipolar Einthoven leads, such as leads I and II (solid lines). Since all three Einthoven leads relate by Einthoven’s law (II = I + III), the Cardiac Axis Extension algorithm (ADInstruments) derived the remaining lead III (dashed line). For grounding, we inserted the fifth electrode, the reference (ref) electrode, either in the damp sponge or in the zebrafish anal region. (C) We numbered the four 90°-quadrants of the Cabrera circle clockwise, roughly corresponding to human “normal QRS axis,” “right axis deviation,” “extreme axis deviation,” and “left axis deviation.” The human QRS axis defines the main heart axis, which normally averages +60° but the range of normal values is wide, encompassing from –30° to +105°, spilling from Quadrant I onto Quadrants II and IV. The zebrafish electrical heart axes and main heart axis were not known prior to our study. RA/LA, right/left atrium; RV/LV, right/left ventricle; BA, bulbus arteriosus; A, atrium; V, ventricle.

Representative simultaneous Einthoven ECG recordings from an adult man and an adult male zebrafish. Zebrafish lead III was software-derived based on Einthoven’s Law (II = I + III).

Lead- and species-comparison. (A) No lead differences existed in the durations of any waveforms for either species. However, for both leads, corrected QT (QTc) interval was the only waveform of similar duration in both species and corrected ST (STc) segment was the only waveform longer in zebrafish than in humans. (B) Lead II consistently registered larger amplitudes than lead I for all three human ECG waveforms, but only for the zebrafish P-wave. Not surprisingly, all ECG voltages were smaller in zebrafish than in humans. (C) The P, QRS, and T polarities were predominantly positive in both human leads I-II and zebrafish lead I, but negative in zebrafish lead II. Consequently, T-wave concordance was positive in human leads I-II and zebrafish lead I, but negative in zebrafish lead II. (D) The human and zebrafish cardiac electrical axes in the frontal plane are reflections across 0° in the Cabrera system, in Quadrants I and IV, respectively. In both species, the dominant QRS axis defined the main heart axis. Symbols: (A,B) Sample sizes: Human (n = 24), zebrafish (n = 30). Box-plot bar, mean; whiskers, SD; circles, subjects. P-values in blue for lead comparison of the same subject (Wilcoxon paired signed rank test), in red for species comparison of the same lead (Mann-Whitney-Wilcoxon test). ^nsP > 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. (D) Line, mean; wedge, (95% CI).