Light-cardiogram, a simple technique for heart rate determination in adult zebrafish, Danio rerio

Current techniques for heart rate determination in adult zebrafish require specialist expertise and are often invasive, technically challenging and not readily transferable to other laboratories for routine assessment. Here, we present a simple, noninvasive and inexpensive light-cardiogram technique to assess heart rate and frequency in adult zebrafish. Brightfield microscope paired with a high-resolution camera and ImageJ (an open source software) were employed as core recording and processing platforms respectively. The heart was visualised ventrally and located by juxtaposing an isosceles triangle between the opercula as reference to analyse pixel intensity fluctuations generated by each cardiac cycle to derive heart rate and frequency. Compared to transparent embryos, the cardiograms generated reverse light signal oscillations, with contraction and relaxation of the heart (ventricle) corresponding to reduced and increased pixel intensities respectively. The heart rates (♂ 122.58 ± 2.15 and ♀ 121.37 ± 2.63 beat/min) and mean dominant frequency (♂ 2.04 ± 0.035 and ♀ 2.05 ± 0.048 Hz) between the sexes were not significantly (P > .05) different at 28 °C. However, the F_D amplitudes between males (0.26 ± 0.03) and females (0.45 ± 0.05) were significantly different (P < .05) suggesting sex specific diastolic cardiac outputs. Collectively, the technique can be used to measure heartbeats as well as readily adaptable to record relative cardiac outputs and compare differences between physiological states (e.g. sexes). Moreover, the approach could be amenable to automation and applicable to other fish species, enabling researchers the flexibility to measure these and other critical heart health endpoint with relative ease.