While straightforward monitoring and ethological interpretability has made behavior the most commonly used parameter for inferring and characterizing emotional states in animals, there is a long history of cardiac readouts as emotion indicators in the fields of neurosciences and psychology.Conceptual frameworks such as the threat imminence theory, postulating that the distance to a threat influences the selection of a defensive strategy, defined a convincing model to explain the range of possible behavioral responses. However, while autonomic responses proved equally variable and dynamic, there did not seem to be a way to reconcile seemingly contradictory results, leading to diverging opinions and partial disinterest in the field.By leveraging the benefits of integrated analyses offered by our customized and low-cost system for tethered electrocardiogram recordings (ECG) in freely moving animals, we were recently able to show that stereotypical cardio-behavioral associations (microstates) such as defensive immobility-associated bradycardia were sensitive to latent states (macrostates) in part driven by environmental threat level. This in turned allowed us to attribute a role of microstate generator to a specific neuronal population within the midbrain’s periaqueductal gray.By providing a description of technical aspects together with key results to discuss critical analytical points and concepts, we highlight the potential and simplicity of implementing and combining ECG recordings with existing paradigms and setups. We show that this approach offers insights into latent neuronal activity and brain states theoretically extending beyond defensive states (e.g. attentional processes, threat detection, etc) and improves translatability to and from humans.