Different mathematical tools, mostly the heart rate variability, the heart rate spectral analysis were studied to assess the depth of anesthesia and pain (6-7). Most of the works done on human patients were done on pain recognition in neonates ( ). Some works on adult patients have been done too, trying to define pain using the heart rate and its analysis. Since the acute effect of pain to increase heart rate is well known, it was hypothesized by Storella et al. (10), in their work that there may be adaptive effects of chronic pain on the autonomic regulation of the cardiovascular system that could be reversed by analgesia. They examined whether the acute relief of chronic pain affects heart rate variability and concluded their study, based on experimental data that acute relief of chronic pain is accompanied by an analgesia-specific increase in heart rate variability in many patients. Ray et al. in their work used this method for ECG signal processing and developed a new monitor for pain assessment during anesthesia (11). Using the heart rate signal, the R-R intervals and the spectral analysis were calculated. The output containing the information about the respiratory cycles from which the RSA (respiratory sinus arrhythmia) was drawn to which they assumed the level of consciousness was proportional. That study has demonstrated the feasibility of this method, but their article states many drawbacks and restriction for this system and clinically this method is not used today.