Effective analgesia in the Intensive Care Unit (ICU) leads to a reduction in complications in critically ill patients and improves the outcome of treatment. For effective analgesia with the applied analgesic methods, it is necessary to use tools for assessing the intensity of pain. The main goal of the research is to evaluate the effectiveness of systemic analgesia in critically ill patients using pain assessment tools. The research was conducted as a retrospective data analysis in the period from June 1, 2022 to July 1, 2022 in the Intensive Care Unit of the Department of Anesthesiology, Reanimation and Intensive Care Medicine, General Hospital Karlovac. The patients were divided into two groups. Group A conscious, verbally responsive, on spontaneous respiration, group B analgosedated on invasive mechanical ventilation. The intensity of pain in group A was assessed using the NRS (Numeric Rating Scale) scale and SPI (Surgical Plethysmographic Index), and in group B using the BPS (Behavioral pain scale) scale and SPI for group B. The intensity of pain was analyzed in each group of 10 patients, 40 measurements were performed. The research results showed that group A had a higher percentage of ineffective analgesia (52.5%), assessed through the NRS. While in group B it amounted to only 10%, assessed by the BPS scale. Ineffective analgesia for NRS was any sum greater than four and on the BPS scale greater than six and required analgesic intervention. Ineffective analgesia assessed by SPI was equal, in group A it was 45.0% and in group B 47.5%. The effect of pain on the vital parameters SAP, DAP, HR assessed with monitoring of nociception-SPI and shows a statistically significant correlation. The results of this research indicate the insufficiently effective analgesia of critically ill patients, the introduction of pain assessment tools into clinical practice with staff education in order to achieve more effective analgesia of critically ill patients.