After breast cancer surgery, more than 60% of patients experience persistent pain (1). Although opioids are the main method of use in postoperative pain management, there are side effects such as constipation, sedation, respiratory depression, urinary retention, itching, postoperative nausea and vomiting (2). Erector spina plane block (ESP) is a technique used in many surgeries for anesthesia and analgesia. It is frequently preferred by anesthesiologists for postoperative analgesia recently because it is easy to apply and no complications are reported in the literature. Erector spina muscle, with arms of iliocostalis, longissimus and spinalis, is located parallel to the spine, from the back of the skull to the pelvis. The injected local anesthetic is distributed in many levels in the cranio-caudal direction and blocks the intercostal spinal nerves, causing sensory block (3). Providing the necessary perioperative analgesia due to the complex innervation of the breast tissue is a big controversy among anesthesiologists. Multimodal analgesia methods with various regional blocks are more preferred. Erector Spina Plane block is also preferred as another method since it provides both perioperative and postoperative analgesia (4). In studies conducted on cadavers, it has been shown that local analgesia spreads to the paravertebral area when applied under the Erektor Spina muscle. Technically, compared to the paravertebral block, the Erektor Spina block has proven to be more reliable and easier in terms of pleural puncture risk (5).

ANI is a monitoring method used in the evaluation of acute nociception and pain (Figure 2.1). Analyzes the instantaneous changes in heart rate due to the activation of the parasympathetic nervous system using respiratory sinus arrhythmia. A value between 0-100 is obtained. If parasympathetic modulation (stress level eg presence of pain) is very low, a value of 0 is obtained, if it is high, a value of 100 is obtained (7). ANI detects the noxious stimulus more specifically and sensitively than heart rate and blood pressure changes (6).

The "yellow colored value" on the upper right corner of the ANI monitor shows the instantaneous ANI value, while the orange colored one shows the average ANI value in the last 4 minutes. (Figure 2.1). Application of anesthesia and having ANI value between 50-70 in the early postoperative period indicates that analgesia is sufficient, and values \<50 are the following 10 minutes. A hemodynamic reactivity (20% increase in heart rate or blood pressure) will occur and the level of analgesia is insufficient, and values\> 70 indicate that there is no painful stimulus or that more analgesic drugs are used (6). After breast cancer surgery, more than 60% of patients experience persistent pain (1). Although opioids are the main method of use in postoperative pain management, there are side effects such as constipation, sedation, respiratory depression, urinary retention, itching, postoperative nausea and vomiting (2). Erector spina plane block (ESP) is a technique used in many surgeries for anesthesia and analgesia. It is frequently preferred by anesthesiologists for postoperative analgesia recently because it is easy to apply and no complications are reported in the literature. Erector spina muscle, with arms of iliocostalis, longissimus and spinalis, is located parallel to the spine, from the back of the skull to the pelvis. The injected local anesthetic is distributed in many levels in the cranio-caudal direction and blocks the intercostal spinal nerves, causing sensory block (3). Providing the necessary perioperative analgesia due to the complex innervation of the breast tissue is a big controversy among anesthesiologists. Multimodal analgesia methods with various regional blocks are more preferred. Erector Spina Plane block is also preferred as another method since it provides both perioperative and postoperative analgesia (4). In studies conducted on cadavers, it has been shown that local analgesia spreads to the paravertebral area when applied under the Erektor Spina muscle. Technically, compared to the paravertebral block, the Erektor Spina block has proven to be more reliable and easier in terms of pleural puncture risk (5).

ANI is a monitoring method used in the evaluation of acute nociception and pain (Figure 2.1). Analyzes the instantaneous changes in heart rate due to the activation of the parasympathetic nervous system using respiratory sinus arrhythmia. A value between 0-100 is obtained. If parasympathetic modulation (stress level eg presence of pain) is very low, a value of 0 is obtained, if it is high, a value of 100 is obtained (7). ANI detects the noxious stimulus more specifically and sensitively than heart rate and blood pressure changes (6).

The "yellow colored value" on the upper right corner of the ANI monitor shows the instantaneous ANI value, while the orange colored one shows the average ANI value in the last 4 minutes. (Figure 2.1). Application of anesthesia and having ANI value between 50-70 in the early postoperative period indicates that analgesia is sufficient, and values \<50 are the following 10 minutes. A hemodynamic reactivity (20% increase in heart rate or blood pressure) will occur and the level of analgesia is insufficient, and values\> 70 indicate that there is no painful stimulus or that more analgesic drugs are used (6).