Caudal block is a common method of regional anesthesia. In pediatrics, it works as a supplement to general anesthesia and is a good method for control postoperative pain. By offering superior analgesia, it decreases the use for intravenous opioids, thereby reducing related adverse reactions like nausea, vomiting, and respiratory impairment. Consequently, caudal block is a safe and appropriate method for managing pain in children undergoing different types of surgery. Usually, it is given following general anesthesia in children, making a some difficulty in evaluating the block's effectiveness The success of a caudal block is traditionally predicted by hemodynamic changes in blood pressure and heart rate, although these variables are thought to be unreliable, many tests, like 'swoosh' and 'whoosh' tests, and ultrasonography, can be used to guarantee precise needle placement and enhance the probability of block success.

Other variables are sometimes used, such as laxity of the anal sphincter tone and loss of the cremasteric reflex; however under general anesthesia, their dependability is doubtful and the latter can only be used for male children . Technological developments in pulse oximeters have improved their capacity to monitor other parameters, such as the surgical pleth index (SPI) and perfusion index (PI).

Perfusion index (PI) is a non-invasive numerical evaluation of the pulsatile strength at a monitoring site that offers helpful data regarding the patient's peripheral perfusion status .

Local vasoconstriction may induce a decrease in PI, whereas vasodilatation causes an increase in PI in the skin at the monitoring site Successful anesthesia onset is indicated by an increase in PI, but failure to do so may be indicated by a lack of increase.

Surgical pleth index (SPI) is another noninvasive, dimensionless score whose value is derived from pulse wave amplitude and heartbeat interval as measured by a pulse oximetry probe The sympathetic reaction of the patient to the surgical stimulus is reflected in SPI , research on SPI indicates that adults with values between 20 and 50 have good nociception, but its usefulness in children is unclear.

The aim of this study This study aims to determine the time to change of value of PI and SPI in assessing the effectiveness of a caudal block in pediatric patients during infraumbilical surgeries under general anesthesia

This prospective observational study, will be conducted in Fayoum University hospital after the local Institutional Ethics Committee and local institutional review board approval.

Inclusion criteria

• Children aged 1 to 6 years of either sex
• With American Society of Anesthesiologists (ASA) physical status I-II
• undergoing elective infraumbilical surgery. Exclusion criteria
• parental refusal
• Patients with liver disease (e.g. elevated bilirubin >2mg/dL, albumin<3.0g/dL)
• Patients with renal disease (e.g. serum creatinine >1.5-2mg/dL)
• Cardiac disorders (low EF>50%, sever valvular heart disease)
• Recent respiratory tract infection within 1-2 weeks and bronchial asthma
• Known organic brain disease (cerebral palsy, tumors)
• patients with any contraindication to caudal anesthesia (local infection, coagulation abnormality e.g. INR > 1.5, known structural abnormality e.g. kyphosis)
• Known allergy to local anesthetics. Anesthesia procedure Children will be assessed for enrollment the day before surgery. After enrollment, written informed consent will be obtained from the patient's parents. Children will be fasted according to standard nil per oral (NPO) guidelines: Solids and infant feeds will be withheld for 6 hours, breast milk for 4 hours, and clear fluids for 2 hours before surgery.

There will be three observers in the study: Observer 1 will be a postgraduate in anesthesia and the principal investigator, responsible for screening patients preoperatively to ensure they met the inclusion criteria and will not have any of the exclusion criteria. Observer 2 will be a consultant anesthetist who will administer the anesthesia, Observer 3 will be a postgraduate in anesthesiology who will be unaware of whether the caudal epidural working. Observer 3 will assess the analgesia and the caudal site postoperatively.

An appropriately sized intravenous (IV) peripheral line (22G or 24G) will be secured in the ward, and IV antibiotics will be administered 1 hour before the procedure, all children will undergo routine preoperative preparations by taking full history, general examination and doing investigations: Complete blood count, Coagulation profile, liver and kidney function tests All patients will receive midazolam premedication 0.05 mg/kg (IV) in the preoperative holding area. On shifting to the operating room, standard monitors including electrocardiogram (ECG)monitoring (Lead II and V5), non invasive blood pressure (NIBP), pulse oximetry (SpO2), end tidal carbon dioxide (ETCO2), and volatile anesthetic monitoring will be instituted. NIBP cuff will be attached on the arm opposite to the side of intravenous access in all the patients. The core temperature will be monitored using a nasopharyngeal probe in all the patients and will be maintained between 36.5 and 37.5°C. All these parameters and PI will be recorded by putting pulse oximeter on big toe using Nihon Kohden (Life Scope model BSM-3562, Tokyo, Japan), PI is calculated as (AC × 100/DC%), which is the ratio of infrared light absorbed by pulsing arterial flow (AC) to non-pulsatile signals in blood and tissues (DC). Its range is wide, ranging from 0.02 to 20.

SPI will be recorded by putting pulse oximeter on the thumb using GE Healthcare Carestation 620 anesthesia machine (Madison, WI, USA), SPI is calculated using the following formula: SPI = 100 - (0.33 x HBI + 0.67 x PPGA), where PPGA is the plethysmographic waveform amplitude and HBI is the heartbeat interval, The range of values is 0 to 100.

After 3 minutes of preoxygenation with 100% oxygen anesthesia will be induced using IV fentanyl (1 mcg/kg) and IV propofol (2-3 mg/kg) until loss of consciousness, this will be followed by the insertion of an appropriately sized supraglottic airway device (i gel or LMA). Anesthesia will be maintained with a mixture of oxygen (50%), air (50%) and isoflurane titrated minimum alveolar concentration (MAC) of 1%, IV fluids (Ringer's lactate with 1% dextrose) will be administered at 4-6 mL/kg/hr.

Caudal Block procedure:

Three minutes after anesthesia induction the patient will be positioned in the lateral decubitus position, a caudal block will be performed using the landmark technique under aseptic conditions. After carful preparing and cleaning the area using 10% povidone-iodine, the sacral hiatus will be identified, a 22G, 1 to 1.5 inch hypodermic needle will be inserted until the sacro-coccygeal ligament will be pierced a distinct "pop" indicated entry into the caudal space, a test dose of lidocaine 1% with epinephrine (1:200,000) will be administered at a dose of 0.5 mL/kg (maximum 3 mL) to exclude inadvertent intravascular or intrathecal injection. After confirming negative aspiration for blood or cerebrospinal fluid (CSF) then a 0.25% bupivacaine (1 mL/kg) will be injected at a rate of 1 mL per 3 seconds. The patient will be immediately repositioned supine; The surgical incision will be made approximately 10 min after caudal block administration.

Monitoring and data collection PI, SPI, heart rate (HR), and mean arterial pressure (MAP) will be monitored throughout the procedure. It will be recorded at baseline after induction just before caudal then every minute for 10 minutes after caudal block as well as at skin incision then every 5 minutes for 30 minutes The anesthesiologist will administer a rescue dose of IV fentanyl (0.5-2 mcg/kg) and paracetamol if they believe the block to be ineffective A working caudal block will be defined as any one of the following criteria achieved, whichever will be occurred earlier: A surgical pleth index of ≤ 43.5 ,100% increase of PI value from baseline, 15% decrease of MAP, or 15% decrease of HR from the baseline Postoperatively, 30 min after surgery, an anesthesiologist, who will be blinded to the intraoperative PI and SPI or the course of anesthesia throughout surgery, will assess the success of the caudal block when the child is calm or sleeping. A gentle pinch will be applied to the skin starting from the distal part of both lower limbs, and the patients will be observed for signs of grimacing, wincing, withdrawal, or changes in heart rate. If no response will be observed, the stimulus will be progressively applied more proximally until the expected level of blockade will be reached (inguinal region). The verbal response of a communicating child will be used to assess the success of the block.

Primary outcome measures:

Time to change of value of PI and SPI after caudal block

Secondary outcome measures:

• The recorded values of PI, SPI, HR and MAP at baseline after induction just before caudal then every minute for 10 minutes after caudal block as well as at skin incision then every 5 minutes for 30 minutes
• ROC curve of PI, SPI, HR and MAP
• AUC of PI, SPI, HR and MAP
• Best cut of value of PI, SPI, HR and MAP in assessing the effectiveness of a caudal block in pediatric patients during infraumbilical surgeries under general anesthesia
• Sensitivity and specificity of PI, SPI, HR and MAP Statistical analysis and sample size estimation The primary outcome is the difference in time to change between the Perfusion Index (PI) and the Surgical Pleth Index (SPI) following caudal block. Based on previous studies, the PI study reported a significant change at 3 minutes after caudal block (Bejugama & Khatavkar, 2025), while the SPI study identified a clinically useful discrimination threshold (SPI ≤ 43.5) at approximately 5 minutes (Harchandani et al., 2025). Although the observed mean difference between the two indices is 2 minutes, we conservatively target a minimal detectable difference of 0.5 minutes to ensure the study is adequately powered for smaller, yet clinically relevant, differences. Assuming a standard deviation of the paired differences of 1 minute, a two sided significance level α = 0.05, and 80% power, the required sample size for a paired samples t test was calculated using G*Power software (version 3.1.9.7). The minimum sample size obtained is 34 patients. Accounting for a 15% failure of caudal block, the final target sample size is 40 patients

Inclusion Criteria:

• • Children aged 1 to 6 years of either sex

  • With American Society of Anesthesiologists (ASA) physical status I-II
  • undergoing elective infraumbilical surgery

Exclusion Criteria:

• • parental refusal

  • Patients with liver disease (e.g. elevated bilirubin >2mg/dL, albumin<3.0g/dL)
  • Patients with renal disease (e.g. serum creatinine >1.5-2mg/dL)
  • Cardiac disorders (low EF>50%, sever valvular heart disease)
  • Recent respiratory tract infection within 1-2 weeks and bronchial asthma
  • Known organic brain disease (cerebral palsy, tumors)
  • patients with any contraindication to caudal anesthesia (local infection, coagulation abnormality e.g. INR > 1.5, known structural abnormality e.g. kyphosis)
  • Known allergy to local anesthetics

study will be conducted in Fayoum University hospital after the local Institutional Ethics Committee and local institutional review board approval.

Inclusion criteria

• Children aged 1 to 6 years of either sex
• With American Society of Anesthesiologists (ASA) physical status I-II
• undergoing elective infraumbilical surgery. Exclusion criteria
• parental refusal
• Patients with liver disease (e.g. elevated bilirubin >2mg/dL, albumin<3.0g/dL)
• Patients with renal disease (e.g. serum creatinine >1.5-2mg/dL)
• Cardiac disorders (low EF>50%, sever valvular heart disease)
• Recent respiratory tract infection within 1-2 weeks and bronchial asthma
• Known organic brain disease (cerebral palsy, tumors)
• patients with any contraindication to caudal anesthesia (local infection, coagulation abnormality e.g. INR > 1.5, known structural abnormality e.g. kyphosis)
• Known allergy to local anesthetics.