Spinal anesthesia, a widely practiced regional anesthesia technique, is frequently employed for surgeries involving the lower abdomen, pelvis, and lower limbs. Among the local anesthetics available for intrathecal use, hyperbaric bupivacaine remains a preferred agent due to its long duration of action, reliable sensory and motor blockade, and relatively low cost. However, one limitation of bupivacaine is its finite duration of analgesia, which may be inadequate for prolonged procedures or in the postoperative period. To overcome this limitation, various adjuvants have been studied and used to enhance the quality and duration of spinal anesthesia, improve intraoperative conditions, and extend postoperative analgesia without increasing adverse effects ^[1,2]. Among these adjuvants, opioids have traditionally been the agents of choice. Fentanyl, a lipophilic μ-opioid receptor agonist, is commonly used as an intrathecal adjuvant due to its rapid onset of action and minimal rostral spread. When combined with bupivacaine, fentanyl has been shown to improve the quality of intraoperative analgesia and extend postoperative pain relief ^[3]. Nevertheless, its use is not without drawbacks. Side effects such as pruritus, nausea, vomiting, urinary retention, and, in rare cases, respiratory depression limit its universal applicability ^[4,5].

In recent years, attention has shifted toward non-opioid alternatives that can provide effective analgesia with fewer opioid-related adverse effects. One such promising agent is dexmedetomidine, a highly selective α2-adrenergic receptor agonist. Dexmedetomidine has gained clinical interest due to its sedative, anxiolytic, analgesic, and sympatholytic properties ^[6]. When used intrathecally, dexmedetomidine acts on the dorsal horn neurons of the spinal cord to inhibit nociceptive transmission, resulting in prolonged sensory and motor blockade without significant neurotoxicity ^[7]. In contrast to opioids, it is associated with minimal respiratory depression and has shown favorable results in early clinical studies evaluating its role as an intrathecal adjuvant ^[8]. Several comparative studies conducted before 2010 have begun to explore the relative efficacy and safety of dexmedetomidine and fentanyl as adjuvants to hyperbaric bupivacaine. These studies have demonstrated that both agents can enhance the quality of spinal anesthesia, but dexmedetomidine may provide a more prolonged duration of sensory and motor block, as well as extended postoperative analgesia ^[9,10]. Moreover, dexmedetomidine has been associated with a lower incidence of side effects, such as pruritus and nausea, compared to fentanyl, although it may cause mild bradycardia and hypotension due to its sympatholytic effects ^[11]. The pharmacological mechanisms underlying these differences further explain their clinical profiles. Fentanyl binds to μ-opioid receptors in the substantia gelatinosa of the spinal cord, inhibiting pain transmission pathways, while dexmedetomidineproduces analgesia through hyperpolarization of interneurons in the spinal cord by activating α2-adrenoceptors ^[12].  This difference in site and mechanism of action contributes to their varied onset and duration of effects. Given the growing body of evidence supporting the benefits of dexmedetomidine and the limitations of opioid-based intrathecal adjuvants, a comparative study is essential to evaluate their clinical efficacy and safety profiles more rigorously. Such studies can inform anesthetic practice by identifying optimal combinations that enhance patient comfort, reduce postoperative analgesic requirements, and minimize adverse events. Understanding the balance between efficacy and side-effect profile is especially important in resource-limited settings and in patient populations where prolonged analgesia is particularly desirable, such as the elderly or those undergoing major orthopedic procedures. In conclusion, both fentanyl and dexmedetomidine offer valuable benefits when used as adjuvants to hyperbaric bupivacaine in spinal anesthesia. However, evolving evidence suggests that dexmedetomidine may provide superior analgesic duration and a more favorable side-effect profile. This comparative study aims to build on the existing literature to further delineate the relative advantages and potential limitations of these two agents when used intrathecally, ultimately guiding anesthetic best practices.

Following approval from the hospital's Institutional Ethics Committee, a prospective, randomized, double-blind study was conducted involving 100 adult patients classified as ASA physical status I and II. Standard exclusion criteria applicable to spinal anesthesia were observed. Each participant underwent a comprehensive pre-anesthetic evaluation, including relevant laboratory investigations. Patients deemed fit for surgery were prescribed alprazolam 0.5 mg and ranitidine 150 mg to be taken the night before the procedure. They were also instructed to maintain a minimum fasting period of six hours. The spinal anesthesia procedure and the Visual Analogue Scale (VAS) for pain assessment were explained in detail to all patients to ensure informed cooperation.

On the day of surgery, after obtaining written informed consent, patients were assigned to two groups through computer-generated randomization:

• Group D (n = 50): Received 15 mg of 0.5% hyperbaric bupivacaine combined with 5 mcg dexmedetomidine diluted in 0.5 ml of normal saline.
• Group F (n = 50): Received 15 mg of 0.5% hyperbaric bupivacaine along with 25 mcg fentanyl.

Both groups received a total volume of 3.5 ml of intrathecal solution.

In the operating room, standard monitoring was initiated using a multiparameter monitor to record heart rate (HR), oxygen saturation (SpO₂), non-invasive blood pressure (NIBP), and electrocardiogram (ECG). Baseline vital parameters were documented. An intravenous (IV) line was established using an 18G cannula, and patients were preloaded with crystalloids at 10 ml/kg body weight. Under strict aseptic conditions, spinal anesthesia was administered in the sitting position at the L3-L4 interspace using a 25G Quincke spinal needle.

After drug administration, patients were immediately placed in the supine position. Both the patient and the administering anesthesiologist were blinded to the group allocation.

Parameters Assessed

1. Sensory Blockade
2. Motor Blockade
3. Pain Assessment
4. Hemodynamic Monitoring: Parameters including HR, SpO₂, NIBP, and ECG were monitored every 5 minutes intraoperatively and every 30 minutes postoperatively for 8 hours.
5. Perioperative Adverse Effects: Monitored and recorded side effects included nausea, vomiting, tremors, headache, and transient neurological symptoms, if any.
6. Sedation Level: Evaluated using the Modified Ramsay Sedation Scale [8].

STATISTICAL ANALYSIS

Data collected were analyzed using SPSS version 16.0. Continuous variables were compared using the unpaired t-test, while categorical and ordinal data were analyzed using Fisher’s exact test. A p-value < 0.05 was considered statistically significant. Results were expressed as mean ± standard deviation, and frequencies were reported as numbers and percentages.

Table 1: Demographic Profile (Per 100 Patients)

This table compares demographic data between two groups, D and F. Group D shows slightly older patients and slightly higher average weight. The duration of surgery is significantly longer in Group D (P = 0.04), suggesting potential procedural or patient differences.

Table 2: Comparison of Subarachnoid Block Characteristics

This table reflects the modified subarachnoid block outcomes. Group F had faster onset times and shorter regression periods, indicating a potentially faster-acting agent. Group D exhibited longer analgesia durations, suggesting prolonged relief possibly due to adjuncts.

Table 3: Comparison of Intraoperative Sedation

Sedation levels during surgery were higher in Group D, where 67% of patients reached RSS 3, indicating deep sedation. Group F mostly had moderate sedation (RSS 2), which may be more suitable for cooperative patients under regional anesthesia.

The present randomized, double-blind comparative study evaluated the efficacy and safety of intrathecal dexmedetomidine versus fentanyl as adjuvants to 0.5% hyperbaric bupivacaine in adult patients undergoing surgery under spinal anesthesia. The findings demonstrated that while fentanyl facilitated a faster onset of sensory and motor block, dexmedetomidine significantly prolonged the duration of sensory and motor block, extended postoperative analgesia, and was associated with a higher level of intraoperative sedation. The demographic parameters such as age, sex distribution, height, and weight were comparable between the two groups, ensuring the validity of the comparative analysis. The slightly longer surgical duration in the dexmedetomidine group (P = 0.04) may reflect inter-patient variability or differences in surgical complexity, but the clinical relevance of this difference appears minimal and is unlikely to confound the study's primary outcomes. Patients in Group F (fentanyl) achieved a faster onset of both sensory (mean 4.25 ± 0.47 min) and motor block (8.22 ± 0.62 min), which aligns with previous studies showing fentanyl’s rapid action due to its high lipid solubility and fast spinal cord penetration ^[13]. This rapid onset is advantageous in procedures requiring quick establishment of anesthesia. Conversely, Group D (dexmedetomidine) had a slower onset but achieved a longer duration of block and analgesia, with two-segment regression times of 160.3 ± 10.9 minutes versus 105.4 ± 8.6 minutes in Group F (P < 0.001), and a significantly prolonged duration of analgesia (305.4 ± 12.8 vs 190.6 ± 9.9 minutes; P < 0.001).

These results are consistent with earlier literature suggesting that dexmedetomidine, a highly selective α2-adrenergic agonist, prolongs local anesthetic action by hyperpolarizing interneurons in the dorsal horn, inhibiting nociceptive transmission ^[14,15]. Al-Ghanem et al. similarly reported significantly extended sensory and motor block duration when dexmedetomidine was used intrathecally with bupivacaine, compared to fentanyl ^[10].

The higher sensory block levels achieved in Group D (predominantly T4–T5) compared to Group F (mostly T5–T6) suggest a greater cephalad spread or potentiation of local anesthetic bydexmedetomidine, which has also been previously observed in urological and lower abdominal surgeries ^[9,14]. Sedation during regional anesthesia can provide patient comfort, reduce anxiety, and enhance procedural acceptance. In our study, a higher proportion of patients in Group D reached RSS 3 sedation (67% vs. 25% in Group F), indicating moderate to deep sedation. This is a well-known effect of dexmedetomidine, which has sedative properties through activation of locus coeruleus α2-receptors, without causing significant respiratory depression ^[6,16]. Kanazi et al. noted that dexmedetomidine produces a calm, cooperative sedation state favorable in the intraoperative setting ^[8]. In contrast, the predominantly RSS 2 scores observed in the fentanyl group correspond to light sedation, which might be preferred when active patient feedback or minimal sedation is desired. These differential sedation profiles can guide anesthetic planning depending on surgical and patient requirements. Although hemodynamic variables were not detailed in the tables, previous studies show that dexmedetomidine can induce mild bradycardia and hypotension due to its sympatholytic effects ^[14,17]. However, these effects are generally predictable and manageable, especially at lower intrathecal doses such as 5 mcg, as used in this study. Fentanyl, by contrast, has minimal hemodynamic effects at the spinal level, which may make it preferable in patients where cardiovascular stability is critical ^[5,18]. One of the most significant findings was the prolonged duration of postoperative analgesia in Group D, emphasizing the superior analgesic-sparing effect of dexmedetomidine. This extended relief may contribute to better postoperative pain control, reduced opioid use, and higher patient satisfaction. Studies by Shukla et al. and Gupta et al. reported similar findings, establishing dexmedetomidine as a superior agent in extending analgesia and reducing pain scores ^[1,9].  Moreover, although not quantified in the tables, the known opioid-related side effects such as nausea, vomiting, and pruritus are more commonly associated with fentanyl ^[3,4]. The literature suggests that dexmedetomidine has a lower incidence of such side effects and does not typically induce respiratory depression, making it a safer alternative in many clinical scenarios ^[16]. Based on these findings, dexmedetomidine appears to be a valuable adjuvant for prolonged surgical procedures, especially where extended postoperative analgesia is desirable. Its sedative profile is also beneficial in reducing intraoperative discomfort without compromising airway safety. Fentanyl, with its faster onset and lighter sedation, may be preferred in short-duration surgeries or where early ambulation and discharge are priorities.

This prospective, randomized, double-blind study demonstrated that dexmedetomidine, when used as an intrathecal adjuvant to 0.5% hyperbaric bupivacaine, offers significant clinical advantages over fentanyl. While fentanyl was associated with a faster onset of sensory and motor blockade, dexmedetomidine provided a more prolonged duration of sensory and motor block, extended postoperative analgesia, and deeper intraoperative sedation, without major adverse effects. These findings suggest that dexmedetomidine is a superior intrathecal adjuvant in procedures where prolonged surgical anesthesia and postoperative pain control are desired. Its favorable side-effect profile and sedative properties make it a valuable alternative to opioids, particularly in patients where opioid-related adverse effects are a concern.

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