The Prolonging Effect of Dexmedetomidine on Spinal Anesthesia Through Different Routes if Administrations

Dexmedetomidine, an α2-adrenergic agonist, is commonly used in anesthetic practice for various applications, including premedication, as an adjunct to general anesthesia, sole anesthetic agent, and sedation in intensive care settings. Its effects include enhancing the efficacy of other anesthetic drugs, inducing perioperative sympatholysis, and lowering blood pressure by stimulating central α2 and imidazoline receptors.

Spinal anesthesia is a type of regional anesthesia that involves administering a local anesthetic directly into the cerebrospinal fluid surrounding the spinal cord and nerve roots. It is primarily used for procedures below the umbilicus, such as orthopedic surgeries involving joints and bones.

The 0.5% isobaric formulation of bupivacaine is characterized by a slower onset compared to hyperbaric formulations but provides a longer duration of both sensory and motor blockade. Achieving successful spinal anesthesia (SA) depends on selecting the appropriate dose of isobaric bupivacaine. Administering an excessive dose can lead to high spinal anesthesia, while an insufficient dose may result in inadequate cephalad spread.

A retrospective observational study involving 1,079 adult patients who received spinal anesthesia with 0.5% isobaric bupivacaine between 2018 and 2021 investigated the relationship between the bupivacaine dose and block height. The study concluded that for patients younger than 60 years, doses of 15-17 mg (3.0-3.4 mL) were optimal, while doses of 10.5-16 mg (2.1-3.2 mL) were suitable for patients aged 60 years or older. These doses achieved a T5-T10

Over the years, various agents have been combined with local anesthetics (LA) to extend the duration of action in both spinal anesthesia via intrathecal (IT) administration and peripheral nerve blocks (PNB), achieving varying levels of success. Dexmedetomidine, a highly selective α2-adrenergic receptor agonist, offers several advantages as an adjuvant. These include its sedative and analgesic effects (acting at both spinal and supraspinal levels), anti-anxiety properties, inhibition of sympathetic activity, mild respiratory depression, and hemodynamic stability.

Intrathecal α2-adrenergic receptor agonists, including dexmedetomidine, have been shown to exhibit antinociceptive properties for both somatic and visceral pain. Studies using small doses of intrathecal dexmedetomidine (e.g., 3 µg) in combination with bupivacaine for spinal anesthesia demonstrated faster motor block onset, prolonged motor as well as sensory block durations, and preserved hemodynamic stability without causing significant sedation.

The dose-dependent effects of dexmedetomidine have been extensively studied. Randomized clinical trials (RCTs) comparing doses (e.g., 3 µg vs. 5 µg with isobaric Ropivacaine and 5 µg vs. 10 µg with isobaric bupivacaine) consistently concluded that higher doses of dexmedetomidine enhanced the onset and prolonged the duration of sensory and motor block when used as an adjuvant in spinal anesthesia.

Intravenous dexmedetomidine has also been shown to prolong the sensory and motor block of bupivacaine through its supraspinal anesthetic and analgesic actions. However, its intravenous administration is associated with a dose-dependent risk of bradycardia. A systematic review and meta-analysis conducted in 2013, which included 364 patients from seven randomized controlled trials of intermediate to high quality, examined the effects of intravenous dexmedetomidine on spinal anesthesia duration. The analysis found that motor block duration increased by approximately 21% (P < 0.00001). Importantly, no significant differences in the incidence of hypotension or postoperative sedation were observed between the dexmedetomidine and placebo groups, and none of the patients experienced respiratory depression.

Inclusion criteria

• Male and female patients in adult age group more than 18 years old
• ASA classification I, II, and III.
• Orthopedic surgery, including total hip replacement (THR) and total knee replacement (TKR).
• No history of Obstructive sleep apnea

Exclusion criteria

• Patient below 18 years old of both genders.
• ASA classification of IV and more.
• Patients with a height of less than 150 cm.
• Patients on alpha-two antagonist ACE inhibitors and CCB.
• Patient received intrathecal morphine, intravascular dexamethasone, or Midazolam
• Patient who can't tolerate fluid challenge. Example: ESRD, HF…etc.
• Any contraindication for neuraxial procedure. Examples: coagulopathy, severe AS, sepsis…etc.
• Psychiatric patients.