Comparison of the Effect of Intrathecal Dexmedetomidine-Magnesium Sulfate Combination and Intrathecal Dexmedetomidine Alone on Subarachnoid Blockade with 0.75% Isobaric Ropivacaine in Knee Arthroplasty: A Prospective Randomised Double Blind Study

Background, Aims & Objectives: Intrathecal local anesthetic alone is associated with relatively short duration of action and early analgesic intervention is needed in post-operative period. The addition of adjuvants provides a dose sparing effect of local anesthetics and accelerates the onset of sensory blockade as well as prolongs the duration of spinal block for long procedures and also provides postoperative pain relief. The aim of this study was to study the adjuvant effect of magnesium sulfate on ropivacaine and dexmedetomidine combination when used intrathecally with 0.75% isobaric Ropivacaine in lower limb surgeries. Block characteristics (onset, level and duration of sensory block and onset, level and duration of motor block), postoperative analgesia, hemodynamic parameters and adverse effects were noted. Material & Methods: In this observational study, 100 adult patients (18-65 years) ASA-PS I & II undergoing total knee replacement surgeries under spinal anesthesia were included and randomized into two groups of 50 patients each. In Group D patients received 2ml of 0.75% isobaric Ropivacaine plus 10 micrograms Dexmedetomidine (in 1.0 ml) Total volume= 3ml. In Group DM patients received 2ml of 0.75 % isobaric Ropivacaine and 10 mcg dexmedetomidine (in 0.5ml) and 50 mg of preservative-free magnesium sulfate 50% diluted in 0.5ml. Total volume=3ml. The sensory block was assessed by loss of temperature discrimination to ice packs and pain sensation to needle-prick. The peak sensory level was noted. Motor blockade was assessed by Modified Bromage Scale. Adverse effects -hypotension (>30% decrease in SBP or SBP< 90 mmHg), bradycardia (heart rate <50bpm), nausea, vomiting, pruritus were noted. Results: It was observed that the mean time to onset of sensory block was 4.45+0.38 minutes and mean time to onset of motor block was 6.12+0.5 minutes in Group DM compared to 2.15+0.63 minutes and 2.72+0.46 minutes in Group D respectively. Duration of sensory and motor blockade observed was 297.2+30.9 minutes and 202.3+0.6 minutes in Group DM and 191.6+0.71 minutes and 152.5+4.77 minutes in Group D respectively. Duration of postoperative analgesia was 441.2+0.83 minutes in Group DM and 302.47+0.21 minutes in Group D. Conclusions: Though addition of Magnesium sulfate to Ropivacaine and Dexmedetomidine combination delayed the onset of subarachnoid blockade but it prolonged the duration of sensory and motor blockade and the duration of postoperative analgesia significantly.