Effect of Injection Rate on Sensory Level and Duration of Hypobaric Bupivacaine Spinal Anesthesia for Total Hip Arthroplasty

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Abstract
We prospectively studied 18 male patients undergoing total hip arthroplasty in the lateral decubitus position to determine the effect of injection rate on sensory level and duration of 0.3% hypobaric bupivacaine spinal anesthesia. Patients were randomized into one of two groups according to rate of injection. Dural puncture was performed in the midline at the L3–4 interspace with a 22-gauge needle, with the patient in the lateral decubitus position (operative side uppermost). Local anesthetic solution in all patients was composed of 2 mL 0.75% bupivacaine, 0.2 mg epinephrine (1:1000), and 2.8 mL sterile water, resulting in 5 mL 0.3% bupivacaine. Injection of the solution was performed by an electrically driven pump at one of two calibrated rates: 0.5 mL/s (fast) and 0.02 mL/s (slow). Segmental spread of analgesia was determined by pinprick in the anterior axillary line by a blinded observer. Fast rate of injection (0.5 mL/s) of the hypobaric bupivacaine solution produced a more cephalad spread of spinal analgesia than the slow rate (0.02 mL/s). The difference was statistically significant from 5 to 180 min on the nondependent (operative) side and from 10 to 180 min on the dependent (nonoperarive) side. Fast rate of injection also resulted in a higher maximum sensory level on both nondependent and dependent sides compared to the slow rate of injection (mean difference four segments). The specific gravity of the local anesthetic solution was significantly less than that of patient cerebrospinal fluid (CSF), indicating that 0.3% bupivacaine is hypobaric. We conclude that fast injection of 0.3% bupivacaine at room temperature results in a higher maximum sensory level than slow injection.

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