Effect of injection rate on level and duration of hypobaric spinal anesthesia

The purpose of this study was to determine whether injection rate affects the spread of hypobaric spinal anesthesia. Hypobaric spinal anesthesia was performed on 20 patients for total hip arthroplasty. Dural puncture was performed with a 22-gauge Whitacre needle. All patients received 10 mg of hypobaric tetracaine with epinephrine. An electrically driven syringe pump was used to inject the anesthetic solution at either slow (250 sec) or fast (10 sec) rates. Ten patients received slow injections, and 10 received fast injections. Anesthetic levels, duration of anesthesia, and specific gravities of injectate and CSF were measured. Slow injection resulted in less spread of spinal anesthesia. Four-segment regression of anesthetic levels took significantly longer in the slow injection group. Local anesthetic mixtures used were consistently hypobaric compared to patient CSF. We conclude that slow injection of hypobaric tetracaine through a 22-gauge Whitacre needle produces lower levels of spinal anesthesia that tend to be of longer duration than levels resulting from fast injection.