Washout of local anesthetic during arthroscopy

The recommended dose of lidocaine is often insufficient to obtain adequate local anesthesia when performing knee arthroscopy, particularly in bilateral procedures. A prospective study was conducted to identify the quantity of anesthetic washed out of the knee by irrigation solution. Fourteen patients had knee arthroscopy performed with local anesthetic, either for diagnosis or reparative surgery. Anesthesia was obtained using 20 ml of 0.5% lidocaine injected intraarticularly at the beginning of each procedure, augmented in four cases by an additional 20 ml of 0.5% lidocaine during the procedure. Each of three portals was also infiltrated with bupivacaine 0.25% with epinephrine. Analysis of the normal saline effluent from knee irrigation during the procedure revealed that washout of lidocaine was 50% by weight (range, 42-66%). This percentage did not vary with age, operative time, or quantity of irrigation. We conclude that: (a) safe local anesthesia can be obtained in the average patient with less than the maximum dose of lidocaine, and (b) because of the 50% washout, the safe dose of lidocaine can be increased substantially if needed.