Premedication with fentanyl and midazolam decreases the reliability of intravenous lidocaine test dose

This study was performed to determine whether premedication with midazolam and fentanyl prevents reliable detection of an TV lidocaine test dose. Thirty ASA physical status I or II patients received either 3 mt of saline or 1.5 mg of midazolam (1.5 mt) plus 75 mu g of fentanyl (1.5 mt) IV in a randomized, double-blind fashion. Five minutes later, lidocaine 1 mg/kg was injected N. At 1.5 min before and every minute after lidocaine administration, each subject was questioned regarding the presence of four symptoms of systemic lidocaine toxicity. Any new tinnitus, perioral numbness, metallic taste, or light-headedness within 5 min after lidocaine administration was considered a positive response. All 15 patients in the saline group (100% sensitivity) had a positive response to IV lidocaine, but only 9 of 15 patients in the sedation group had a positive response (60% sensitivity; P = 0.017). We conclude that midazolam and fentanyl premedication decreases the reliability of subjective detection of TV lidocaine. Implications: Anesthesiologists often rely on subjective symptoms to prevent local anesthetic toxicity while performing regional anesthesia. Sedatives are often administered during the administration of regional anesthesia. This study demonstrates that typical sedation decreases the reliability of detection of local anesthetic toxicity by subjective symptoms.