Optimal Lidocaine Propofol Mixture for Painless Induction of Anesthesia

Rapid onset of action, smooth induction, favorable recovery and low rate of adverse effects have made propofol the most commonly used intravenous anesthetic for induction and maintenance of anesthesia. However, pain associated with propofol injection is still considered by many patients as the worst prioperative unpleasant experience.1 Therefore, a diversity of techniques have been suggested in the literature to solve this problem including pretreatment with local anesthetics, opiates, ketamine and alfentanil. 2-5 New propofol formulation was another way to alleviate this pain.6 Among these techniques lidocaine was investigated extensively whether as a pretreatment with the use of tourniquet, which is a time consuming technique, or mixed with propofol. Evaluation of optimal dose of lidocaine in propofol lidocaine mixture was widely assessed. However, evaluation of optimal lidocaine dose in propofol lidocaine mixture to prevent pain with newer formulations of propofol should be further evaluated. We hypothesize that optimal dose of lidocaine to alleviate pain with propofol injection might need further evaluation.

Patients and methods

This prospective randomized and double blinded study will be conducted in Sohag University Hospital after obtaining approval from Institutional Review Board and written informed consent from all patients before enrolment in this study. Patients scheduled for elective surgery under general anesthesia, aged 18 to 60 years and American Society of Anesthesiologists (ASA)physical status I \& II will be selected for this study. Patients with history of drug allergy, hepatic or renal impairment, cardiac problems, neurologic deficits, psychiatric disorders or anxious patients who need sedation preoperatively will be excluded from the study.

Before entering the operating room premedication will not be allowed. In the operating room 20 gauge cannula will be inserted into a vein on the dorsum of the non-dominant hand \& basic monitoring (pulse oximetry, electrocardiography, noninvasive blood pressure \& end tidal carbon dioxide) will be applied to the patient before induction and throughout the procedure.

Mixtures of propofol, lidocaine and saline will be prepared by an anesthetist who is not involved in the study team according to the lidocaine dose sequence as shown in table (1) resulting into eight mixtures. The mixture will be injected at a rate of 20ml/minute using syringe pump until patient loose consciousness. The anesthetist who inject the propofol and the patient will be blinded to the lidocaine dose in the mixture.

Table 1 propofol lidocaine mixtures Mixture Propofol Lidocaine 2% (ml)(20mg/ml) Lidocaine concentration (%)

1. 20ml 0.5
2. 20ml 1 0.095
3. 20ml 1.5 0.139
4. 20ml 2 0.181
5. 20ml 2.5 0.222
6. 20ml 3 0.26
7. 20ml 3.5 0.297
8. 20ml 4 0.333

Pain scale will be explained to all patients preoperatively and they will be asked to report for pain sensation (no pain or mild, moderate or severe pain) The dose of lidocaine, in propofol lidocaine mixture, administered to each patient will be determined by the response of the previous patient using a modified Dixon's up-and-down method (using 10mg as a step size). The first patient will receive a mixture with 10mg lidocaine. If the patient report pain with propofol injection, then the next patient will receive a mixture with 10mg higher dose of lidocaine, if the patient report no pain with injection, then the next patient will receive a mixture with 10mg lower dose of lidocaine. The study will continue until we obtain seven crossover midpoints (No limited number).

The minimal effective dose of lidocaine that results in painless injection of propofol for 50% (ED50) of patients will be determined by calculating the midpoint dose of all independent pairs of patients after at least seven crossover points.

The following data will be recorded; Primary outcome: pain scale during injection and volume of propofol-lidocaine mixture used.

Secondary outcome: increase in heart rate and/or hand or forearm withdrawal during injection, patient satisfaction(will be assessed in the post anesthesia care unit (PACU) after regaining consciousness), demographic data, type and duration of surgery, Time for unconsciousness.

Aim of the work To determine the Minimal effective dose of lidocaine in propofol lidocaine mixture to achieve painless induction of anesthesia using propofol.