The Effect of Intravenous Lidocaine on Normal Sensation and Pain in Healthy Volunteers

Primary outcomes

1. Ischemic Pain

   Time frame: baseline, during 20 minute lidocaine infusion, and 30 minutes after discontinuation of lidocaine infusion

   The right arm was exsanguinated by elevating it above heart level for 30 seconds, after which the arm was occluded with a standard blood pressure cuff positioned proximal to the elbow inflated to twice the participant's mean arterial pressure. Participants then performed 20 handgrip exercises of 2-second duration at 4-second intervals at 50% of their maximum grip strength. Pain was rated on a scale from 0 - 10 with 0 being no pain to 10 being the worst pain imaginable.

2. Electrical Pain

   Time frame: Baseline, during 20 minutes lidocaine infusion, and 30 minutes after completion of lidocaine infusion

   Peripheral nerve stimulation electrodes were attached to the base and the tip of the third digit and connected to a constant current stimulator (DS7A, Digitimer Ltd, Hertfordshire, England). Ascending electrical stimuli of 2000 mu duration, ranging from 0.5 to 35 mA (ampere) was administered one per second in 0.5 mA increments. Participants were instructed to indicate when they first felt the slightest sense of pain (electrical pain threshold, EPTh) and when they were unable to tolerate a further increase (electrical pain tolerance, EPTo). For each measure, the average of three trials was computed for use in subsequent analyses. Each of the three electrical pain stimuli were presented three times and balanced in order using a Graeco-Latin square design. The pain scale is between 0 and 10, with 0 being no pain and 10 being the worst pain imaginable

3. Heat Pain

   Time frame: baseline, during 20 minute lidocaine infusion, and 30 minutes after completion of lidocaine infusion

   The thermal procedure involved a baseline assessment of heat pain threshold and tolerance. Contact heat stimuli were delivered using a computer-controlled Medoc Thermal Sensory Analyzer (TSA-II; Ramat Yishai, Israel), which is a peltier elementbased stimulator. Temperature levels were monitored by a thermistor and returned to a preset baseline of 32°C by active cooling at a rate of 10°C/s. The 3 × 3 cm contact probe was applied to the right forearm. The pain scale is between 0 and 10 with 1 being no pain and 10 being the worst pain imaginable

4. Cold Pain

   Time frame: baseline, during 20 minute infusion, and 30 minutes after lidocaine infusion

   The participant's foot was immersed up to the ankle into a container filled with ice water of 3°C. Participants were instructed to maintain their foot in the container until the cold pain became intolerable (cold pain tolerance). The length of time was recorded in seconds. This procedure was repeated once with a gap of at least fifteen minutes in-between repeated tests. The range was 0 seconds to 120 seconds

5. Tactile Sensation

   Time frame: baseline, during 20 minute infusion, and 30 minutes after completion of infusion

   Pin prick sensory thresholds (PPT) were obtained by touching the skin in-between the first and second metacarpal bone with a 23-gauge needles which moved freely out of a 10 mL plastic syringe barrel. The pin prick sensation was modified by adding small weights to the 23-gauge needles (from 0.2 to 5.2 mg). A syringe barrel of tuberculin (TB) needles that were cut to different lengths to add the desired weight to the 23-gauge needle. The PPT was determined using the weighted 23-gauge needle in ascending order, according to the method of limits. This assessment was to evaluate whether participants were able to feel the touch of the needle. The participant's arm was placed on a tray table. A linen sheet was suspended in-between two IV poles in such a fashion that the subject's view of his/her hand was blocked. Normal values are between 0.21mg and 5mg.