Minimum Effective Concentration of Bupivacaine in Ultrasound-guided Axillary Brachial Plexus Block
Condition: Pain
Study type: Interventional

Summary: 
      Background: The use of ultrasound in regional anesthesia enables a reduction in the local
      anesthetic volume. The present study aimed to determine the minimum effective concentration
      (EC50 and EC95) of bupivacaine for axillary brachial plexus block (ABPB). Methods: Following
      approval by the Research Ethics Committee, patients with a physical condition of I or II
      according to the American Society of Anesthesiologists, between 21 and 65 years of age were
      recruited and subjected to elective surgery of the hand and ABPB. The concentration of the
      anesthetic was determined using a step-up/step-down method and was based on the outcome of
      the preceding block.
    

Detailed description: 
      The demographic data of all included patients were recorded. Routine monitoring of the
      surgical procedure, including electrocardiography, non-invasive blood pressure and pulse
      oximetry, was performed. Peripheral venous access was established in the nonoperative upper
      limb and standard intravenous premedication (0.03 mgkg of midazolan) was administered to all
      patients.

      Brachial plexus blocks were performed through the axilla on patients in a supine position
      using a ultrasound device with a 13-6 MHz linear transducer (SonoSite, Bothell, WA, USA) and
      a Stimuplex® DIG RC peripheral nerve stimulator (B. Braun, Melsung, Germany).

      The end of the local anesthetic injection was considered time 0 for evaluating the
      effectiveness of the block. A blinded observer, who was not present during the injection and
      was unaware of the concentration of local anesthetic used, assessed motor, thermal and
      sensory blocks. These assessments were performed every 5 minutes after the procedure for 30
      minutes or until the block was deemed effective.

      Motor function was assessed using a modified Bromage scale. The following muscles were
      assessed: finger flexors (median nerve), finger extensors (radial nerve), finger adductors
      (ulnar nerve) and elbow flexion (musculocutaneos nerve).The assessment of thermal sensation
      in the upper limb was performed using gauze and alcohol. Pain sensation was assessed by a
      pinprick test using a 23 G needle. The thermal and pain sensitivities of the ulnar, median,
      radial and musculocutaneos were examined.

      Surgical anesthesia was defined as a motor scale of 2 or lower, with absent of appreciation
      of cold and pinprick sensation. Patients who exhibited block failure received an
      ultrasound-guided complement to the nerve block distal to axilla or conversion to general
      anestehesia.

      All patients received a subcutaneous injection of 3 mL of a 2% lidocaine solution with
      epinephrine as a supplementary intercostobrachial block due to the use of a pneumatic
      tourniquet on the middle third of the arm. During surgery, an infusion of 25-50
      mcg.kg-1.min-1 of propofol was used to achieve proper sedation.

      Postoperative analgesia was assessed in the recovery room using a numeric pain rating scale
      (0 indicating no pain, 10 indicating the worst pain ever experienced) and the amount of
      analgesic used at four hours after the ABPB.

      After the surgical procedure, the patients were admitted to the recovery room, where they
      were monitored until they met the conditions for discharge into an outpatient regimen.

      Statistical Analysis

      In this study, the primary objective was to estimate the minimum effective concentration of a
      5-mL bupivacaine solution without epinephrine for ultrasound-guided ABPB. Volume assignment
      was carried out using a biased coin design up-and-down sequential method.The initial
      concentration of local anesthetic was 0.35%. This dose was chosen based on our clinical
      experience and statistical simulation at various doses. Each subsequent dose was based on the
      response of the preceding subject, as per a biased-coin design up-down sequential method.6
      The dosing changes were in increments of 0,05%. The anesthesia provider was blinded to the
      concentration of bupivacaine, as was the subject. If a failure was observed in the previous
      subject, the dose was stepped up in the next subject. If a success was observed, the next
      subject was randomized with probability of 0.1 to the next lower dose and with probability of
      0.9 to the same dose.

      The ED95 with 95% confidence intervals (CIs) and estimated probabilities were calculated
      using logistic regression with SAS software. A sample size of at least 40 patients was
      selected after testing a variety of scenarios, each with simulations of both the responses
      and the corresponding doses selected by the sequential allocation method described above, and
      beginning with various starting doses.

      The means and standard deviations were used to analyse parametric data, whereas medians and
      quartiles were used to analyse non-parametric data.
    

Intervention name(s): Bupivacaine
Intervention type(s): Drug
Intervention MeSH terms: Bupivacaine
Intervention model: Single Group Assignment
Masking: Single (Investigator)
Primary purpose: Basic Science
Keywords: Axillary brachial plexus block; Ultrasound; Minimum effective concentration; Bupivacaine; ultrasound-guided axillary brachial plexus block