Ultrasound-guided Axillary Plexus Block - Dose Reduction of Prilocaine

• Number of Participants With Complete Sensory Block [ Time Frame: 60 minutes after administration of the local anesthetic ] [ Designated as safety issue: No ]
  The number of outpatients with complete sensory block of all 4 nerves (n.musculocutaneous, n.radialis, n.ulnaris,n.medianus) was registrated in each group.
  
  
• Number of Participants With Complete Motor Blocks [ Time Frame: Within 60 minutes after administration of the local anesthetic ] [ Designated as safety issue: No ]
  To examine the extent of the motor block the manual muscle function test after Vladimir Janda was used. As a complete motor block was defined, when no motion (grade zero after Janda) of muscles innervated by the four blocked nerves (musculocutaneous, median, radial and ulnar nerve) was observed within 60 minutes after administration of the local anesthetic.
  
  
• Onset Time. [ Time Frame: within 60 minutes after administration of the local anesthetic ] [ Designated as safety issue: No ]
  Time from beginning of administration of the local anesthetic until complete sensoric block.
  
  

• Maximum Concentrations of Methemoglobin [ Time Frame: 0,1,2,3,4 hours post-dose ] [ Designated as safety issue: Yes ]
  Concentration of Methemoglobin (Met-Hb) was measured using spectrophotometry prior to the anesthesia (baseline value) and then hourly after performing the block until a clear decrease became apparent. The maximum amount was reached in every case two or three hours after administration of the local anesthetic.
  
  
• Number of Participants With Objective Adverse Events as a Measure of Safety and Tolerability [ Time Frame: Outpatients were followed for the duration of hospital stay, an average of six hours. ] [ Designated as safety issue: Yes ]
  In groups A, B and C was determined the rate of objective clinical signs of increased Met-Hb-levels : drops in oxygen saturation <93% using pulseoximetry or lip cyanosis.
  
  
• Number of Participants With Subjective Adverse Events as a Measure of Safety and Tolerability. [ Time Frame: Outpatients were followed for the duration of hospital stay, an average of six hours. ] [ Designated as safety issue: Yes ]
  In groups A, B and C was determined the rate of subjective clinical signs of increased Met-Hb-Levels : headaches or dizziness, when correlated with the peak-Met-Hb-Level.
  
  

Background

With the availability of high resolution portable ultrasound equipment, the use of sonography in the field of anesthesia has become more widespread in recent years. The investigators would like to draw attention to a meta-analysis of prospective randomized studies by Abrahams et al. as being representative of the numerous publications available. Compared to nerve stimulation, ultrasound-guided nerve blocks have a higher success rate with significantly fewer vascular punctures and shorter procedure times. However, the administration of ultrasound-guided regional anesthesia necessitates considerable equipment acquisition and training expenses. Nevertheless, the suggested benefits resulting from savings in time and local anesthetic, increased patient satisfaction and avoidance of complications should be considered in addition to the results from Abrahams et al.

The aim of the present work was to investigate whether an ultrasound-guided block combined with a reduced dose of the local anesthetic prilocaine altered the success rate or other parameters of block quality. Any adverse effects and the methaemoglobin concentration were also recorded.

Methodology

After receiving ethics committee approval and after informing patients, ultrasound-guided axillary plexus anesthesia was carried out on 120 adult outpatients in ASA risk groups I-II. The surgical spectrum covered procedures on the distal forearm or the hand (carpal tunnel syndrome, ganglion extirpation, removal of material, etc.). Provision of information and inclusion in the study was done in the anesthesia outpatients' clinic as part of the standard preoperative preparation. Prior to administering the regional anesthesia the investigators administered midazolam as a premedication by mouth (3,75-7,5mg) or intravenously (2-3mg). Application of the local anesthesia was done prospectively and single blind in 3 cohorts with decreasing dosage. The patients received either 400mg (group A), 300mg (group B) or 200mg (group C) of prilocaine 1%. The block was performed under sterile conditions using a portable ultrasound device (Sonosite S-Nerve) and a 24 G short bevel cannula with flexible tubing. The block was performed using combined out-of-plane (n. musculocutaneous) and in-plane techniques (n.radialis, n.medianus, n.ulnaris), usually from a single puncture site. The outpatients were cared for in the anesthesia care unit preoperatively and postoperatively until their discharge. NIBP, pulse oxymetry and ECG were continuously monitored. The discharge criteria were widespread regression of the block with a subjective feeling of well-being including adequate analgesia and stable vital signs with methemoglobin (Met-Hb) concentrations ≤ 5%. Met-Hb levels were measured using spectrophotometry prior to the anesthesia (baseline value) and then hourly after performing the block until a clear decrease became apparent. A complete sensory block of all 4 nerves within 60 minutes of administration of the local anesthetic was rated as a successful block.

For the statistical analysis the investigators selected distribution-free, nonparametric test methods. The Mann-Whitney U test was used when comparing 2 groups and the Kruskal-Wallis test or Fisher's exact test was used when comparing more than 2 groups. The level of significance was defined with α ≤ 0.05. With multiple paired comparisons the significance level was adjusted using the Bonferroni correction. The Bravais-Pearson correlation coefficient rho describes the relationship between the maximum Met-Hb value and the weight-based prilocaine dose in mg/kg.