Physiological variables for the objective detection of intraoperative nerve block failure in dogs

Abstract

Objective To identify physiological variables that can be used to objectively detect intraoperative nociception to indicate peripheral nerve block failure. Study design A prospective blinded randomized clinical study. Animals A sample of 14 male (40.8 ± 12 kg; mean ± SD) and 16 female (34.3 ± 11.4 kg) client-owned dogs undergoing a stifle arthrotomy. Methods Dogs were randomly assigned to one of three treatment groups for psoas compartment and proximal sciatic nerve blocks (0.2 mL kg-1 per site): guided bupivacaine (GBB) or saline (GSB) blocks or blind bupivacaine block (BBB). Guided blocks were performed using ultrasound and nerve stimulation. Dogs were premedicated intramuscularly with 0.01 mg kg-1 medetomidine and 0.3 mg kg-1 morphine. General anaesthesia was induced with propofol (to effect to achieve tracheal intubation) and maintained with isoflurane in oxygen (targeted end-tidal concentration of 1.6%). The assigned investigator, based on randomisation, allotted a confidence score [1 (poor) to 4 (high)] that the block will be successful after administering the assigned nerve block treatment. The blinded investigator allotted a binomial subjective score of the nerve block outcome (“Yes”: response to surgical stimulation; “No”: no discernible response) at each time point. Receiver of operator characteristic curve analysis was used to compare actual values and change in values of physiological variables between GSB (Yes nociception) and GBB (No nociception) at the time of the arthrotomy. The Youden index and associated criterions for each physiological variable were used as an objective measure. Fishers exact t-test, McNemar's test and Cohens kappa statistical analysis were used to determine association, differences and inter-score reliability, respectively between the objective and subjective scoring for the BBB. The subjective score was compared to objective scores after being stratified into the assigned confidence scores using Kendall’s tau-b rank correlation coefficient. Results The cardiovascular variables had good discriminating ability in distinguishing a nociceptive response (p < 0.01). The Youden indexes for MAP and DAP had the best potential effectiveness in detecting a response to surgical stimulus. The highest sensitivity was that of delta MAP (100%). Good agreement was indicated between the subjective and objective scores with delta HR or SAP. The use of delta MAP (> 6 mmHg), delta SAP (> 10 mmHg), delta DAP (> 8 mmHg) had the best ability in indicating peripheral nerve block failure (p < 0.001). Conclusions and clinical relevance The use of delta MAP, delta SAP or delta DAP can be considered as objective measures to detect intraoperative peripheral nerve block failure in anaesthetised dogs undergoing stifle arthrotomy. The determination of criterion values for different populations and conditions will benefit future clinical trials.