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

To identify physiological variables for objectively detecting nociception indicative of intraoperative peripheral nerve block failure. A double-blinded randomized clinical study. A sample of 14 male (40.8 ± 12 kg; mean ± standard deviation) and 16 female (34.3 ± 11.4 kg) client-owned dogs. Dogs were randomly assigned to one of three groups for psoas compartment and proximal sciatic nerve blocks (0.2 mL kg–1 per site): guided bupivacaine (GBB), or saline (GSB) block or a blind bupivacaine block (BBB). Guided blocks were performed using an ultrasound-peripheral nerve locator combination. Premedication consisted of medetomidine 0.01 mg kg–1 and morphine 0.3 mg kg–1. General anaesthesia was induced with propofol and maintained with isoflurane in oxygen. Receiver operator characteristic curve analysis was used to compare actual values and change in values of physiological variables between GSB and GBB. The Youden index and associated criterion for each physiological variable were used to determine an objective measure for nociception. Fisher’s exact t test, McNemar’s test and Cohen’s kappa statistical analysis were used to determine association, differences and inter-score reliability between the objective and subjective scoring for BBB. Cardiovascular variables had good discriminating ability to identify a nociceptive response (p < 0.01). The Youden indices for mean (MAP) and diastolic (DAP) arterial pressure were most reliable in detecting nociception. The highest sensitivity was that of ΔMAP (100%) with good agreement between the subjective and objective scores of Δheart rate or systolic arterial pressure (SAP). The use of ΔMAP, ΔSAP, ΔDAP had the best ability in indicating peripheral nerve block failure (p < 0.001). Blood pressure values can detect a response to surgical stimulus in adequately anaesthetized dogs. The use of ΔMAP, ΔSAP or ΔDAP may be considered as objective measures to detect nerve block failure.