Duration of Peripheral Nerve Blocks in Opioid Tolerant Individuals - A Volunteer Blinded Matched Case-control Study

Do peripheral nerve blocks work shorter if you are on opioids? Background: Peripheral nerve blocks effectively alleviate postoperative pain. Animal studies and human research indicate that opioid tolerance results in reduced effectiveness of local analgesics. This applies to both central nerve blockade and infiltration anaesthesia. The impact on peripheral nerve block in humans has not been evaluated. The aim of the study is to assess the onset time and duration of a radial nerve block in opioid tolerant individuals compared to opioid naive individuals. We hypothesise that peripheral nerve blocks cause shorter sensory and motor block in opioid-tolerant compared to opioid-naive counterparts. Methods: Twenty opioid-tolerant individuals will be matched by sex and age with opioid-naïve counterparts. Participants will undergo an ultrasound-guided radial nerve block. The onset time and duration of motor and sensory blockade will be tested and measured. The primary outcome is difference in duration of sensory nerve blockade between the two groups. The secondary outcomes include the onset time of sensory blockade, onset time of motor blockade, and difference in duration of motor nerve blockade. Perspectives: The data from this study will provide evidence for the duration of peripheral nerve block in opioid tolerant individuals and support if a specific postoperative protocol for opioid tolerant individuals is needed or it should be of special attention.

Inclusion criteria for opioid group * Age \> 18 years old * ASA 1-3 * Chronic pain, defined as persisting pain for more than three months * Daily use of more than 60 mg of morphine equivalent in more than 7 days * Legally competent and able to give informed consent Inclusion criteria for the opioid naive control group * Age \> 18 years old * ASA 1-3 * No use of opioids within 30 days prior to participation * Legally competent and able to give informed consent Exclusion Criteria: * Age \> 84 years old * Volunteers who cannot cooperate with the study * Volunteers who cannot understand or speak Danish * Allergy to the lidocaine or the preservatives used in the study * Alcohol or drug abuse other than opioids * Peripheral neuropathy at the upper limb non-dominant arm * Pathology or previous major surgery to the upper limb * Active signs of infection in the cutaneous area of injection * Pregnancy at the time of the trial * Partial or complete 2nd or 3rd degree atrioventricular heart block without a pacemaker * Severe liver disease * Severe kidney disease and reduced kidney function (eGFR below 30 mL/min) * Daily treatment with drugs that are structurally related to local analgesics or class IB antiarrhythmics, as the toxic effects are additive * Daily treatment with class III antiarrhythmics e.g., amiodarone.