The intertransverse process (ITP) block is a new ultrasound-guided peripheral nerve block modality designed for peri- and postoperative pain amelioration for patients undergoing surgery on the thoracic wall. The modality mimics the well-known thoracic paravertebral block but, potentially, with a significantly lower risk of adverse events. However, evidence for the ITP block efficacy and the clinical applicability, e.g. the number of injections, dermatomal coverage etc., is still sparse and needs further investigation.

In this noninferiority, short-term, randomised, blinded, procedure-related crossover trial we will further investigate the ITP block in healthy volunteers to see whether it is possible to reduce the number of injections from three to one and substantiate the number of anaesthetised dermatomes with either modality as the primary outcome. Secondary outcomes include sensory mapping of the thorax, non-invasive thermography, changes in non-invasive blood pressure and satisfaction with block application.

At Zealand University Hospital, twelve healthy volunteers will be randomised to receive either a single injection or multiple injection blockade with long-lasting local anaesthetic. All twelve participants will, on day one, receive active blockade with the well-known marketed drug Ropivacaine 7.5mg/ml, and all participants will receive 21 ml; that is six participants receiving 1x21ml and six participants receiving 3x7ml. The six participants receiving 1x21 ml will also receive two sham injections to ensure the blinding. No placebo is used. On day two the intervention is crossed over and the participants will receive the other modality. Within the following hour after block application, relevant standard cutaneous testing is performed; pinprick and cold sensation test for dermatomal coverage, the anaesthetised skin area is pen marked on the thorax and photo documented, thermography to measure temperature differences between each hemi thorax (blocked side vs. non- blocked side) and standard non-invasive blood pressure measurements are performed.

After 60 minutes of relevant testing the trial ends and the participant are free to leave the hospital 2 hours after block application if no adverse events are recorded. With such series of test procedures, we will generate new knowledge of the ITP block before future patients undergo breast cancer surgery.

Inclusion Criteria:

Male sex Age ≥ 18 years at the date of inclusion American Associations of Anaesthesiologist (ASA) class I-II Have received thorough information, orally and in written form, and signed the "Informed Consent" form on participation in the study.

Weight > 52,5 kilograms (chosen due to the international maximal single dose of Ropivacaine (3mg/kg))

Exclusion Criteria:

• Substantial co-morbidity: American Associations of Anaesthesiologist (ASA) class > II
• Inability to speak and understand Danish
• Inability to cooperate
• Allergy to study drugs
• Daily intake of analgesics (investigators decision)
• Alcohol and/or drug overuse (investigators decision)
• Previous thoracic trauma or thoracic surgery
• Previous/current piercing of the nipple of any kind on blockade side
• Tattoos on the thorax (according to the investigators decision)
• Any systemic muscular or neuromuscular disease
• Any use of vasodilatory substance (investigators decision)
• Local infection at the site of injection or systemic infection
• Difficult sonoanatomical visualisation of the target area (SCTL, ITTC etc.) necessary for the block execution
• Severe hypovolemia