Regional anesthesia techniques (or nerve blocks) are used to provide sensory blockade over the chest wall. The most established technique is thoracic epidural. This provides bilateral spread but is also associated with neuraxial complications, which has led to a decrease in clinical usage. Alternative peripheral nerve block techniques, such as intercostal nerve blocks, eliminate the neuraxial complications but, in turn, require numerous injections to provide unilateral coverage. The introduction of ultrasound-guided (USG) nerve blocks has generated clinical interest in the development of novel paraspinal (non-epidural) nerve block techniques, which will provide multiple dermatomal coverage with a single injection. Two such techniques are the Erector Spinae Plane (ESP) block and the Intertransverse Process (ITP) block. The Intertransverse Process (ITP) block targets a slightly deeper plane than the Erector Spinae Plane (ESP) block and was specifically designed to address the ESP block's variable anesthetic spread. Despite its potential for more localized and precise dermatomal coverage, no prior studies have directly compared the two techniques in terms of their cutaneous distribution.

Thus, this randomized, double-blinded trial aims to compare the cutaneous distribution of both these blocks in healthy adult volunteers. To compare the dermatomal sensory block distribution, the investigators will use detailed mapping with pinprick, cold, and heat stimuli.

In addition to assessing the distribution and coverage of the ESP and ITP blocks, this study also seeks to investigate the pharmacokinetics (PK) of lidocaine in each technique. The rationale for this lies in the anatomical differences between the two blocks. Theoretically, there may be a difference in the anatomical target for both blocks, but the investigators hypothesize that there is no difference in the local anesthetic absorption. Therefore, the investigators expect similar systemic absorption, the chances of local anesthetic systemic toxicity (LAST) should be equally low, and both blocks should be similarly safe.

A total of 14-18 healthy adult volunteers will be included. There will be two different procedure days, separated by at least one week. On each day, the subjects will receive a paraspinal block, according to the randomization process. Each volunteer will undergo pre-procedure screening on the first visit. The blocks will be performed on the same side, at the same level, by the same anesthesiologist, at least one week apart. An ultrasound-guided ESP block will be administered in one day, and an ITP block on the other. The order will be randomized, and both the subject and the research team member assessing the block will be blinded. Sensory testing will be performed at baseline and 60-90 minutes after the block injections. Serial samples of blood will be drawn to analyze lidocaine PK (between 0 and 240 minutes post-block).

Inclusion Criteria:

• Ability to provide written informed consent
• Ability and willingness to comply with the study procedures and duration requirements
• ASA physical status 1 or 2
• Age ≥ 18 years

Exclusion Criteria:

• BMI > 35kg.m2
• Use of analgesics within 24 hours before the procedure
• History of thoracic trauma or surgery
• Thoracic deformities or abnormalities that may prevent proper block performance
• Thoracic tattoos
• Systemic neuromuscular disease
• Contraindications to regional anesthesia (e.g., infection, allergy, challenging sonoanatomy)
• Other known health conditions that would affect the participant's ability to successfully complete the study