The aim of this clinical trial is to investigate the effect of different injection speeds on the spread of local anesthetic during ultrasound-guided erector spinae plane block (ESPB) in patients undergoing CT-guided lung nodule localization. The primary question addressed is whether a high injection speed (30 ml delivered within 30 seconds) produces a different local anesthetic distribution compared with a lower injection speed (30 ml delivered within 180 seconds).

Senventy-four patients were selected to undergo CT guided puncture localization of lung nodules under local anesthesia, and the patients were divided into groups using a computer-generated random number in a ratio of 1:1. To ensure objectivity, a nurse who was not involved in the study prepared a sealed opaque envelope containing grouping information. Patients were randomly divided into two groups: control group (group C, 37 patients), and experimental group (group S, 37 patients). All patients underwent ultrasound guided ESPB before CT-guided nodule localization. Ultrasound guided ESPB method: Using a high-frequency linear probe (5-13 MHz, Sonosite, USA), the probe is placed parallel to the spine on the surface of the transverse process tip of the seventh thoracic vertebrae. Under ultrasound, the transverse process and spinal muscles are clearly exposed. Then, a long beveled needle is used, and inserted from the cephalad to caudal with in-plane technique. After the needle tip reaches between the transverse process and erector spinae muscles, 2ml saline is injected using water separation technique to confirm the position of the needle tip, then injecting 30ml local anesthetic solution (0.75% ropivacaine 15ml+iohexol 15ml). In the control group, the injection is administered at a rate of 30 ml within 180 seconds. In the experimental group, the injection is delivered at a faster rate of 30 ml withinr 30 seconds. After 30 minutes of block completion, CT scan and puncture localization were performed, following with 3D reconstruction. The primary outcome was mixture spread to the paravertebral space.The second outcomes were as follow: 1.spread to the intercostal space. 2. spread to the epidural space 3.spread to the neural foramina 4. cranio-caudal spread

Inclusion Criteria:

Patients scheduled for CT-guided lung nodule localization under local anesthesia will be selected

1. aged 18-80 years
2. BMI 18-30 kg/m²
3. ASA classification I-III

Exclusion Criteria:

1. Allergy to the study drug or to local anesthetics
2. History of opioid abuse
3. Previous infection at the ESPB or PVB puncture site
4. Peripheral neuropathy
5. Dysfunction of blood coagulation