Objective <p>To evaluate the impact of preoperative pleural effusion on the ultrasound visualization of the paravertebral space (PVS), thoracic paravertebral nerve block administered by anesthesiologists, and to investigate whether ultrasound combined with pressure guidance can assist in locating the paravertebral space in patients with pleural effusion.</p> Methods <p>This prospective observational study enrolled patients undergoing thoracic surgery at Beijing Jishuitan Hospital between September 2021 and September 2022. Patients were categorized into two groups based on preoperative CT findings: the pleural effusion group (<i>n</i> = 40) and the non-pleural effusion group (<i>n</i> = 40). Prior to the induction of general anesthesia, all patients were placed in a lateral position. Thoracic paravertebral nerve block (TPVB) was administered using ultrasound guidance combined with pressure monitoring, with a 20&#xa0;ml of 0.5% ropivacaine.</p> Results <p>Parameters recorded included the duration of puncture and ultrasound pre-scan for TPVB, the ultrasound image definition score of the PVS, the pressure in the external intercostal muscle and PVS, and additional relevant indicators. Mean arterial pressure (MAP) and heart rate (HR) were measured before anesthesia induction, post-induction, and during skin incision. Compared to the non-pleural effusion group, the pleural effusion group demonstrated prolonged ultrasound pre-scan and puncture durations. The PVS definition score, the ventral displacement of the pleura, and the accuracy of resident anesthesiologists in identifying the PVS were all significantly lower in the pleural effusion group (<i>p</i> &lt; 0.05). Compared to non-pleural effusion group, the pleural effusion group had significantly higher pressure in PVS. In the pleural effusion group, the pressure in PVS was significantly lower than that in external intercostal muscle (<i>p</i> &lt; 0.05). No significant differences were observed in MAP and HR between the two groups before anesthesia induction, post-induction and during skin incision (<i>p</i> &gt; 0.05).</p> Conclusion <p>Preoperative pleural effusion is associated with reduced clarity of ultrasound visualization of the PVS, and extended procedural durations for anesthesiologists, thereby increasing the complexity of TPVB. Pressure detection during TPVB implementation can assist in locating the position of the puncture needle. For anesthesiologists with less experience, TPVB should be carefully performed in patients with preoperative pleural effusion.</p> Trial registration <p>The trial was prospectively registered with the Chinese Clinical Trial Registry under registration number ChiCTR2100050582, on August 30, 2021.</p>