Diffusion and clearance of ropivacaine after serratus anterior plane block by CT imaging: a prospective, single-arm trial

Abstract

Objective

This study aimed to provide the direct in vivo CT-based visualisation and quantification of local anaesthetic distribution and clearance within the fascial plane following a single-injection serratus anterior plane block (SAPB).

Methods

A total of 16 participants received an ultrasound-guided SAPB with 40 ml of 0.375% ropivacaine. Computed tomography (CT) scans were performed at 30 and 60 min post-injection. The primary outcome was the clearance rate of ropivacaine from the fascial space between 30 and 60 min. Secondary outcomes included the three-dimensional diffusion range of injectate.

Results

CT-based volumetric analysis showed that the volume of the ropivacaine within the fascial plane was 29.1 (28.3, 29.4) ml at 30 min post-injection and decreased to 26.0 (25.2, 26.9) ml at 60 min after a single 40 ml injection of 0.375% ropivacaine, resulting in a clearance rate of 6.4 ± 1.5 ml/h. CT imaging revealed that the injectate was primarily located within the deep serratus anterior plane, showing craniocaudal and anteroposterior spread along the thoracic wall. In the absence of postural restrictions, the injectate exhibited a tendency to migrate caudally over time.

Conclusions

This study demonstrates that CT imaging can directly visualise the distribution pattern and quantitatively assess the clearance of local anaesthetic within the fascial plane after a single-injection SAPB.

Introduction

The serratus anterior plane block (SAPB) has gained increasing application for analgesia of the anterolateral chest wall in breast surgery, rib fracture, thoracoscopic surgery and thoracotomy [1–4], since the first description by Blanco et al. [5]. Specifically, single-shot deep SAPB involves administering 20–40 ml of local anaesthetics (LAs) into the fascial plane between the serratus anterior and the rib/external intercostal muscles [6–8], targeting the lateral cutaneous branches of T2–T9 intercostal nerves to block sensation of the anterolateral chest wall [5,9]. Using intermediate-acting LAs like ropivacaine or levobupivacaine provides analgesia for about 6 to 10 h by binding to receptors on nerve fibre [10,11], but the effect diminishes as LAs are absorbed or diffuse away.

Continuous peripheral regional analgesia techniques have demonstrated superiority over single-injection blocks in providing prolonged postoperative pain control [12,13]. When applied to continuous regional blockade, programmed intermittent bolus (PIB) administration produces a more extensive sensory blockade and provides better analgesic efficacy compared with continuous basal infusion (CBI) [14–17]. Importantly, the clinical analgesic efficacy of fascial plane blockade, especially the analgesia range, is influenced by the extent of LA diffusion [6,8,18]. However, the early dynamic processes of LA distribution and clearance within the fascial plane remain poorly understood, largely due to the lack of direct visualisation tools. Most existing studies have employed dye injections in cadaveric specimens to observe the spread of injectate, whereas others have evaluated LA clearance based on changes in plasma concentration [4,19,20]. However, these findings do not accurately capture changes in volume or the dynamic distribution of LAs within the target fascial plane.

Although no prior imaging studies (CT or MRI) have investigated the spread and clearance of LAs following a single-injection SAPB in living participants, such methodology has been validated in studies of drug diffusion within interfascial space elsewhere [21]. After a single-injection SAPB with 0.25%–0.5% ropivacaine, plasma concentration peaks within 30 min, remains relatively stable or decreases slightly between 30 and 60 min, and declines rapidly thereafter [3,22]. Accordingly, we hypothesised that the diffusion of a 40 ml bolus of 0.375% ropivacaine is largely complete within 30  minutes, followed by a stable clearance phase. Hence, this study selected 30 and 60 min post LAs-injection as key observational time points for CT imaging.

This exploratory, descriptive study presents the first direct in vivo visualisation and quantification of ropivacaine distribution and clearance within the fascial plane after a single-injection SAPB, using CT imaging.

Patients and methods

Trial design

This prospective, open, exploratory, single-arm clinical trial was approved by the Ethics Committee of the First Affiliated Hospital of USTC, University of Science and Technology of China, on July 30, 2025 (2025KY360) and registered at ClinicalTrials.gov (ChiCTR2500107321), which was performed in accordance with the Helsinki Declaration. All the patients were informed of the full details of the study protocol and signed informed consent forms.

Participants

From August to December 2025, 16 patients scheduled to undergo CT-guided percutaneous pulmonary nodule localisation under local anaesthesia before thoracoscopic surgery (segmentectomy or lobectomy) were recruited. Participants were recruited based on the following inclusion criteria: age 18–65 years, both sexes, body mass index (BMI) of 18–24 kg/m², American Society of Anaesthesiologists (ASA) class I–III, and voluntary participation with the ability to accurately complete pain assessments. The exclusion criteria included the following: (1) systemic or puncture site infection; (2) known allergy or contraindication to local anaesthetics; (3) coagulation dysfunction; (4) severe impairment of heart (New York Heart Association classes III-IV), pulmonary (forced expiratory volume in 1 s <50% of the predicted values), kidney, or liver functions; (5) pregnancy or lactation; (6) peripheral nervous system disease or nerve damage in the region targeted by the SAPB-blocked area, or not suitable for nerve block; (7) inability to communicate or cooperate with study procedures; and (8) other circumstances under which the patients were ineligible for this study considered by the investigators.

Standard procedures

A specialised anaesthesiologist performed ultrasound-guided SAPB in the ward. The participant was positioned laterally, and an ultrasound system (Mindray, Shenzhen, China) equipped with a high-frequency linear transducer (10–13 MHz) was utilised to locate the serratus anterior and latissimus dorsi muscles, which were situated over the fourth to sixth ribs between the anterior and posterior axillary lines. A single-injection technique was performed at the fifth intercostal space in the midaxillary line. After skin preparation, a 22-G 90-mm nerve block needle (Tuoren, Henan, China) was introduced in the caudal-cephalad direction via an in-plane approach. Negative aspiration was confirmed when the needle tip was positioned at the deep fascial surface of the serratus anterior muscle adjacent to the target rib. One millilitre of saline was injected to test the location of the needle tip and open the potential fascial spaces. Subsequently, 40 ml of 0.375% ropivacaine (100 mg/10 ml, Shijiazhuang No. 4 Pharmaceutical Co., Shijiazhuang, China) was injected into the interfascial plane over a period of 40 s, with the needle tip maintained at the surface of the fifth rib on the midaxillary line throughout the injection. Ultrasound imaging confirmed adequate distribution of the LAs into the fascial plane between the serratus anterior muscle and the external intercostal muscle. Thereafter, CT-guided percutaneous nodule localisation was performed by the thoracic surgeon. Standard monitoring, including electrocardiography, heart rate, blood pressure, and pulse oximetry, was monitored and recorded at 10-min intervals throughout the procedure. During the waiting period for the CT scan, the patient could freely change their position without any specific position restrictions.

CT imaging study

Image acquisition and reconstruction

The CT scans were performed using a low-dose, thin-slice spiral protocol on a 64‑slice Philips scanner, covering at least from C7 to L1 levels. All scans were acquired at end-expiration breath-hold, with respiratory motion limited to <3 mm. CT images were acquired at 30 min and 60 min after SAPB, allowing a 2‑min timing window.

Definition of injectate boundaries

Three-dimensional digital processing of these images was performed on the Philips IntelliSpace Portal workstation to quantify the volume of fluid spread within the fascial plane (V_30min and V_60min). First, on axial CT images (slice thickness 1.25 mm), the boundaries of the injectate were delineated slice-by-slice. The injectate (ropivacaine mixed with saline) appeared as a slightly hypodense area (approximately 0–10 HU) within the fascial plane, bordered laterally by the serratus anterior muscle and medially by the ribs and intercostal muscles. This appearance allowed it to be clearly distinguishable from adjacent bone (150–1000 HU) and muscle (30–150 HU) on CT images. Using a slice‑by‑slice segmentation approach, the boundaries of the injectate were manually delineated on each CT section (Figure 1). Subsequently, 3D reconstruction and volumetric calculation were performed automatically (Figure 2).

Figure 1.

Delineation of LAs on axial CT images. (A) Schematic of axial CT magnification showing anatomical compartments—brown for peritoneal cavity, green for pleural cavity, red for vasculature, white for bone, yellow for muscle, blue for LAs, and gray for subcutaneous adipose tissue; (B) Axial CT magnification with measured HU values at representative sites—muscle at 42 HU, bone at 554 HU, and fluid at 0 HU; (C) Axial CT magnification with the LAs region (blue) delineated. CT: computed tomography; LAs: local anaesthetics; HU: Hounsfield unit.

Figure 2.

The 3D-reconstructions of LAs distribution in a single patient at 30-minutes (A) and 60-minutes (B) post SAPB by CT. The distribution of the injectate is shown in the green area in the figures. LAs: local anaesthetics; SAPB: serratus anterior plane block.

Volume measurement

The clearance rate (kabs) was calculated as: $\mathit{\text{kabs}}=(V_{30\mathit{\text{min}}}-V_{60\mathit{\text{min}}})/30$. Additionally, three-dimensional post-processing with pseudo-colour rendering was applied to visually distinguish the injectate from surrounding tissues, thereby illustrating its spatial relationship to adjacent bony structures and facilitating determination of its diffusion range.

Prior to study initiation, a senior radiologist with expertise in CT imaging and an engineer proficient in the Philips IntelliSpace Portal workstation conducted standardised training for one investigator (an anaesthesiologist) to ensure accurate delineation methodology. Subsequent delineation of the injectate boundaries on CT images was performed by the trained investigator. All delineation results underwent a comprehensive review by the experienced radiologist who conducted the initial training, ensuring verification and confirmation of the final delineation accuracy.

Assessment of sensory block

Cold sensation was assessed by the same anaesthesiologist using alcohol-soaked degreased gauze at 25 and 55 min after SAPB, with comparison to the contralateral area. Sensory block assessment was conducted according to a standardised dermatomal map (T2–T10). Testing was performed along five anatomical landmarks: the midclavicular, anterior axillary, midaxillary, and posterior axillary lines and the scapular line. At each point, sensation was graded on a 3-point scale: 2 (normal sensation), 1 (diminished sensation), or 0 (absent sensation). A successful sensory block for a specific dermatome was defined as a score of 1 or 0.

Other outcomes

Adverse events, including haematoma, infection, allergy, local anaesthetic systemic toxicity, and block failure, were monitored and documented.

Statistical analysis

All the statistical analyses were performed via SPSS (version 23.0). The normality of continuous variables was assessed with the Shapiro-Wilk test. Quantitative data were expressed as mean ± standard deviation (SD) for normally distributed data or as medians with interquartile range (IQR) for skewed data. Categorical variables are described as frequencies and percentages (n, %). A P value of less than 0.05 was considered statistically significant.

Results

A total of 16 patients met the eligibility criteria and were enrolled. All participants completed SAPB and underwent scheduled CT scans at 30- and 60-minutes post-SAPB. Their demographic characteristics were detailed in Table 1.

Table 1.

Participant characteristics.

Variable	Total cohort (n = 16)
Gender, male, n (%)	4 (25%)
Age, y, median (IQR)	56.5 (36.0, 60.0)
Weight, kg, mean ± SD	58.6 ± 6.4
Height, cm, mean ± SD	162.1 ± 6.9
BMI†, kg/m2, median (IQR)	22.9 (20.7, 23.7)
ASA, n (%)
II	7 (43.8%)
III	9 (56.2%)

IQR: Inter-quartile range; SD: Standard deviation; BMI: Body mass index; ASA: American Society of Anaesthesiologists.

^†Calculated as weight in kilograms divided by height in metres squared.

CT-based volumetric analysis showed that the quantified volume at 30 min (V_30min) was 29.1 (28.3, 29.4) ml, which decreased to 26.0 (25.2, 26.9) ml at 60 min (V_60min) after a single 40 ml injection of 0.375% ropivacaine. Given this, the clearance rate (kabs) was 6.4 ± 1.5 ml/h.

LAs was predominantly located within the deep serratus anterior plane, with craniocaudal and anteroposterior spread along the thoracic wall. Thirty minutes after the administration of SAPB, the diffusion of ropivacaine ranged from the 2^nd to 7^th rib at the anterior axillary line, from the 2^nd to 9^th rib at the midaxillary line, and from the 3^rd to 9^th rib at the posterior axillary line, respectively. The LAs moved towards the caudal direction in the fascial space over time. The diffused pattern of LAs was shown in Figures 2-3, and 4C and 4D. Sensation levels to cold were variable at different anatomical landmarks of the chest wall (Figure 4A and 4B).

Figure 3.

Distribution of LAs across the thoracic wall after SAPB. The distribution of LAs along the anterior axillary (A), midaxillary (B), and posterior axillary (C) lines is shown. The vertical axis indicates the rib segments and intercostal levels covered by the LAs, while the horizontal axis denotes the number of participants in whom spread was observed at each segmental level. LAs: local anaesthetics; ICS: intercostal space; SAPB: serratus anterior plane block.

Figure 4.

Sensory loss to cold and distribution of LAs after SAPB. (A and B) Sensory loss to cold at 25 minutes (A) and 55 minutes (B) post-SAPB. The intensity of colour correlates positively with the success rate; a deeper shade indicates a higher likelihood of successful sensory loss. The y‑axis corresponds to dermatomal levels from T2 to T10 (top to bottom), while the x‑axis represents the midclavicular line, anterior axillary line, midaxillary line, posterior axillary line and scapular line. (C and D) The distribution of LAs at 30 minutes (C) and 60 minutes (D) post-SAPB. Colour intensity indicates the number of participants with detectable injectate at each rib level along each vertical line. The y‑axis represents rib levels from T2 to T10 (top to bottom). The x‑axis represents the anterior axillary line, midaxillary line, and posterior axillary line. LAs: local anaesthetics; SAPB: serratus anterior plane block.

One participant experienced a pneumothorax during CT-guided percutaneous pulmonary nodule localisation. Symptoms were limited to mild chest tightness, with stable hemodynamics and oxygen saturation under monitoring. The protocol was completed without further incident. No other complications, such as haematoma, local infection, allergic reaction, or local anaesthetic systemic toxicity, were observed. Furthermore, there were no instances of SAPB failure.

Discussion

Our study clearly delineated the spatiotemporal distribution and clearance of ropivacaine within the first hour following a single-injection SAPB using CT imaging. The distribution of LAs within the fascial plane are characterised by free diffusion through the tissue, spread along fascial fissures, and partial systemic absorption via blood and lymphatic vessels [5,23,24]. This pharmacokinetic profile results in a gradual reduction of the dosage and volume of the LAs. After a single 40 ml injection of 0.375% ropivacaine, the volume of the LAs within the anterior serratus fascia plane demonstrated a time-dependent decline. Our imaging-based approach directly measured the volume loss from the fascial plane, yielding the clearance rate (kabs) of 6.4 ± 1.5 ml/h.

Until now, there have been no reports investigating the changes in local anaesthetic volume post-injection during SAPB using imaging tools such as computed tomography (CT). To obtain objective and reproducible measurements, we adopted CT-based volumetric analysis, a technique well-validated for quantifying pleural effusion, peritoneal effusion, and intracranial haemorrhage volume [25–27]. This approach can directly visualise the diffusion and volume changes of LAs, thus represents an ideal tool for real-time monitoring of the distribution and clearance of LAs within fascial space. To avoid both the potential influence of a contrast agent on LAs fluidity and absorption and its possible uncertain effects on neural tissues [28], contrast agents were not utilised in this study. Although the absence of a contrast agent increased the difficulty of identifying the LAs to some extent, precise measurement of CT attenuation still allowed reliable differentiation from surrounding tissues and ensured the validity of the results. The feasibility of this method has previously been demonstrated [21,29].

Previous pharmacokinetic studies of SAPB have demonstrated that plasma concentrations of ropivacaine increase rapidly within the first 30 min, peak at approximately 30 min, and then stabilise, indicating a shift from an initial rapid absorption phase to a slower and more stable phase [20,22]. This pharmacokinetic profile, coupled with clinical observations that sensory blockade is largely established by the same time point, supports the results that the primary phase of inter-fascial diffusion is substantially complete within 30 min post-injection [30,31]. Accordingly, we selected the 30- and 60-minute time points (i.e. the plateau phase after peak concentration) to measure LA volume. Our results showed that V_30min was 29.1 (28.3, 29.4) ml and V_60min was 26.0 (25.2, 26.9) ml after a single bolus of 40 ml injectate. The observed volume change over time was generally consistent with the trend of plasma concentration changes reported in previous pharmacokinetic studies.

Our imaging-based approach not only measured the volume loss from the fascial plane but also provided visual evidence. Notably, the craniocaudal distribution observed in our in vivo CT study differed from the ranges typically reported in cadaveric studies [23,32], which most commonly describe dye spread from T2 to T7. In our participants, the LAs spread from the 2^nd rib to the 7^th rib at the anterior axillary line, from the 2^nd rib to the 9^th rib at the midaxillary line, and from the 3^rd rib to the 10^th rib at the posterior axillary line, with variation among the sixteen participants. Furthermore, in anteroposterior diffusion, in the cadaveric studies, 100% (10/10) of specimens showed dye spreading anteriorly to the anterior axillary line, whereas none (0/10) showed posterior spread to the posterior axillary line. In contrast, in our study, injectate spread anteriorly to the anterior axillary line in 14 out of 16 participants, and posteriorly to the posterior axillary line in 12 out of 16 participants. Overall, the distribution in our participants tended to be more posterior and caudal compared to cadaveric findings. While unexpected, this discrepancy can be explained by patient position. Diffusion within the fascial plane is not fixed but can be influenced by gravity and posture. A previous study demonstrated that the choice of patient’s position after surgery significantly affects the distribution of LA-contrast mixture in the erector spinae plane block [33]. In our protocol, participants maintained a self-selected, head-up position (e.g. supine, semi-recumbent, seated, or lateral) during the first hour post-injection, rather than a strict supine position. In such postures, gravity likely promoted caudal drift of the LAs within the potential fascial space. The observed dermatomal blockade correlated well with this imaging pattern of LAs diffusion. It appeared that sensory blockade over the axillary region was slightly less extensive than that described in previous SAPB studies [34]. Notably, postoperative patient positioning has often not been standardised in SAPB studies, and because the serratus anterior plane forms an open anatomical space, leading to less predictable spread along the thoracic wall [35–37]. Therefore, postoperative patient positioning should be actively incorporated into the clinical management of patients undergoing single-injection or continuous SAPB.

In the present study, the clearance rate (kabs) of the LAs was calculated from the measured reduction in LA volume over time. This calculated reduction in LA volume includes potential minimal diffusion into adjacent anatomical spaces during SAPB. As established, the injectate diffuses within the fascial plane between the serratus anterior muscle and the ribs/intercostal muscles [7]. Currently, no evidence indicates that the spread extends beyond the SAP. However, even if such diffusion occurs, LAs that spread beyond the SAP would be unlikely to block the target nerves and may still reasonably be regarded as part of the hourly volume reduction, which aligns with the primary objective of this study.

This study has several limitations. First, it was a single-center trial. Second, the variable interval between SAPB performance and the end of surgery precluded standardised early postoperative pain assessments at specific time points, as commonly performed in other regional anaesthesia studies. It is important to note that the primary aim of this study was to quantify LAs distribution and clearance rather than to reconfirm the established analgesic efficacy of SAPB, which has been well-documented in earlier reports including our own [34,38]. Third, we did not perform subgroup analyses based on factors such as posture, sex, or age. Despite these limitations, we believe they do not undermine the validity of our primary conclusions. Future studies should consider multicentre collaborations and stratified analyses to further extend our findings.

Conclusion

This study provides the first direct in vivo CT-based visualisation and quantification of local anaesthetic distribution and clearance within the fascial plane after a single-injection SAPB. Notably, the injectate spread more posteriorly and caudally than reported in cadaveric studies, a difference likely attributable to the influence of patient positioning. These findings are descriptive and exploratory. Further prospective investigations are warranted to explore the potential clinical implications of these imaging findings, including aspects related to postoperative patient positioning, as well as dosing intervals and volumes of local anaesthetic for continuous postoperative analgesia.

Ethics approval and consent to participate statement

This trial was approved by the Ethics Committee of the First Affiliated Hospital of USTC, University of Science and Technology of China (approval NO. 2025KY360) and written informed consent was obtained from all participants in the trial.

Funding Statement

The study is supported by the National Natural Science Foundation of China. (grant number 82501480).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this research are available from the corresponding author, J.Y., upon reasonable request.