Abstract

Background: This study aims to investigate the effects of deep parasternal intercostal plane block on left internal thoracic artery vasospasm in coronary artery bypass grafting patients.

Methods: Between March 2024 and August 2024, a total of 29 patients (24 males, 5 females; mean age: 60.1±8.3 years; range, 56 to 66 years) who were scheduled for elective coronary artery bypass grafting were included in this prospective study. An ultrasound-guided bilateral deep parasternal intercostal plane block was performed with 15 mL of 0.25% bupivacaine per side after anesthesia induction. Left internal thoracic artery peak systolic velocity, end-diastolic velocity, and resistive index, along with heart rate and mean arterial pressure, were recorded immediately before the block (T0) and 30 min after (T1). Demographics, body mass index, and comorbidities of the patients were noted.

Results: After deep parasternal intercostal plane block administration, the left internal thoracic artery resistivity index decreased significantly (p=0.041), and the left internal thoracic artery diameter increased significantly (p=0.004). Although the peak systolic velocity increased and the end-diastolic velocity decreased following the block, these changes were not statistically significant (p=0.145 and p=0.135, respectively).

Conclusion: Our study findings suggest that deep parasternal intercostal plane block administration may prevent left internal thoracic artery vasospasm by increasing arterial conduit diameter and reducing the resistivity index. Based on these findings, we believe that this method can be safely applied under ultrasound guidance without complications.

Coronary artery bypass grafting (CABG) is among the most frequent cardiac operations, restoring coronary flow with autologous conduits and lowering myocardial infarction (MI) risk. Currently, the left internal thoracic artery (LITA) remains the preferred graft owing to its 90% long-term patency and survival benefit, although perioperative vasospasm of the LITA can dramatically reduce graft flow.[1,2] Vasodilators such as calcium-channel blockers, nitroglycerin, papaverine, and sodium nitroprusside are routinely administered,[3] while regional blocks that decrease sympathetic tone also enlarge LITA diameter and flow.[4,5]

Sternotomy, drains, and retraction cause considerable postoperative pain; consequently, non-opioid multimodal analgesia, particularly fascialplane blocks, has gained traction in cardiac surgery. The deep parasternal intercostal plane (DPIP) block, formerly known as the transversus thoracis muscle plane block, targets the anterior cutaneous branches of the intercostal nerves (T2-T6), which innervate the sternum.[6] Local anesthetic is administered between the internal intercostal and transversus thoracis muscles, approximately 2 to 3 cm lateral to the sternal border.[6] The DPIP block provides effective opioid-sparing analgesia in cardiac, breast and thoracic procedures.[7-11] Regional anesthesia techniques may provide not only analgesia, but also vasorelaxation by modulating sympathetic activity. Still, LITA vasospasm remains a challenge during CABG, and fascial plane blocks such as the DPIP may improve graft flow by reducing arterial spasm. However, evidence regarding the vasodilatory effects of DPIP block remains limited, warranting further investigation.

To the best of our knowledge, there is no study to investigate arterial conduit vasorelaxation in the context of the DPIP block in the literature. In the present study, we, therefore, aimed to investigate the potential hemodynamic benefits of the DPIP block by assessing its effect on LITA vasospasm and to evaluate changes in LITA diameter and Doppler flow parameters prior to harvesting in order to determine whether DPIP block could attenuate sympathetic-mediated vasospasm.

Methods

This single-center, prospective study was conducted at Ankara Bilkent City Hospital, Department of Cardiovascular Surgery between March 2024 and August 2024. A total of 29 patients (24 males, 5 females; mean age: 60.1±8.3 years; range, 56 to 66 years) who were in the American Society of Anesthesiologists (ASA) Class II-III and scheduled for elective CABG were included. Each patient had a median sternotomy and three- or four-vessel grafting using the LITA plus a saphenous vein. We excluded emergency or combined/redo cases, ASA Class ?IV, ejection fraction (EF) of <45%, bradycardia (<60 bpm), failed LITA harvest, allergy or infection at the block site, and any vasoactive drug use before LITA flow was measured. Calcium-channel blockers and angiotensin-converting enzyme (ACE) inhibitors were discontinued 24 h before surgery, while long-term beta-blockers were continued. Written informed consent was obtained from each patient. The study protocol was approved by the Ankara Bilkent City Hospital Medical Research Ethics Committee (Date: 28.02.2024, No: TABED 1-24-28). The study was conducted in accordance with the principles of the Declaration of Helsinki.

Perioperative management
After arrival in the operating theatre, patients received no pre-medication. Standard ASA monitors, an arterial line, bispectral index (BIS) (BIS VISTA™, Covidien, Mansfield MA, USA) and cerebral oximetry (NIRS INVOS™, Somanetics, Troy MI, USA) were applied. Anesthesia was induced with propofol 1 to 3 mg/kg, fentanyl 2 ?g/kg, lidocaine 1 mg/kg, and rocuronium bromide 0.6 mg/kg, followed by endotracheal intubation, central venous, urinary and nasopharyngeal temperature catheters. Maintenance comprised 50% O₂-air, propofol 5 to 7 mg/kg/h and remifentanil 0.5 to 1 ?g/kg/min, titrated to a BIS of 40-60. Volume-controlled ventilation used tidal volumes of 6 to 8 mL/kg, a rate of 12 to 14 min^-1, and end-tidal carbon dioxide (EtCO₂) 35 to 45 mmHg.

After placing patient supine, the intercostal spaces were marked (Figure 1), baseline LITA peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI) were measured at the T5 level. The DPIP block was, then, placed bilaterally by a single anesthesiologist using a color Doppler ultrasound system (PHILIPS Affiniti 50, Philips Ultrasound Inc., Reedsville, PA, USA) with a 50-mm linear transducer (Philips L12-5), and an 80-mm echogenic needle (SonoTAP®; PAJUNK, Geisingen, Germany) (Figure 2). Under sterile conditions, the probe was positioned 2 to 3 cm lateral to the sternum in the sagittal plane, and 20 mL of 0.25% bupivacaine was injected between the intercostal and transversus thoracic muscles on each side. Doppler confirmed spread without pleural breach. Doppler ultrasonographic measurements were repeated 30 min after local anesthetic administration at the previously marked site. The PSV, EDV, and RI values remeasured. A right DPIP and femoral block were also administered for saphenous harvest.

Figure 1. Determination of deep parasternal intercostal plane block location and left internal thoracic artery measurement location.

Figure 2. Ultrasound-guided deep parasternal intercostal plane block and Doppler assessment of the left internal thoracic artery. (a) Ultrasound-guided image of the ICM, TTM, ribs and pleura were obtained in the T4-T5 intercostal space. (b) Ultrasound imaging of deep parasternal intercostal plane block applied between the ICM and the TTM. (c) Ultrasound imaging of pleural descent during deep parasternal intercostal plane block. (d) The diameter, peak systolic velocity, end-diastolic velocity and resistivity index of left internal thoracic artery were measured.
ICM: Intercostal muscle; TTM: Transversus thoracic muscle; PM: Pectoral muscle.

The same surgeon harvested every LITA using electrocautery (Force 2™ Electrosurgical Generator; Valleylab, Boulder CO, USA) and hemoclips, preserving a wide pedicle. After systemic heparin 300 U/kg, the artery was divided proximal to its bifurcation. A topical solution of 20 mg of papaverine in 20 mL saline was applied to the pedicle; no peri-arterial or intraluminal injection was performed. The patients were kept normothermic; the mean hematocrit was set at 44±5.4%. Additional heparin (300 U/kg) maintained an activated clotting time (ACT) of >400 sec, until cardiopulmonary bypass.

Statistical analysis
Study power analysis and sample size calculation were performed using the G*Power version 3.1.9.2 software (Heinrich Heine University Düsseldorf, Düsseldorf, Germany). We used a paired-samples t-test on LITA diameter data as described by Gopal et al.[5] W ith a n e ffect s ize o f d=1.127, ? =0.05, and power=0.95, the required sample was 13. To cover additional variables, 29 CABG patients were ultimately enrolled.

Statistical analysis was performed using the IBM SPSS version 27.0 software (IBM Corp., Armonk, NY, USA). Continuous data were presented in mean ± standard deviation (SD) or median (Q1-Q3), while categorical data were presented in number and frequency. The comparison of pre- and postoperative measurements for LITA was performed using the paired sample t-test. Mixed linear models were established to determine the effect of the demographical and clinical data on the change of flow measurements in LITA before and after the surgery. A p value of <0.05 was considered statistically significant.

Results

Among 29 patients included in the study, no damage to the LITA or pleura was observed following the DPIP block. The majority of the patients (75.9%) had hypertension (HT). A total of 34.5% of the patients had a history of smoking. Body mass index (BMI) values indicated that the patients were mildly overweight, and the mean EF was 54.44±7.80% (Table 1).

Table 1. Characteristics of the patients

In the LITA flow velocity measurements, PSV increased following the DPIP block compared to preblock values, while EDV decreased. However, these changes were not statistically significant (p=0.145 and p=0.135, respectively). A notable finding was a significant decrease in the RI (p=0.041), with RI reducing from 0.95±0.04 before the DPIP block to 0.91±0.11 after the procedure. Furthermore, the LITA diameter showed a significant increase from 0.21±0.05 cm pre-block to 0.26±0.07 cm post-block (p=0.004) (Table 2).

Table 2. Comparison of repeated flow measures

Regression analysis was conducted to explore factors influencing changes in PSV, EDV, RI, and LITA diameter before and after the DPIP block. Although changes in PSV and EDV were not statistically significant overall, age was found to significantly affect the increase in PSV (p=0.030). A significant decrease in EDV was associated with male sex (p=0.017), the presence of HT (p=0.001), diabetes mellitus (p=0.027), and smoking (p=0.026). Similarly, the decrease in RI was significantly influenced by male sex (p=0.021), increasing age (p=0.026), HT (p=0.001), and hyperlipidemia (p=0.040). Additionally, time-dependent RI demonstrated significant differences in repeated measurements (p=0.042). Regarding LITA diameter, a statistically significant change was observed before and after the block (p=0.039). However, no other variables significantly affected diameter changes. When LITA flow measurements were compared according to demographic and clinical characteristics, patients with HT exhibited significantly higher flow values (p=0.025). However, flow values did not differ significantly based on other demographic or clinical factors. Similarly, LITA diameter values before and after the DPIP block did not show significant differences across demographic or clinical characteristics (Table 3).

Table 3. Linear mixed regression models for LITA flow measurements

No statistically significant differences were observed in the hemodynamic data of the patients before and after the block (Table 4). Moreover, none of the patients experienced postoperative MI or block-related complications such as hematoma or pneumothorax.

Table 4. Hemodynamical parameters of the patients

Discussion

The main drawback of arterial conduits is perioperative graft spasm, particularly in LITA and radial artery (RA) used in myocardial revascularization. Various factors contribute to graft spasm intraoperatively, including endothelial injury, local manipulation, temperature changes, and vasoconstrictor substances. Many drugs, techniques or blocks have been used to reduce this arterial vasospasm, but research on this subject is still ongoing.[5,12-16] In the present study, we demonstrated that the DPIP block increases the diameter of the LITA in patients undergoing CABG surgery, potentially preventing vasospasm by decreasing EDV, RI, and flow velocity. Although the increase in LITA PSV with DPIP was not statistically significant, the observed decrease in RI is a critical finding, supporting the hypothesis of reduced vasospasm.

The thoracic intercostal nerves (T1-T11) divide into dorsal and ventral ramus in each spinal nerve, interact with the sympathetic trunk and provide sensory innervation of the anterior and lateral chest, with the dorsal ramus traveling between the middle back and the ventral ramus traveling first between the pleura and endothoracic fascia and then between the intercostal muscles. As the intercostal nerve divides into the internal and external intercostal nerves in the midaxillary line, it gives its lateral cutaneous branch and innervates the lateral chest and gives its anterior cutaneous branch in the sternum and innervates the anterior chest.[17] Previous studies have shown that DPIP block is effective in blocking the anterior cutaneous branches of the thoracic intercostal nerves (T2-6) through a singular intercostal site injection.[18] Therefore, the analgesic efficacy of DPIP block in cardiac surgery has been demonstrated in recent articles.[19,20] We believe that it can also be used for vasodilation of the LITA. In our study, the increase in LITA diameter and decrease in RI support this hypothesis.

To provide optimal analgesia with minimal side effects in cardiac surgery, regional anesthesia techniques, particularly fascial plane chest wall blocks, continue to become more popular increasingly.[17] However, the effect of fascial t runk blocks on the arterial grafts used, particularly in CABG operations, has not been sufficiently investigated in the literature. One of the studies performed for this purpose is the effect of stellate ganglion block (SGB) on vasospasm on LITA and this block is performed correctly, with rare complications, and can increase blood flow in LITA and RA diameter and prevent spasm, potentially improving surgical outcomes.[5,15] Chandran et al.[21] showed that LITA with SGB and papaverine and LITA with papaverine alone did not increase blood flow, but decreased the radial-femoral arterial pressure difference. In this case, it is a study with conflicting results showing that SGB is effective on arterial vasospasm, but not on LITA spasm.[21] Another method to manage pain after CABG is the erector spinae plane block (ESPB) as a novel approach where anesthetic is administered between the erector spinae muscles, providing blockade of thoracic spinal nerves and sympathetic fibers. Darçın et al.[16] also showed that ESPB was superior to other methods of preventing arterial graft spasm, such as medication and stellate ganglion block. As the transversus thoracis muscle is a muscle located on the inside of the rib cage, DPIP block technique involves injecting the local anesthetic drug into this muscle plan. This type of block can affect the blood flow of the LITA and may provide vasodilation and reduce artery spasm. The effects of blocking on LITA may improve surgical outcomes such as LITA diameter, changes in blood flow velocity and long-term effects on postoperative graft patency.[17,18] In our study, DPIP showed a decreasing effect on ITA vasospasm, suggesting that trunk blocks performed in cardiac surgeries may have a positive effect on related studies.

Thoracic epidural anesthesia (TEA) has also been used to increase LITA flow in studies. In a study, TEA increased vascular endothelial growth factor (VEGF), endothelial nitric oxide synthase (e-NOS), and adenosine-A2B receptor expressions not only in flow velocities, but also in immunohistochemical and histological analysis.[4] This is a result which supports the innervation of the LITA and the favorable effect of anterior intercostal nerve blockade with DPIP, which interacts with the sympathetic trunk, on vasospasm.

Temporary spasm or mechanical damage during surgery can be considered as the main causes of low LITA free flow. Postoperative Doppler studies have shown an improvement in LITA flow, indicating that transient spasms subside in the acute postoperative phase. It is recommended against using intraluminal maneuvers (such as papaverine injection or balloon dilation) for low free-flow LITA, as these may cause endothelial damage. Its long-term results are favorable: 94.3% graft patency rate at four years, 93.3% cardiac-related event-free rate at five years, and 97.1% actuarial survival rate at five years.[13,22]

A study examines the effects of four vasodilator agents (nitroglycerin, diltiazem, papaverine, and adenosine) on LITA blood flow during CABG.[23] The results of the study showed papaverine increased LITA flow 1.82-fold, with diltiazem and nitroglycerin showing slightly less effect, and adenosine the least (1.57-fold increase). The study also emphasized the importance of using vasodilator solutions at body temperature (37°C) rather than room temperature, as it may enhance the vasodilatory effect.

In the limited studies in the literature, while examining the effect of regional anesthesia methods on LITA flow, risk factor analysis of demographic characteristics was not performed. In our study, male sex, advancing age and HT comorbidity were found to have an increasing effect on the change in RI. However, further well-designed, large-scale, prospective studies are needed to confirm these findings.

Nonetheless, there are several limitations to this study. First, the small sample size may have limited the statistical significance of the observed increase in PSV. Second, more frequent LITA measurements at additional intervals could provide clearer insights. Third, the lack of a comparison group employing alternative spasm prevention methods limits the generalizability of our findings. Finally, the study lacks long-term follow-up data on graft patency and outcomes.

In conclusion, our study findings suggest that deep parasternal intercostal plane block may prevent vasospasm by increasing left internal thoracic artery diameter and reducing resistive index. Taken together, we propose that deep parasternal intercostal plane administration, under ultrasound guidance, offers a safe and effective alternative for managing pain and arterial spasm in post-coronary artery bypass grafting patients, with comparable outcomes to other pharmacological agents. Further research with larger sample sizes and long-term follow-up is needed to validate these results and expand upon the potential benefits of deep parasternal intercostal plane in cardiac surgery.

Data Sharing Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author Contributions: Conceived, designed and did statistical analysis & editing of manuscript, is responsible for integrity of research: S.M.; Did data collection and manuscript writing: E.B.G., A.Y., N.G., N. S.; Did review and final approval of manuscript: S.M, N.S.

Conflict of Interest: The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding: The authors received no financial support for the research and/or authorship of this article.

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