Study design and patient recruitment

This prospective observational study was conducted at the Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea, between July and December 2019. The trial protocol was registered at ClinicalTrials.gov (https://clinicaltrials.gov/ct2/show/NCT03890640) on March 26, 2019. Participants were enrolled from July 4, 2019 to December 12, 2019. Adult patients (20–79 years) who were scheduled for upper abdominal surgery and required patient-controlled epidural analgesia in the thoracic region were considered eligible for enrollment. Patients with allergy to local anesthetics and contrast dye or steroids, infection at the insertion site, neurological or psychiatric disorders, prior spine instrumentation near the insertion site, or coagulopathy or those taking anticoagulants or antiplatelet medication were excluded.

Protocol for real-time US-TECP

According to the previous reports of US-TECP,6 14 patients were placed in the sitting position during the procedure. However, based on our experience, maintaining the proper position of US probe is sometimes challenging when the patients are seated; hence, patients were placed in the prone position with a pillow under the upper abdomen to widen the target interlaminar space. Routine monitoring was established prior to the procedure. Although the interlaminar space between T6 and T8 has been traditionally recommended as the target space for epidural catheterization in patients undergoing upper abdominal surgery,15 we considered the interlaminar space between T9 and T12 as the target space for US-TECP in this study. This is because programmed intermittent epidural bolus (PIEB) infusion is used as our institutional protocol for patient-controlled epidural analgesia, which provides a more extensive cephalic spread of epidural medication.16 17 In addition, accessing the thoracic epidural space was relatively easy through the interlaminar space between T9 and T12 compared with the mid-thoracic region. After preprocedural scanning of the interlaminar space between T9 and T12, the interlaminar space with the best visualization of epidural structures such as the ligament flavum, posterior dura, and anterior complex (anterior dura, posterior longitudinal ligament, and vertebral body) was considered the target space. According to our standard of care, the T10–T11 interspace was most commonly selected for real-time US-TECP. Two investigators (D-HK and J-HL) with >2 years of experience in performing this procedure performed all placements.

First, a high-frequency linear US probe (12 MHz; NextGen LOGIQe, GE Healthcare, Madison, Wisconsin, USA) was placed in the longitudinal plane near the scapular line of thoracolumbar junction. Thereafter, the 12th rib on the target side was identified by caudal-to-cranial scanning. Subsequently, the probe was moved cephalo-medially to identify the intercostal space in line with the target interspace (figure 1A). In this view, the round-shaped rib and hyperechoic pleural line were detected (figure 1B). Second, the probe was further moved medially to obtain the paramedian sagittal transverse process view (figure 1C). In this view, the square-shaped transverse processes were observed (figure 1D). Third, the probe was moved slightly medially from the transverse process view to obtain the paramedian sagittal articular process view (figure 1E) to visualize the corresponding laminae, which resembled wave-like structures, and superior articular process (SAP) of inferior vertebrae between the laminae (figure 1F). Then, to obtain the paramedian sagittal oblique view, the physician laterally tilted the probe (figure 1G). Consequently, the posterior complex (ie, the ligamentum flavum and posterior dura) was observed as linear hyperechoic structures between the laminae (figure 1H). Moreover, the intrathecal space and spinal cord (anechoic) and anterior complex (hyperechoic) were visualized in this view. Subsequently, the cephalad end of the probe was pivoted medially (figure 1I). The height of laminae of the inferior vertebral body in this view was lesser than that in the paramedian sagittal oblique view (figure 1J). This ensured that the pathway of the epidural needle tip was not interrupted by laminae. The final location of the probe is indicated in figure 1I. The real-time US scanning for identification target epidural space was displayed in online supplemental video 1.

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Figure 1

Location of the ultrasound probe and the corresponding ultrasound views, and movement of the ultrasound probe to obtain proper ultrasound views. (A) The probe positioned to identify the intercostal space in line with the target interspace. (B) Round-shaped rib (R) and hyperechoic pleural line are seen on the intercostal space view with the probe positioned as in A. (C) The probe moves medially to obtain the paramedian sagittal transverse process (TP) view. (D) Square-shaped TPs are seen on the paramedian sagittal TP view with the probe positioned as in C. (E) The probe moves slightly medially from the TP view to obtain paramedian sagittal articular process view. (F) Laminae (L) resembling wave-like structures and superior articular process (SAP) of inferior vertebrae between the L are seen on the paramedian sagittal articular process view with the probe positioned as in E. (G) The probe laterally tilts to obtain the paramedian sagittal oblique view. (H) The ligamentum flavum (LF) and posterior dura (PD) of the posterior complex are seen as linear hyperechoic structures between the L on the paramedian sagittal oblique view with the probe tilted laterally from the position in G. (I) The cephalad end of the probe is pivoted medially to decrease the height of L of the inferior vertebral body. (J) A decrease in the height of L of the inferior vertebral body compared with that in the paramedian sagittal oblique view is observed when the cephalad end of the probe is pivoted medially; the probe is positioned as in I. AC, anterior complex.

The target interlaminar space was identified and the center of the interlaminar space and both the cephalad and caudal ends of the probe were marked on the overlying skin. After local infiltration with 2% lidocaine at the intended needle entry site, an 18-gage Tuohy needle (Perifix, B Braun Melsungen AG, Melsungen, Germany) was inserted from the caudal end of the probe and was advanced in plane view under real-time US guidance until the needle tip reached in front of the posterior complex in interlaminar space (figure 2A and online supplemental video 2). Needle-beam alignment and tip visualization was more challenging than usual because it was necessary to compensate for the lateral-to-medial tilt of the probe and beam. Needle-beam alignment can be maintained by advancing the needle in a similar lateral-to-medial trajectory. The probe and beam position should be held constant to keep the target in view, and the needle trajectory adjusted to keep the tip within the beam and view at all times.

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Figure 2

Real-time ultrasound views for needle insertion (A) and identification of epidural catheter during the injection of the air-saline mixture (B). PC and AC indicate posterior complex and anterior complex, respectively. Arrow and open arrow indicate epidural needle and the needle tip, respectively. Arrow head indicates the epidural catheter. L, laminae.

The advancement of the needle under real-time US guidance should be stopped just in front of the posterior complex; thereafter, the needle was further advanced using the loss-of-resistance (LOR) techniques to normal saline without US guidance. When the needle engages the ligamentum flavum, the physician can sense the ligamentum flavum due to its tire rubber texture,18 which can be helpful to identify the needle location without US guidance. After accessing the thoracic epidural space, the epidural catheter was advanced through the needle such that 3–5 cm of the catheter remained in the epidural space. Finally, a 0.5 mL mixture of normal saline and air was administered through the epidural catheter under real-time US monitoring. The hyperechoic epidural catheter could be observed behind the posterior complex under US guidance (figure 2B and online supplemental video 3). Hence, if the epidural catheter was present outside the epidural space, bulging around the laminae was observed.

Fluoroscopic findings

After real-time US-TECP, the position of the catheter was determined using the fluoroscopy. As the epidural catheter was radiolucent, correct epidural access was confirmed by injecting 1 mL of non-ionic, low-osmolar contrast medium (Omnipaque 300, GE Healthcare, Little Chalfont, UK). To identify the correct position of catheter tip and to verify that there was no intravascular and subarachnoid spread of the contrast medium, fluoroscopic continuous pulsed imaging was performed for 3 s during the contrast administration (online supplemental figure 1A). Then, the catheter tip and contrast dispersion in the epidural space (patchy and honeycomb appearance) were observed. After confirming the presence of catheter in the epidural space, 4 mL of contrast medium and 2% lidocaine (1:1 mixture) was administrated to evaluate contrast dispersion (online supplemental figure 1B). Bilateral and craniocaudal dispersion of contrast were also observed.

Protocol for thoracic epidural infusion

To identify the patency of epidural catheter and provide TEA, epidural bolus of 4 mL of 0.125% ropivacaine was administered before the surgical incision. Subsequently, intraoperative epidural analgesia was instituted with an infusion of 4 mL bolus of 0.125% ropivacaine with sufentanil 0.4–0.6 µg/mL using the PIEB mode every 60 min. For postoperative analgesia, a bolus option was programmed to allow for 2 mL of epidural medication with a 20-minute lockout interval. As part of our institution’s standard of care at post-anesthesia care unit (PACU) and ward, if hypotension was developed, the amount of bolus could be reduced or TEA could be temporarily stopped. The amount of bolus could be increased if a patient exhibited insufficient pain improvement.

Primary and secondary outcomes

The primary outcome was the success rate of real-time US-TECP based on the fluoroscopic evidence (epidural spread of contrast medium), which was validated by the third investigator (JHS). Secondary outcomes were as follows: (1) time to mark spaces (the time to obtain the best US images of the target, the interlaminar space, and mark the overlying skin); (2) needling time (time to access the epidural space after skin insertion); (3) number of needle passes (first needle pass plus additional needle passes, that is, readvancement of the needle after any needle withdrawal for changing the direction); (4) number of skin punctures, (first skin puncture plus additional skin punctures, that is, reinsertion at a new location after complete needle withdrawal from the skin); (5) first-pass success rate (achievement of LOR and successful catheter placement on first needle pass and first skin puncture); (6) complications such as epidural hematoma, dura puncture, intravascular or intrathecal local anesthetic injection, and pneumothorax; (7) distance between skin and epidural space; and (8) fluoroscopic findings such as location of catheter tip at vertebral body and cranial and caudal contrast dispersion. All secondary outcomes, except fluoroscopic findings, were assessed by the fourth investigator (H-MK). Additionally, postoperative clinical outcomes such as the occurrence of hypotension and opioid requirements in the PACU at 0 hour (on admission to the PACU) and 1 hour post-surgery were assessed. Postoperative pain intensity was assessed using a single 11-point Numeric Rating Scale (NRS: 0=no pain, 10=worst pain imaginable).

Statistical analysis

Data are expressed as mean±SD, median (IQR), or number (proportion), as appropriate. Data were analyzed using the SPSS, V.21.0 (IBM SPSS Statistics).