In a variety of thoracic and abdominal surgeries, thoracic epidural placement is associated with better pain relief, less opioid consumption, a decrease in adverse perioperative cardiac events, and improved intestinal perfusion and motility. Unfortunately, epidural catheter placement is challenging and is not always successful. The epidural space in the thoracic region is especially difficult to access due to the steep and inferior angulation of most of the spinous processes.
The vast majority of thoracic epidurals are placed using a landmark-based technique (i.e. without ultrasound guidance or fluoroscopy). In addition, a "loss of resistance" technique is typically used to confirm the needle has entered the epidural space. With a paramedian technique, the thoracic spinous processes are palpated (if possible), the needle is advanced through the skin just lateral to the spinous processes until lamina is contacted. The needle is then redirected medially based on an estimate of the location of the midline at the interlaminar space. The needle is then "walked" cephalad or caudad to blindly search for the interlaminar space. The depth to the lamina and interlaminar space are not known which introduces additional uncertainty in the mind of the operator, who must proceed with caution so as to not advance the needle too far and puncture the dural sac. It is not surprising that the landmark approach is associated with a significant number of unsuccessful attempts, long procedure times, and ineffective catheters, with the percentage of failed thoracic epidurals reported as high as 32%. In one study, the failure rate confirmed by both epidural waveform analysis and a sensory test after local anesthetic injection, the primary failure rate was found to be 23%-24%. A higher number of procedural attempts can also lead to an increased risk for complications including paresthesia, epidural hematoma, and dural puncture headaches. The high failure rate of this procedure highlights the need for more studies on ways to improve the techniques for thoracic epidural access.
Ultrasound (US) has proven to be useful in the imaging of the spine, and has also helped improve our ability to access the lumbar epidural space. Ultrasound measurement of the epidural space depth before epidural catheter placement decreased the rate of lumbar epidural catheter replacements, and reduced the number of epidural attempts when performed by first year residents compared to attempts without ultrasound guidance. In another study the use of ultrasound to identify the depth and location of the interlaminar and epidural space improved placement of labor epidurals with a lower number of block failures, a higher number of complete blockade, a higher rate of subject satisfaction, and lower VAS scores. Ultrasound has also been used to image the thoracic spine and epidural space.
The superior medial border of the thoracic transverse process aligns with the interlaminar space. This anatomical fact, combined with the ability to use ultrasound to identify the thoracic transverse processes and other bony landmarks forms the basis of this study. In the transverse plane, ultrasound can be used to identify the thoracic spinous processes and the midline. In the paramedian plane, the lamina, articular process and transverse process can be identified. In the lumbar spine, once the articular process is identified, a paramedian angulation of the ultrasound beam is used to identify an "acoustic window" between the lamina and into the spinal canal, thus identifying the interlaminar space. In the thoracic spine, because of the tight space between the lamina and overlapping of the lamina, an acoustic window is infrequently visualized and cannot be used as a reliable method to identify the interlaminar space. In contrast, both the superior border of the transverse process and the step-off between lamina can be readily identified with ultrasound. Both of these landmarks align with the interlaminar space.
In clinical practice, there is significant variability in techniques used to identify the correct epidural space. Although the loss of resistance technique is the most commonly used, variability exists in methods to confirm presence in the epidural space. Once such technique routinely used at this institution is the manometry technique. This involves using a three-way stopcock connected to clear plastic tubing IV extension tubing filled with normal saline. When the loss of resistance is detected, it can be confirmed by connecting the prefilled tubing to the Tuohy needle and opening the stopcock so the fluid inside is freely flowing. If the Tuohy needle tip is located in the epidural space, the saline column will initially fall, then exhibit pulsatility associated with heartbeat and respiration.
The identification of the epidural space by respiratory and heartbeat fluctuations in the air-fluid level has been previous described. The heart rate variability is believed to be due to arteriolar pulsations transmitted within a closed epidural space, while the respiratory variation in air fluid level is believed to be due to venous engorgement of the epidural veins with changes in thoracic pressure during inspiration. In the past 30 years, a handful of studies have been published evaluating epidural pressure measurement. Many of the studies using gravity to confirm presence in epidural space also use a pressure transducer to produce a visual waveform showing increase in pressure upon entry into the ligamentum flavum, and decrease in pressure upon entry into the epidural space. Two of these studies also incorporated an audible alarm to indicate change in pressure. One study correlated epidurals placed with pressure transducers to confirm the epidural space with presence or absence of block post-op and contrast spread on CT cathetergram showing a complete correlation between pressure waveform and catheter positioned in epidural space, further establishing the validity of pressure measurement to confirm entrance in the epidural space. Epidural pressure waveform analysis has not become routine practice, likely due to equipment, cost, and time constraints. We hypothesize that the simple, rapid, and inexpensive technique of extension tubing manometry will offer similar benefits for confirmation of epidural placement and will be applicable to placement at the thoracic levels.
