Consider the Paramedian Approach for Thoracic Epidural Placement, Especially at the Midthoracic Level



Consider the Paramedian Approach for Thoracic Epidural Placement, Especially at the Midthoracic Level


Amit Sharma MD



You are scheduled to do the anesthesia for a lung resection for a 68-year-old man with severe chronic obstructive pulmonary disease. You usually use intrathecal preservative-free morphine for this type of case. Today, however, you hear from the surgical resident, surgical fellow, and surgical attending, all within a 15-minute interval and all requesting a thoracic epidural. It isn’t your favorite type of catheter to place, but you decide to give it a try. Unfortunately, all you and your junior resident seem to hit is “bone everywhere.” You and your patient are both getting a bit uncomfortable and discouraged. You call your favorite senior pain attending, who is doing cases in the outpatient center and, fortunately, he can come within 5 minutes. He arrives and does one of the “Mr. Miyagi” moves he is famous for—and the catheter is in, taped, and test dosed within 2 minutes. You wonder to yourself, “How the heck does he do that?”

Neuraxial (spinal and epidural) techniques are a vital component of anesthesia and pain-management practice. Since August Bier first described spinal anesthesia in 1898, neuraxial techniques have been in constant use and have been the subject of a vast amount of clinical research and inquiry directed at maximizing both safety and efficacy. Today, epidural analgesia is widely used for postoperative pain control after thoracoadominal and lower-extremity procedures. Insertion of a thoracic epidural catheter is often considered more challenging than placement of its lumbar counterpart. Fortunately, a thorough understanding of the anatomy of the thoracic spine and adherence to a few simple principles will alleviate trepidation and facilitate placement.

The spinal cord is divided into eight cervical (C), twelve thoracic (T), five lumbar (L), five sacral (S), and a few coccygeal (Co) segments. Each spinal segment (except C1) involves a dorsal (sensory) and a ventral (motor) nerve root on either side that join to form a spinal nerve. Each spinal nerve subsequently exits through an intervertebral foramen of the vertebral column. The bony vertebral column surrounds and protects the spinal cord and the spinal nerves encased by arachnoid membrane filled with cerebrospinal fluid. The epidural space is a potential area just outside the dura mater,
bound anteriorly by the posterior longitudinal ligament; posteriorly by the vertebral laminae and the ligamentum flavum; and laterally by the vertebral pedicles and intervertebral foramina. At the thoracic level, the epidural space is around 3 to 5 mm in width and contains fat, veins, arteries, lymphatics, and connective tissue. Local anesthetic solution administered in the epidural space bathes the spinal nerves, leading to sensory and motor blockade. Similarly, opioid medications are either absorbed by the venous plexus and act systemically or they cross the dura mater and bind to the opioid receptors on the spinal cord.

Thoracic dermatomes are organized in circumferential bands from axilla to iliac crest. The T4 dermatome (sensory distribution of T4 spinal nerve) is often located at the level of nipples and T10 at the level of umbilicus (these are universally used reference points). There is a degree of dermatomal overlap, which can cause difficulties with insufficient block for all areas of the surgical site. This problem has been overcome by introduction of newer multibore epidural catheters that enhance the local anesthetic spread and therefore are expected to cover a wider dermatomal distribution.

Technically, both the upper (T1-T2) and the lower thoracic (T10-T12) vertebral interspaces are functionally equivalent to lumbar area since spinous processes at these levels are almost angled horizontally. In contrast, placing epidural catheters at the T3 to T9 level is challenging because of the acute downward angulation of these respective spinous processes (Fig. 103.1). Since successive spinous processes are angled less acutely, the interspinous space is narrower at the distal end (superficial), making the needle entrance harder.

Most anesthesia providers who do not have experience in treating pain start with the midline approach, even in the midthoracic region and that is perhaps why they often have such difficulty in placing the catheter. A senior pain anesthesiologist or an experienced regional anesthesia provider would probably suggest using the paramedian approach to facilitate placing the thoracic epidural catheter at these middle thoracic interspaces. With this approach, the needle is entered slightly inferolateral to the spinous process and angled cephalomedial, thus avoiding the crowded interspinous space. With this technique, the needle tip travels through the skin, subcutaneous tissue, and ligamentum flavum before entering the epidural space, unlike the classic interspinous technique in which supraspinous and interspinous ligaments are also encountered in needle trajectory.

The epidural catheter should be placed on the unilateral side, approximately at the mid-dermatomal level to the corresponding surgical incision. For instance, if the intended thoracotomy incision is planned at the right T5-T7 dermatomes, the epidural catheter should be inserted at the right T5-T6 interspace. The unilateral catheter placement is perhaps necessary
since Blomberg described midline epidural septa or dorsomedian connective tissue bands in 1986 and are believed to be one of the causes of unilateral anesthesia after an adequate catheter placement.

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Jul 1, 2016 | Posted by in ANESTHESIA | Comments Off on Consider the Paramedian Approach for Thoracic Epidural Placement, Especially at the Midthoracic Level
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