A Factorial Trial of Ultrasound and Manometry to Improve the Success of Thoracic Epidural Placement

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.

Inclusion Criteria:

- Epidural indicated for a T4-T10 placement site

- American Society of Anesthesiologists (ASA) physical status I to IV

Exclusion Criteria:

- Non-English speaking subjects in situations when an interpreter or consent in their
native language is not available.

- Pregnant women

- Decsionally impaired

- Prisoners

- Children