Transverse Abdominis Plane (TAP) Block After Cesarean Delivery

Cesarean delivery is the most commonly performed surgical procedures in the United States
today, with over 1.2 million cases performed in 2005.1 One of the most important aspects of
cesarean delivery is the provision of safe, effective postoperative analgesia for the
mother, while simultaneously ensuring minimal side effects for both the mother and neonate.
Studies have suggested that a multimodal approach to post-cesarean pain utilizing both
intravenous, oral, and neuraxial opioids and non-steroidal anti-inflammatory drugs is highly
effective in providing effective analgesia.2-4 A significant component of post-cesarean pain
is incisional pain from the Pfannenstiel incision on the anterior abdominal wall. The
sensory supply to the skin, muscles, and parietal peritoneum of the anterior abdominal wall
is derived from the anterior rami of T7-L1. After exiting the spinal column, these nerves
proceed through the lateral abdominal wall within the transversus abdominal fascial plane,
terminating in the anterior abdominal wall.6,7 Recent studies have suggested that blocking
these afferent sensory nerves with local anesthetics, as part of a multimodal postoperative
pain regimen, provides superior pain relief in terms of decreased pain scores and morphine
consumption for up to 48 hours postoperatively.8-10 The technique utilized for these studies
employed a surface anatomical approach to the transversus abdominal fascial plane via the
lumbar triangle of Petit, a technique validated in both cadaveric and radiologic studies.11
However, as ultrasonography has emerged as the "gold standard" for initiating many nerve
blocks, reports have described the successful use of ultrasound imaging for initiation of
transversus abdominis plane (TAP) blocks for both abdominal surgeries and cesarean
deliveries.

In the published studies investigating the use of the TAP block for post-operative
analgesia, either ropivacaine or bupivacaine was utilized in concentrations of 0.75% and
0.375%, respectively. Studies comparing ropivacaine with bupivacaine for use in
interscalene, femoral, or sciatic nerve blocks have found no difference in terms of potency,
time to onset or duration of postoperative analgesia between the two local anesthetics.
Although no similar studies have been done with TAP blocks, one can assume that utilization
of ropivacaine for this nerve block would yield similar results in terms postoperative
analgesia. Moreover, the cardiotoxicity of ropivacaine has been shown to be significantly
less than bupivacaine, making it a safer alternative for use in nerve blocks when used in
high doses.Risk factors for respiratory depression after the administration of neuraxial
opioids in the non-obstetric population include morbid obesity and obstructive sleep apnea.
For the obstetric population, a study of 856 patients revealed that all 8 patients who
experienced respiratory depression after intrathecal morphine for cesarean delivery were
markedly obese. Furthermore, the onset of respiratory depression after intrathecal morphine
can occur up to 12 hours after administration, a time period when the patient is not being
as closely monitored as she is during the 1:1 nursing care in the recovery room. Therefore,
it is the investigators policy on the Labor and Delivery unit to not administer intrathecal
morphine to any parturient with a history of obstructive sleep apnea or a BMI > 40 kg/m2. As
such, these patients often require intravenous opioid patient-controlled analgesia
postoperatively, which has been shown to provide inferior pain relief and greater opioid
consumption than neuraxial opioids. Moreover, the current clinical standard is to administer
the TAP block to those patients who have not received morphine in their neuraxial
anesthetic. Hence, the TAP block offers a novel addition to the management of post-cesarean
pain for this patient population.

Since there have been no published dose-response studies investigating the effective
analgesic dose of ropivacaine for use in a TAP block for post-Cesarean delivery analgesia,
the investigators propose a study primarily examining the effect on 24 hour post-Cesarean
delivery opioid consumption of using either a placebo, 0.25% ropivacaine, 0.5% ropivacaine,
or 0.75% ropivacaine for TAP blocks.