Ultrasound anatomy of the transversus abdominis plane region in pregnant women before and after cesarean delivery
No systematic description of the TAP ultrasound anatomy in pregnant women exists in the literature. We studied the ultrasound anatomy of relevant structures in pregnant women before and after elective cesarean section. In this study, we found that the relevant muscular layers were visible in all ultrasound examinations before as well as after CD. The TAP was found to more superficial before CD and also the external and internal oblique muscles were located closer to the surface.
This finding may be explained by the specific changes in the abdominal wall during pregnancy. At term, due to the significantly increased volume of the uterus, the circumference of the abdominal wall is considerably increased. Accordingly, the layers of the abdominal wall are under tension and both subcutaneous fat as well as the muscular layers are thinned out. As a consequence, the TAP is shifted closer to the skin. Also, the firmness of the gravid uterus gives a good support for the ultrasound probe. With the surgical opening of the uterine cavity and delivery during cesarean section, the abdominal volume is decreased and the tension on the abdominal wall is released. Thus the slackened muscles and subcutaneous layer increase again in thickness.
A TAP block is a has been used in a variety of abdominal surgical procedures, including cesarean delivery [12, 16, 17]. If morphine is not used as intrathecal adjunct, like e.g. in Germany [18], a TAP block may be of potential benefit in this setting.
A TAP block for any surgical procedures may be initiated before or after the surgical procedure. Given the relatively slow onset of long acting local anesthetics, performing TAP block before surgery may potentially beneficial. However, in all randomized controlled studies on TAP block for postoperative pain control after cesarean delivery, the block was initiated after the surgical procedure [12, 19]. As shown in our study, the relevant muscular layers and the TAP compartment were visualized after delivery in all participants. Regarding distance of the TAP compartment to the skin administering the TAP block before delivery might be a meaningful strategy, as the target plane can be reached by the needle in a shorter distance than after CD, however, as local anesthetic systemic toxicity is a major concern with TAP blocks, we do not recommend such strategy. TAP associated seizure has been reported within 10 min of injection, thus it might have occurred before delivery, inflicting additional risk as compared to after delivery [20].
If the anesthetist decides to perform the TAP block after delivery it may be useful to perform an ultrasound examination of the abdominal wall preoperatively to minimize the time needed to establish the block after surgery, e.g. by preparing the ultrasound machines settings for focus depth and frequency. When employing this strategy one needs to keep in mind that, as seen in our study, the TAP shifts position after cesarean delivery. It is then found approximately 0.7 cm deeper in reference to the skin. Patient positioning also affects TAP depth, possibly because with left lateral displacement, the gravid uterus provides a firmer base for the ultrasound probe. Of note, we use 110 mm cornerstone needles for TAP blocks. With an angle of 45°, these would have been long enough for all studied patients.
The depth of the TAP depends on the body weight of the parturient. In patients with an increased BMI it can be expected to be observed deeper than average. The patient’s body weight should be taken into account when planning a postoperative TAP block. The depth and the frequency of the transducer may need to be adjusted and more time may need to be allocated for the procedure [21, 22]. Additionally, the choice of needle length may need to be optimized in this way too.
For transverse abdominal incisions as in CD, TAP block needs to be performed on both sides to obtain a bilateral effect. Bilateral TAP injections of ropivacaine have been shown to achieve relevant plasma concentrations of local anesthetic 30 min after injection [23]. Elevated plasma ropivacaine levels and symptoms of mild neurotoxicity have been observed after TAP block for cesarean sections [24]. Individual dosing strategies should be applied to minimize the risk of potentially toxic plasma concentrations [25]. Given the risk of relevant transfer of local anesthetic into the circulation, the sonographical anatomy of the TAP region should be judiciously visualized before puncture, while advancing the needle and during injection, in order to minimize the total applied dose of local anesthetic by poorly targeted injection. Also, patients must be monitored closely for at least 45 min after injection, as peak plasma concentration is reached about 30 min after injection [24]. The main limitation of this observational sonoanatomical study is that no actual TAP blocks were performed.