Reports of trauma to intra-abdominal structures resulting from abdominal wall blocks are rare. However, cases have included trauma to the liver [5–8] and bowel wall hematomas [9] (Figs. 13.2 and 13.3). Colonic perforation has been described following ilioinguinal nerve block in children [10, 11]. Performing nerve blocks of the abdominal wall without ultrasound has been shown to lead frequently to intraperitoneal needle tip placement [12, 13]. Ultrasound does not completely eliminate this risk as needle imaging and identification may be poor. Furthermore, there may be some difficulty in identifying the layers. A particular difficulty may be air or surgical gas in the abdominal wall. There may be greater risk in the morbidly obese (see later) or in pediatric patients, where the transversus plane may only be a few millimeters in thickness. The risk, however, does seem to be very low in experienced hands. If identification of the layers is uncertain or needle tip location undetermined, the needle should not be advanced further.

Retroperitoneal injection in the inguinal area has been associated with retroperitoneal hematoma from deep circumflex iliac artery damage [5, 6, 14, 15]. The iliaca fascia is located immediately deep to the transversus abdominis muscle in the area near the ASIS .

Inadvertent transient femoral nerve block has been described after nerve blocks of the abdominal wall including TAP block as well as ilioinguinal nerve block [16–19]. This potential should be considered in any block around the ASIS . However, it has not been reported after ultrasound-guided TAP or ilioinguinal nerve block. Femoral nerve block has also been described after posterior TAP injection in the area of the triangle of Petit without ultrasound guidance [10]. The authors speculated spread from deep to transversus abdominis to the iliac fossa, posterior to the fascia iliaca where the femoral nerve is located. Another anatomical route from injection in this location would be direct injection into the psoas muscle finding its way to the psoas compartment and femoral nerve. While uncommon, the potential for inadvertent femoral nerve block remains an important consideration when performing nerve blocks of the abdominal wall, especially in ambulatory or day surgery. Patients (and medical staff) are unlikely to anticipate leg weakness and this may increase the risk of postoperative falls.

Catheter entrapment from abdominal wall placement has also been described [20, 21]. The mechanism from this case was not clear as the catheter was eventually removed without a knot. Ultrasound imaging of the entrapped catheter was not undertaken.

Abdominal wall blocks are field blocks requiring the administration of large doses of local anesthetic. Efficacy clearly relies on an adequate volume of injectate to allow spread within the anatomical plane to bathe the required nerves; however, there are limited published data on the relative importance of dose, volume, and concentration of local anesthetic in the injectate. Cadaveric studies have been conducted to correlate the spread of injectate with the dermatomal anesthesia achieved [22–24]. There is a paucity of dose finding studies in the literature; however, Beloeil et al. demonstrated that large doses of local anesthetic were required to achieve ED₅₀ [25].

Emerging evidence suggests that absorption of local anesthetic from the transversus abdominis plane is rapid, with peak levels typically achieved in 30 min (Fig. 13.4) and remaining elevated for up to 90 min [26–28]. If the block is administered following wound closure, it is important to recognize that peak plasma concentrations of local anesthetic are likely to occur in the Recovery Room and potentially after the departure of medical staff.

Plasma concentrations of local anesthetic have been studied following TAP block in several settings including following gynecological laparotomy and caesarean section [26, 27]. Peak total concentrations are comparable with local anesthetic concentrations following regional anesthesia elsewhere, including ilioinguinal nerve, caudal and scalp blocks for awake craniotomy [29–32]. However, these levels are also in the vicinity of those on the threshold of neurological toxicity as described by Knudsen and colleagues [33]. Several case reports of seizures have been described, mainly involving large doses of local anesthetic although one case of seizures postcaesarean section involved only 2.7 mg/kg of ropivacaine [28, 34–36].

It is not known to what extent the accuracy of the deposition of local anesthetic within the transversus abdominis plane influences the resulting local anesthetic plasma concentrations. A variable amount of leakage of local anesthetic into surrounding musculature is occasionally observed during TAP blocks, particularly if the block is difficult, such as in the obese patient. It is possible that this leakage contributes to (or in fact, may reduce) the extent of systemic absorption. There is some evidence that the use of ultrasound guidance may decrease the plasma concentrations of local anesthetic following ilioinguinal nerve block [37].

There is some evidence to suggest that the addition of adrenaline (epinephrine) may reduce the systemic absorption of levobupivacaine [38]. This has not been studied in other local anesthetics, but would seem to offer potential, and also could serve as a marker of intravascular injection.

Catheters placed in the TAP can provide ongoing analgesia for 24–72 h [39, 40]. Hessian monitored bound and unbound plasma levels of ropivacaine for 72 h after an initial bolus and infusion into the TAP . Although one patient with low plasma ropivacaine levels reported symptoms consistent with local anesthetic toxicity, measured levels of ropivacaine remained below accepted toxic thresholds. Total ropivacaine levels continued to increase to 72 h although unbound ropivacaine peaked at 24 h.

The key to safe abdominal wall blockade is anatomical knowledge, ultrasound scanning skill, and adherence to local anesthetic dose limits. Adequate training and equipment are critical to the safe performance of all regional anesthesia [48, 49]. Practitioners need to be aware of the neuroanatomy of the abdominal wall relevant to the site requiring anesthesia [22, 50]. Abdominal wall blocks often require a long needle to traverse a relatively long distance. As with other regional techniques [51], care needs to be taken to visualize the needle throughout its passage in order to avoid intraperitoneal injection and potential trauma to intra-abdominal structures. The transversus abdominis muscle is often very thin, so the risk of entering the peritoneal cavity (and therefore potentially causing trauma to intra-abdominal structures) must always be considered. Abdominal wall blocks are “field” blocks requiring a large dose and volume of local anesthetic solution; therefore, practitioners should always consider the potential for local anesthetic toxicity. Systemic absorption of drug is similar to other comparable techniques, however unexpectedly high plasma levels and toxicity have been reported. Practitioners should always be cognizant of the likely time course of peak plasma concentrations of local anesthetic. Finally, ultrasound has been shown to result in more accurate placement of local anesthetic [12, 37, 41]. It has been proposed that the practice of “blind” placement of transversus abdominis plane and ilioinguinal nerve blocks should be discontinued [13].