Discussion

The cases presented describe the benefits of regional anesthesia in providing high-quality, opiate-sparing analgesia for prolonged aeromedical retrieval.

The use of regional anesthesia prehospitally has been described for the management of femoral fractures;^{Reference Hards, Brewer, Bessant and Lahiri3} and more recently, the use of serratus anterior blocks as analgesia for rib fractures in aeromedical transfer has also been published.^{Reference McLean, Cooke, Burns and Reid4} This is the first report in the literature of single-injection ESB being used in the retrieval environment, despite their established use in-hospital.^{Reference Hamilton and Manickam5}

The ESB is performed with the patient in either the upright sitting or lateral decubitus position, with inline ultrasound guidance. The erector spinae muscle (ESM) complex runs vertically in the back, lateral to the spinal column. It is comprised of the spinale, longissimus thoracis, and iliocostalis muscles. The ESM lies directly on top of the transverse process (TVP) of the vertebral body. The TVP is identified in a longitudinal parasagittal plane approximately 3cm lateral to the midline on the affected side using ultrasound (Figure 1). The TVP appears as a flattened, squared acoustic shadow with minimal pleura visible (Figure 2). In contrast, the image of a rib appears rounded with associated pleura underlying (Figure 3). The needle tip is directed onto the TVP and local anesthetic is injected below the ESM on top of the TVP. The anesthetic spreads in a cranio-caudal plane by one dermatome for approximately each 3.4ml of injectate,^{Reference De Cassai and Tonetti6} so the target injection site should be in the middle of the area to be blocked. The mechanism of action of the ESB is by anterior diffusion of anesthetic into the paravertebral and epidural spaces, and lateral diffusion into the intercostal spaces.^{Reference De Cassai and Tonetti6-Reference Ueshima and Hiroshi8} This blocks the dorsal and ventral rami of the spinal nerves, which provide sensation to both the anterior and posterior chest wall.

Figure 1. Positioning of the Patient in the Upright Sitting Position. Note: 1 = midline/spinous process; 2 = approximate position of the transverse processes; 3 = position of the scapulae.

Figure 2. View of the Transverse Process Deep to the Erector Spinae Muscle in the Longitudinal Parasagittal Orientation with a Linear 13-6MHz Probe. Note: The transverse process is a square acoustic shadow with no pleura visible. Dotted line: desired needle placement for block.

Figure 3. View of a Rib in the Longitudinal Parasagittal Orientation with a Linear 13-6MHz Probe. Note: Deep to the erector spinae muscle, the rib appears as a rounded acoustic shadow with underlying pleura.

Complications of the ESB are rare. The block is superficial and performed in an area without significant vascular structures. In-hospital use has been described in coagulopathic patients without complications.^{Reference Ayub, Talawar, Kumar, Bhoi and Singh9,Reference Adhikary, Prasad, Soleimani and Chin10} The underlying bony anatomy makes identification of the target area easy and protects underlying structures from damage should needle placement be off target. Reported complications include pneumothorax,^{Reference Ueshima11} incomplete blockade,^{Reference Ueshima and Otake12} and motor block.^{Reference Selvi and Tulgar13} Hematoma is also a theoretical complication, but has not been described in the literature. No complications were reported with either case published.

As with any use of ultrasound, performance of an ESB is an operator-dependent procedure. Different levels of training and clinical background my lead to different challenges and success rates in performing ESBs. Challenges while performing an ESB include: identification of anatomic structures in patients under spinal precautions, or with large body habitus, and maintaining continuous needle visualization.

Other alternative regional anesthetic procedures for rib fractures in-hospital include thoracic epidurals and paravertebral blocks. However, these blocks are more complex to perform in a retrieval environment, require a greater level of operator skill, are closer to vulnerable anatomic structures, and are contra-indicated in coagulopathic patients.

The use of serratus anterior block for chest trauma^{Reference May, Hillermann and Patil14} has also been used in retrieval medicine;^{Reference McLean, Cooke, Burns and Reid4} however, it lacks coverage of the chest posterior to the mid axillary line^{Reference Durant, Dixon, Luftig, Mantuani and Herring15} and so may not provide analgesia to posterior rib fractures.

One case report from a military retrieval service reports a multi-injection ESB in a prolonged transfer of a patient with chest trauma.^{Reference Fabich, Greene, Tighe, Devasahayam and Becker16} In contrast, in this case series, a larger injection volume was used to reduce the technique to a single injection.

At the time of publication, no studies have been performed to assess the relative efficacy and safety of myofascial plane blocks (such as the ESB), so it is not possible to ascertain whether the ESB is superior to other forms of regional anesthesia.