Porcine erector spinae plane block model for simulation practice

To the Editor,

Since its original description in 2016,¹ the ultrasound-guided erector spinae plane (ESP) block, a regional anaesthesia technique, has become increasingly popular and the potential clinical applications have been increasing.² A popular indication is analgesia for rib fracture management. Although randomised controlled trial (RCT) -level evidence for the ESP block is lacking in rib fractures, a recent retrospective cohort study³ has shown increased incentive spirometry values and reduced pain scores in the first 24 hours after implementation of the ESP block compared with before block administration. Alternative techniques, such as thoracic epidurals and paravertebral blockade, have greater potential for complications and steeper learning curves. The serratus anterior plane block’s efficacy has been questioned in patients with posterior rib fractures, with potential technical challenges if surgical emphysema or chest drains are present.³ This makes the ESP block an attractive proposition in management of these challenging patients.

At our institution, a survey of senior anaesthetic trainees highlighted the need for training in ESP blockade as a tool in their armamentarium for rib fracture analgesia when a thoracic epidural was not feasible or practical. The knowledge and skills required in placement of an ESP block include the identification of relevant sono-anatomy, the practical skill of advancing the needle while using ultrasound to visualise it in real-time and the recognition of the correct endpoint for a satisfactory technique. Training should ideally be first undertaken in a simulated environment to reduce patients’ exposure to risk of harm,⁴ making simulation an ideal technique for practice. We therefore undertook a ‘Sonoclub’ session incorporating the use of scanning and needling models. We were unable to find any previous literature on an ESP block needling model.

We elected to use the porcine model as the porcine lower thoracic and lumbar spine was found to be a suitable alternative to human spines.⁵ Models were also readily available and inexpensive. The pig was delivered to the butcher hemisected in the sagittal plane, and we were able to obtain a section comprising of the lower six thoracic hemivertebrae in an intact block. This ensured that, as far as possible, the ribs and spinal bony anatomy remained intact. We asked for the skin to be removed and removed approximately a further 4 cm of muscle in order to approximate the tissue thickness that one may expect when performing an ESP block (see figure 1A). Preparation and deodorisation the model was undertaken as per ASRA recommendations,⁴ where the model was maintained in 70% alcohol within a plastic bag, in a refrigerated sealed container before use.

Figure 1.

(A) Porcine needling models prepared. (B) Sonographic image generated using a high-frequency linear probe.

Ultrasound scanning of the models using a linear probe provided images such as that of figure 1B. We were able to clearly delineate the transition between ribs and spinal bony structures, and able to simulate the lifting of the muscle tissue off bone as normal saline was injected. The model was also able to simulate the placement of ESP catheters.

The advantages of the porcine model include similarity with human tissue, similar visibility of the needle, and less distortion with repeated needle passage. The disadvantages are limited shelf life, the requirement for storage and infection risk.⁴ One limitation of our model was the fact that we were unable to obtain a model with the vertebral bodies intact, which meant that tissue and bony disruption could not be ruled out. We believe, however, as the main aim of this model is to simulate the transition in sono-appearance between the rounded ribs and the more ‘block-like’ transverse process in humans, this is of lesser significance. It also does not affect its ability to simulate the tactile feel of needle passage, or injection.

A further limitation is the fact that this is an animal model, and so may present a barrier for those with concerns for animal welfare. However, this model was obtained from a butcher, who would otherwise have sold the materials used for consumption. It is difficult to artificially recreate the complex paraspinal anatomy, the dynamic interactions between fascial planes and the interactions between muscle and bone, as the learner practices needle placement and local anaesthetic injection. These are required for satisfactory simulation practice of the ESP block. In a non-commercial setting, artificial techniques, such as using three-dimensional printed models within a gelatin-base, may not be able to produce a model of sufficient fidelity to mimic these interactions, thus denying the learner the opportunity to appreciate the end-points required for satisfactory block placement. They may also be time-consuming to create and require resources that may not be available. Therefore, until a commercial model becomes available for this block, we believe our model represents an easily reproducible and relatively cheap option for training requirements.

By presenting this model, we hope to encourage development of other models and part-trainers that would facilitate training of this emerging regional technique.