Injectate spread in erector spinae plane block

Yang et al. described an interesting cadaveric study showing the differences and similarities in anatomical spread of injectate for two types of paraspinal block: retrolaminar and erector spinae plane (ESP) 1. Costache et al. describe these blocks as ‘paravertebral by proxy’ as neither require direct entry into the paravertebral space, yet still achieve blockade of the thoracic spinal nerves 2. There is some contention regarding the anatomical basis of the paravertebral spread in these blocks 1, 3-5. The reasons for these differences may stem from the fact that there is currently no standardised method to model the spread of local anaesthetic (LA) in cadaveric studies. There is abundant variation in mixtures of injectate that are used as a surrogate for LA in order to visualise spread upon dissection 1, 3, 4, 6. In addition, the mechanism for how LA travels through tissues is unknown even in living humans. Is spread reliant on conduction by mass effect along tissue planes, or is it by diffusion? The injectates used in these studies are likely to have different densities and biochemical properties in comparison with LA. This leads to difficulties in interpreting injectate spread, as differences in these properties are likely to affect both diffusion and spread within tissues. Other variables that could influence spread are force of injection – which will have an effect on the pressure within tissue compartments, volume of injectate and direction of the needle. Also the various methods of cadaver fixation can influence tissue integrity and therefore the extent of spread. Newer techniques of embalming to create ‘soft fix’ cadavers are available for making cadavers more pliable 7, but there is still no standard for this in injectate studies. In their study, Yang et al. used unembalmed cadavers with a dye latex mix. Dye clearly makes the injectate easy to identify and the addition of latex will limit the diffusion. In our experience 8, 9 this injectate mix is a sensible choice when examining the extent of spread within a fascial plane, though no evidence or consensus exists to support this assertion. Furthermore, caution needs to be executed in drawing conclusions regarding the presence of dye in a region and how this is clinically-relevant. Injectate found within a region does not necessarily correlate that enough LA would be delivered to this area to provide anaesthetic effect in a living human. This information can only be determined using clinical studies in conjunction with cadaveric evidence. Although Yang et al. present evidence of spread to the paravertebral space, the majority of injectate did not spread there. The most efficient method to ensure paravertebral spread remains injection into the paravertebral space. The attraction of the novel fascial plane blocks is the potential for a similar efficacy to a paravertebral block (PVB) but with less technical difficulty and a better risk profile. There is increasing evidence to support the ESP block but it is becoming doubtful that it will prove to be equivalent to the PVB 4. No block is entirely without risk and pneumothorax with ESP has already been described 10. The decision to perform any intervention is based on a risk-benefit analysis. An intervention with even a very low risk may not be appropriate if there is limited benefit. Costache et al. have highlighted the alarmingly high rates of pleural injury in simulations of landmark PVB 2. This is much higher than the published incidence of pneumothorax with landmark techniques 11. In addition, an intrapleural block 12 is a recognised technique for thoracic analgesia so it is unlikely that intrapleural injection is responsible for the failure rate (10%) with the landmark technique. Ultrasound-guided PVB has an excellent safety profile, so perhaps the main benefit of ESP is that it is a technically easier block rather than a safer one. The fact that we have already seen pneumothorax with ESP shows that even an ‘easier’ and ‘safer’ block can be risky if the tip of the needle is not seen throughout. In our institution, ultrasound-guided PVBs have been routinely provided for breast surgery and rib fractures since 2014. Throughout this timeframe, all regional anaesthesia advanced trainees and a number of other trainees have managed to achieve competency in the technique. Although we acknowledge that this is a more technically difficult block, we believe that this would be in the skill set of a competent regional anaesthetist. Anaesthetists routinely learn and perform a number of technically difficult tasks such as thoracic epidurals, fibreoptic intubation and various US-guided lines. We feel that as with developing any skill, the key is sound fundamental technique. All in-plane nerve blocks require the same skill set but the more challenging blocks highlight poor or inadequate techniques. There is currently a limited evidence base on teaching the fundamental skills of regional anaesthesia to novices. Perhaps the real key to increasing the availability of safe and effective regional anaesthetic techniques is high-quality training and high fidelity simulation.