Evaluating clinical success of the newer field-block catheters: Limitations of case reports

Recent editions of this journal have included two separate manuscripts presenting case reports on the use of the newer field blocks.

The first manuscript detailed two instances of the use of serratus anterior plane (SAP) catheters for abdominal surgery. ¹ The catheters were described to have been used for analgesia with ‘good effect’ in the absence of documentation of convincing proof of specific efficacy of the catheter. Therefore, there is a possibility in both cases of a contribution of systemic local anaesthetic effects or placebo effects.

The second manuscript detailed two case reports of use of erector spinae plane (ESP) blocks for lower limb surgery. ² Using bilateral single-shot blocks for one patient and a catheter for another, the authors were appropriately cautious in their conclusion with regards to efficacy. Despite promising clinical effects, and the demonstration of postoperative sensory blockade, there were still several aspects that were not reported and would have greatly assisted in better understanding of the observed effects.

When clinicians seek to demonstrate their intended anaesthesia of peripheral nerves has resulted in benefit solely through targeted anaesthesia of those nerves, then several endpoints should be sought. Our awareness of these endpoints arrives from many decades’ worth of clinical observations and publications in the field of regional anaesthesia. ³ The greater the number of endpoints achieved, the more convincing is the proof the intended nerves have been specifically and effectively anaesthetised. These endpoints are:

Proof of analgesia. Pain is assessed prior to the intervention, the intervention is performed and then pain is reassessed with the expectation of reduction in pain, both static and dynamic. When an intervention is performed under general anaesthesia and before the source of pain has occurred, as in the cases reported, then the ability to prove analgesia by this method is lost, and merely observing a lack of pain will not provide proof of the mechanism of analgesia.

Absence of need for supplemental systemic analgesia. If the nociceptive inputs from a site of injury are all being carried by the targeted nerve(s), then there should be no requirement for systemic analgesia, particularly opioids.

Sensory anaesthesia. When the target nerves include cutaneous nerves, then there is an expectation of concomitant sensory anaesthesia in the distribution of these nerves. This has to be differentiated from the expected numbness surrounding an incision due to surgical transection of cutaneous nerves. This endpoint becomes of importance (a) when the block is being relied on for postoperative analgesia, such as our testing of a thoracic epidural catheter prior to induction; (b) when researchers desire to prove the block they are investigating actually worked; ⁴ and (c) to aid clinical decision-making on duration of catheter use.

Dynamic pain relief. The aim of effective neural anaesthesia is the prevention of increased pain with movement.

As a reference point, the most commonly encountered example where all four endpoints are met is seen routinely by clinicians with our use of lumbar epidural analgesia for relief of labour pain.

The twin influences of the opioid crisis and the preponderance of patients on various coagulation-altering medications has driven an appropriate interest in newer field blocks such as the SAP and ESP. Continual efforts to assess better and report clear endpoints of new techniques will greatly facilitate their optimal uptake.