This study retrospectively compared the analgesic efficacy of a bilateral ESPB versus a fentanyl CRI in dogs undergoing hemilaminectomy. While the overall number of dogs receiving at least one rescue fentanyl bolus was similar between groups, the number of rescue fentanyl boluses administered was higher in group CRI, especially during lumbar hemilaminectomy. A rescue fentanyl bolus was more likely to be administered during phase 1 of surgery in group CRI, and phase 2 of surgery in group ESPB. Postoperatively, a significant methadone sparing effect was found during the first 12 hours in group ESPB. These findings support our initial hypothesis and agree with a previous study in which a unilateral ESPB block reduced the perioperative opioid consumption and intraoperative adjuvant analgesic administration in dogs undergoing hemilaminectomy [13]. However, interpretating our results, we need to acknowledge several limitations. The retrospective nature of the study, and the use of a control group obtained from a previous prospective study performed at our institution few years before, might have affected the standardisation of the anaesthetic and analgesic management between groups. Despite all the neurosurgeons used a similar surgical technique, a different tissue handling caused by a different level of experience gained over these years could have affected the degree of surgical trauma, the level of nociception and the postoperative pain. Multiple anaesthetists with different level of experience in ultrasound guided regional anaesthesia were involved, and the volumes and concentrations of local anaesthetic administered were at their discretion. This variability might have affected the effectiveness of the ESPB and our results, despite junior anaesthetists were always supervised by more senior one during the ESPB execution. Rescue fentanyl boluses were administered by the attending anaesthetist according to his/her clinical judgement. Despite the reason for fentanyl administration was reported in the anaesthetic record, we cannot categorically exclude that some nociceptive events were missed or misinterpreted, especially considering the different level of experience of each anaesthetist involved in the study. A different level of preoperative pain between groups could have affected the postoperative results [22], but this is in our opinion unlikely especially considering that dogs in group ESPB had a higher MFS, therefore a more severe spinal cord compression. Several anaesthetists and ward nurses, with different level of experience and aware if an ESPB was performed, assessed postoperative pain. This, together with the not standardised postoperative management, could have added an element of bias affecting our results. However, the CMPS-SF, that was used during the entire study period at our institution, was clinically validated between three different institutions to produce a score linked to the requirement of additional analgesia, rather than to measure the actual level of pain [20]. Despite all these limitations, we believe that the results of this study might have a clinically value, as they represent a real clinical scenario of single referral centre.
Dogs in group ESPB were less likely to receive rescue fentanyl boluses during phase 1 of surgery. This result could be explained considering that in canine cadaveric studies the injection of dye into the thoracic and lumbar ESPB interfascial plane resulted in staining of the dorsal branches of the spinal nerves [3, 5, 7, 8]. These branches innervate the vertebral laminae, articular facets, epaxial muscles and skin near the dorsal midline [23, 24]. During phase 2 of surgery, when the spinal cord was manipulated and the herniated disc removed, or adjacent discs fenestrated, less dogs required rescue fentanyl boluses in group CRI. The structures within the vertebral canal are innervated by the meningeal branch of the spinal nerves in humans and although, the canine thoracolumbar region lacks a meningeal ramus per se, they are innervated [23, 25]. Therefore, to be effective on the meningeal branches, the local anaesthetic injected in the erector spinae fascial plane should spread paravertebrally and extradurally, as demonstrated by some human cadaveric studies, but neither stain has been found in dogs when a parasagittal approach was used [1, 3, 5, 7, 8]. Previously, several dogs required analgesic interventions when the proximity of the dorsal root ganglion was manipulated [13]. Therefore, unsurprisingly, fentanyl CRI was more effective than the ESPB during phase 2 of surgery. Unexpectedly, 64 out of 93 dogs (68.8%) in group ESPB did not require rescue fentanyl administration during disc removal and fenestrations, even if the number of dogs with more than one disc extrusion was higher. Probably, the ESPB mechanism of action is more sophisticated than what has been postulated in dogs so far [3, 5, 8], and differences between cadaveric studies and the clinical performance of the block should be considered.
In comparison to Portela and colleagues (2021), a bilateral rather than unilateral ESPB was performed, as the cutaneous and vertebral canal nerves can cross or communicate across the midline [13, 23, 26]. Furthermore, a different volume and concentration of local anaesthetic was used in the present study. Theoretically, when performing an interfascial plane block, the greater the local anaesthetic volume and concentration, the higher the success rate [2, 27]. However, no direct correlation between the volume, concentration and intraoperative efficacy has been clinically established in humans [27, 28]. In our study, a high volume and low concentration were employed (1 mL/kg of 0.125% local anaesthetic per site). On the contrary, a low volume and high concentration [median (range) of 0.46 (0.2–0.6) mL/kg of 0.5% bupivacaine] was previously used [13]. Our choice was made considering that the anulus fibrosus of a disc might receive innervation from the spinal nerves as far as two segments cranially and caudally [23]. Furthermore, we considered the length of the skin incision and the generally high number of fenestrations performed at our institution, which were greater than those performed in Portela and colleagues’ study (2021) [13]. Despite these technical differences, the overall prevalence of rescue analgesia administration between the two studies (54.8 and 59.5%) was similar. A prospective randomised clinical trial using different local anaesthetic volumes and concentrations, and comparing unilateral with bilateral, is warranted to better understand the intraoperative effect of an ESPB in dogs undergoing hemilaminectomy.
The MAC multiple of inhalational anaesthetic agent administered was lower in group ESPB. The real clinical value of this result is debatable, if we consider that the FE′Iso was maintained constant in group CRI, and the anaesthetist was probably biased by the knowledge of a performed ESPB. Furthermore, dexmedetomidine was used as pre-anaesthetic medication in 95.7% of dogs in group ESPB. When medetomidine (5 μg/kg IV) was administrated to isoflurane anaesthetised dogs, MAC multiples were similar to baseline values by 60 minutes from administration [29]. In addition, Muir and colleagues (1999) observed a lack of analgesic effect with intramuscular medetomidine at doses of 2–5 μg/kg in dogs [30]. Furthermore, the analgesic effects of intravenous dexmedetomidine were noted at doses above 2 μg/kg in dogs [31]. Considering that the median dose of dexmedetomidine used was 1 μg/kg, and the median time from premedication to start of surgery was 85 minutes, we believe that it is unlikely that administration of dexmedetomidine contributed to decrease the MAC multiple of inhalational anaesthetic agents and to lower the number of rescue fentanyl boluses administered to group ESPB.
Overall, the postoperative methadone requirement was reduced in group ESPB, as reported by previous human and veterinary medicine studies [13, 32, 33]. During the first 12 hours methadone requirement was reduced, but no difference was found between 12 and 24 hours. Therefore, it is plausible that the effect recorded was related to the duration of the levobupivacaine [34]. Nevertheless, the postoperative analgesic protocol was not standardised, and multiple contributing factors should be considered. In group ESPB, for example, 91.2% of the dogs had an opioid, with or without a steroid, splashed extradurally before the end of surgery, and this could have affected the postoperative methadone requirement. In fact, while extradural opioid splash provided additional postoperative analgesia in dogs after hemilaminectomy [35], extradural steroids might have decreased the possible spinal cord inflammation associated with a herniated disc [36]. All dogs in group CRI received a NSAID, while it was only administered to 78.5% of dogs in group ESPB. A lidocaine patch was applied to 51% of the dogs in the group CRI, but this should not have impacted our results as this technique did not provide any extra postoperative analgesia following hemilaminectomies in dogs [15].
Different from our results, a unilateral ESP block produced postoperative benefits up to 48 hours in dogs after hemilaminectomy [13]. This might be related to the greater local anaesthetic concentration administered; in humans, greater reduction in postoperative rescue analgesia was found when higher concentration of local anaesthetic was used for the ESPB [28]. However, while a non-validated pain scoring system was previously used [13], the pain scale employed in our study has been validated to assess acute postoperative pain in dogs [20]. In addition, dexmedetomidine, used as an adjuvant to the local anaesthetic in 21% of their dogs [13], might have prolonged the ESPB effect [37], and the intraoperative infusion of lidocaine and/or ketamine to 23.8% might also have affected Portela’s and colleagues’ results.
General anaesthesia time, but not surgical time, was longer in group ESPB. Unfortunately, the time to execute the ESPB was not measured, but it might have prolonged the overall anaesthetic time. A higher prevalence of hypothermia was found in group ESPB. While this might be the result of a prolonged anaesthetic time, no significant association was previously found between anaesthetic time and hypothermia in dogs undergoing hemilaminectomies [38]. It could also be possible that the bilateral ESPB caused a pronounced peripheral vasodilation, promoting heat loss and hypothermia [39].
A dog developed sinus arrest 10 minutes after a T13 ESPB and it was not responsive to antimuscarinic therapy and intralipid solution was administered as local anaesthetic systemic toxicity (LAST) was suspected [21]. However, we cannot categorically exclude that the response to the intralipid solution was coincidental. In humans, LAST was associated with an ESPB in 1.6% of cases but was described only as a central nervous system toxicity [27, 40]. It might develop following a local anaesthetic spread to the paravertebral and intercostal space, and a fast systemic absorption due to the high vascularisation of these areas [27]. However, while no correlation between LAST and the local anaesthetic volume was noted, LAST occurred mainly by administering low local anaesthetic concentrations [27], as used in this study. No evidence of LAST was found in a previous study using higher concentrations of local anaesthetic [13].
