Ultrasound-Guided Dorsolateral Approach for Quadratus Lumborum Block in Rabbits (Oryctolagus cuniculus): A Prospective, Randomized, Blinded, Cadaveric Study Comparing Four Different Injectate Volumes

SIMPLE SUMMARY: The popularity of rabbits as pets has grown substantially in the UK. However, despite progress, this species still has a much higher anesthesia-related mortality than dogs and cats. Rabbits are prey animals that tend to hide signs of pain, which makes it particularly difficult to assess and to treat it adequately. Perioperative systemic opioid administration is a cofactor in the high perioperative mortality and morbidity rates recorded in rabbits. To reduce post-surgical opioid requirements, a multimodal analgesia regimen is advisable. In such a context, loco-regional anesthesia has gained a central role in daily anesthesia practice as it effectively and significantly reduces the doses of perioperative systemic analgesic drug requirements. The quadratus lumborum block (QLB) is an ultrasound-guided loco-regional anesthesia technique developed in human medicine consisting of the injection of local anesthetic within the quadratus lumborum (QL) fascia that provides analgesia to the abdominal wall and viscera by blocking the ventral branches of the thoraco-lumbar spinal nerves and sympathetic trunk. The efficacy of the QLB in human medicine has driven veterinary research to investigate this block in companion and farm animals. To date, no study has investigated the QLB in rabbits. With the present cadaveric study, these authors aimed to carry out the following: firstly, to investigate the anatomical layout of the thoraco-lumbar area in rabbits; secondly, to design a feasible ultrasound-guided QLB for rabbits; and thirdly, after comparing different volumes of injectate and analyzing the spread obtained, to propose the most suitable volume of injectate to administer. This research was conducted by performing anatomical dissections and computed tomographic studies in rabbit cadavers. ABSTRACT: Ultrasound-guided (US-guided) loco-regional anesthesia can provide significant analgesia and anesthetic-sparing effects when used in rabbits. The aims of this study were to investigate the thoraco-lumbar anatomy of the rabbits, particularly the quadratus lumborum (QL) muscle, to design an appropriate US-guided quadratus lumborum block (QLB) specific for rabbits, and to define the most adequate volume of injectate required to consistently cover the ventral branches of T11 to L3 without affecting the pelvic limb innervation (L4, L5 and L6). Sixteen adult rabbit cadavers were included in the study. After randomization, four different volumes of injectate (0.1 mL/kg, 0.2 mL/kg, 0.3 mL/kg and 0.4 mL/kg) were tested, with these volumes additionally randomized to two sites of injection (right or left QL fascia). An ultrasound-guided QLB was performed with a solution of lidocaine, iodinated contrast and tissue dye (in a proportion of 3:1:1 volume, respectively), with subsequent computed tomography (CT) and anatomical dissection, to evaluate the spread of the injectate. In all but one case, the US-guided QLB performed with a dorsolateral approach using 0.3 mL/kg was adequate, while a dose of 0.4 mL/kg consistently reached the targeted nerves but also extended to L4 and caudally. This may suggest that an injectate volume of 0.3 mL/kg may be the most appropriate to produce adequate spread while not affecting pelvic limb innervation.