This trial was reviewed and approved by the Ethical Committee of the Department of Veterinary Science – University of Turin, Italy (Prot. No. 1515/2020). Written informed consent was signed by the owners at dog admission. Dogs scheduled for TPLO between July 2020 and April 2021 at the Veterinary Teaching Hospital of Turin (Italy) and at Centro Veterinario Fossanese (Fossano, Italy) were recruited. All dogs underwent preoperative physical examination and blood analysis. Exclusion criteria were: American Society of Anesthesiologists classification (ASA) > 2, age < 6 months, infectious skin disease affecting the lumbosacral area, history of bleeding disorders, uncorrected hypovolemia, neurodegenerative central or peripheral diseases, and anatomical spinal abnormality. Two treatment groups SA group and PB group were formed according to simple randomization based on a computer-generated randomization sequence (www.randomizer.org). The estimated sample size to detect a difference in the primary endpoint (power 80% and alpha error 5%) assuming an intraoperative rescue anaesthesia (iRA) of 20% in the SA and 50% in the PB group, respectively, was 36 dogs per group (https://select-statistics.co.uk/calculators/sample-size-calculator-two-proportions) [12]. Accounting for possible dropouts, we decided to enroll 40 dogs per group and to plan an interim statistical analysis after enrollment of at least 60 dogs. At this stage, if a clear superiority of one of the two groups would have been found the study would have been interrupted.
The manuscript conforms to the Consolidated Standards of Reporting Trials (CONSORT) Statement 2010 for reporting randomized clinical trials [13] (Fig. 1). SA and PB were performed by two experienced operators (PF, DS), other two anaesthetists (EL, MS) blinded to treatment monitored the dogs during surgery. All surgical procedures were performed by two surgeons (LP, DM) who were unaware of the assigned group.
Anaesthesia protocol and procedures
Dogs of group PB received an intramuscular (IM) injection of dexmedetomidine (1 μg/kg, Dexdomitor; Orion) and methadone hydrochloride (0.2 mg/kg, Semfortan; Dechra) as pre-anaesthetic medication. In group SA, only dexmedetomidine (1 μg/kg, Dexdomitor; Orion) was used. The cephalic vein was catheterized and a volume of 5 mL/kg/h of lactated Ringer’s solution (lactated Ringer’s; Fresenius Kabi) was administered intravenously (IV). Intravenous propofol (Proposure 1%; Merial) was titrated to effect to induce general anaesthesia. After orotracheal intubation, isoflurane (Isoflo; Esteve) was delivered in a mixture of medical air and oxygen with a fraction of inspired oxygen of 0.4, using a rebreathing system. The dorsal pedal artery was cannulated with a catheter of suitable dimension (Surflo; Terumo), pressure transducer was zeroed at the level of dog’s sternal manubrium. Intraoperatively, a multiparametrical monitor (Infinity Delta; Draeger or Datex AS3, Draeger) was used to monitor cardiovascular (systolic, diastolic, mean arterial pressures [SAP, DAP, MAP], heart rate [HR] and rhythm) and respiratory (end tidal carbon dioxide [PE’CO2], peak inspiratory pressure [PIP], respiratory rate [fR], tidal volume [VT], minute volume [VE], inspired fraction of oxygen [FiO2], end tidal isoflurane fraction [FE’ISO]) parameters, as well as esophageal temperature (T,°C). Perioperatively, an active heating system (Bair Hugger Warmer Model 505; Augustine Biomedical Design) was used to maintain a body temperature above 36 °C. Volume-controlled intermittent positive pressure ventilation (Cato; Draeger, or Primus; Draeger) was applied to maintain values of PE’CO2 between 35 and 40 mmHg. All data were manually recorded every 5 min (min) for the entire duration of anaesthesia. The time to perform RATs was defined as the minutes between the end of surgical preparation and injection of the LA solution (for PB) or the end of 10-min fixation of LA (for SA). The number of attempts was also recorded. Each new skin puncture was recorded as another attempt.
SA group
Dogs were positioned in lateral recumbency with the limb to be operated lowermost. Both hind limbs were extended cranially and kept parallel by an assistant. The ilium wings, together with the dorsal spinous process of L6, were used as anatomical landmarks. After surgical preparation of the skin, the subarachnoid space was reached inserting a Quincke needle (22, 23 or 25 gauge; Becton, Dickinson and Company) on the dependent side using a paramedian approach and with the bevel facing cranially. The needle was directed from the lateral to the cranial aspect of the spinous process of L6 vertebra, until it was in contact with the lamina. The needle was then advanced cranioventrally toward the midline until the intervertebral space was identified and then forwarded through the subarachnoid space. Proper needle placement was ascertained by detection of cerebrospinal fluid in the needle hub. A hyperbaric solution of prilocaine hydrochloride (Prilotekal 2%; B Braun) and morphine hydrochloride (Morphine hydrochloride 1%; Molteni) was injected as a single bolus at a rate of about 1 mL per min.
The prilocaine dose was calculated based on body weight (BW) and spinal cord length (SCL) as described by Sarotti et al. (2013) and assuming that bupivacaine is four times more potent than prilocaine [14]:
$$\mathrm{Prilocaine}\ \left(\mathrm{mg}\right)=4\times \left[0.3\times \mathrm{BW}\ \left(\mathrm{kg}\right)+0.05\times \mathrm{SCL}\ \left(\mathrm{cm}\right)\right]$$
The morphine dose calculated in relation to BW was 0.03 mg/kg.
SA was performed by two experienced anaesthetists (DS, PF). The number of attempts and the time to perform SA was recorded. After injection the dog was kept in the same lateral recumbency for 10 additionally minutes. This fixation time was considered as part of the time taken to perform SA. After three attempts without cerebral spinal fluid outflow, the procedure was aborted and recorded as a procedural failure. Each new skin puncture was recorded as another attempt.
Peripheral Block group (PB)
Psoas-ischiatic nerve blocks were performed combining ultrasound- and nerve stimulator-guided (Stimuplex HNS 12; B Braun) techniques. Ultrasound guidance was performed using a high-frequency 12 MHz linear array transducer (HS 50; Samsung or Mylab 30 gold; Esaote).
The dogs were positioned in lateral recumbency with the limb to be operated uppermost. The anatomical site was then aseptically prepared and the positive electrode for stimulation was applied to the skin of the ventral abdomen.
Ropivacaine (Naropina® 0.75%; Fresenius Kabi) was diluted with an equivalent volume of 0.9% saline solution (Sodium chloride 0.9%; Fresenius Kabi) to obtain a 0.375% ropivacaine solution. A syringe containing 0.45 mL/kg of 0.375% (equivalent to 1.69 mg/kg) ropivacaine was aseptically prepared and connected to an insulated stimulating needle of appropriate length (Stimuplex D; B Braun).
Psoas compartment block (femoral and obturator nerve block)
The iliopsoas muscle was sonographically identified using an in-plane technique, as suggested by Tayari et al. (2017) [15]. When the psoas muscle was visualized, an insulated stimulating needle was inserted cranially to the iliac wing and directed caudomedially a pulse width of 0.1 ms and a stimulation frequency of 2 Hz. The needle was advanced until a contraction of the quadriceps muscle and extension of the stifle were evident [16] and maintained with a current ≥ 0.4 but ≤ 0.6 mA. When contraction was not achieved, the needle was redirected toward other hypoechoic structures. If the electro locator failed to identify where the femoral nerve was, the LA was injected into the area where the nerve was expected. After negative aspiration of blood to rule out intravascular needle placement, a volume of 0.3 mL/kg of 0.375% ropivacaine was injected in the psoas muscle. Any time the femoral nerve was not found by ultrasound nor was the muscle contraction elicited with the nerve locator was recorded as a procedural failure event.
Ischiatic nerve block
An US-guided nerve block of the ischiatic nerve was performed using a medium lateral approach by placing the probe on the lateral aspect of the proximal third of the thigh. When the ischiatic nerve was identified in the fascia between the adductor muscle and the biceps femoris muscle, an insulated stimulating needle was inserted in-plane in caudo-cranial direction through the semimembranosus muscles and advanced until contraction of the tibialis cranialis or gastrocnemius muscle was evident. When contraction was maintained with a current ≥ 0.4 but ≤ 0.6 mA and negative aspiration of blood was detected, 0.15 mL/kg of 0.375% ropivacaine was injected.
Intra-operative evaluation and treatment
The duration of both surgery and anaesthesia (min) were recorded as it was the incidence of bradycardia (HR < 60 beats/min with a MAP < 60 mm Hg for at least 5 min) and hypotension (MAP < 55 mm Hg). Hypotension was treated by reducing by 20% the FE’ISO and giving a 3 mL/kg bolus of lactated Ringer’s solution IV in 60 s (sec). If MAP increased after the first bolus, an additional 2 mL/kg of fluid in 60 s was administered. If hypotension persisted, a bolus of ephedrine (50–100 μg/kg) and/or a continuous rate infusion of norepinephrine (0.1–0.3 μg /kg/min) was given [17].
Intraoperative rescue analgesia (iRA) (fentanyl 1 μg/kg IV) was administered if the MAP rose by more than 30% of the pre-incisional level, which was defined as the mean pressure measured during the 5 min prior to skin incision. The fentanyl bolus (1 μg/kg IV) was repeated every 3 min until the MAP returned to the pre-incisional level. The iRA incidence and the number of fentanyl boluses between the two groups were compared [8].
Arousal events were defined as intraoperative movement, brisk palpebral reflex, or spontaneous breathing against mechanical ventilation. In such cases propofol 1 mg/kg was administered IV. At the end of surgery, IV meloxicam (0.2 mg/kg, Metacam; Boehringer) was administered to all dogs and the urinary bladder was voided manually [8, 17].
Postoperative evaluation and treatment
Two experienced operators (LP, DM) unaware of the treatment evaluated each dog during the first 24 h after RAT. Quality of recovery from general anaesthesia was scored using a recovery scoring system [18] (Additional file 1: Appendix 1A).
Postoperative pain was assessed using the Short Form of the Glasgow Composite Pain Scale (SF-GCPS) every 2 h starting from 3 h after RAT [19]. Methadone (0.1 mg/kg) was administered IM if the pain score was ≥ 5/20. The dogs were re-evaluated 30 min later and a further methadone dose (0.1 mg/kg IM) was administered if needed. The number of rescue methadone administrations during the first 24 postoperative h was recorded. A previously published weight-bearing scoring system was used before (T0) and 24 h after surgery (T24) [20] (Additional file 2: Appendix 2A).
The assessment of the complete motor block recovery was performed at 3, 5, 8, 12, and 24 h after RAT by an operator unaware of the RAT technique (LP, DM). Dogs had to walk on their own, but they were assisted to stand up, if necessary. Dogs that were unable to walk before surgery or that had postoperative bandaging of the leg were not assessed.
Urinary retention was defined as the inability to spontaneously void in the presence of bladder overdistension. If a dog did not spontaneously urinate within 12 h after local anaesthetic injection, both abdominal palpation and US scan were performed to evaluate bladder overdistension [8, 17].
Statistical analysis
Categorical variables are reported as frequency and percentage; Fisher’s exact test was used to evaluate frequency distribution independence between the two groups. The Lilliefors test was performed on continuous variables to check for normal distribution. Not normally distributed data are reported as median and range and were analysed using the Mann–Whitney U test. Comparison of median weight-bearing scores among different time points (T0 vs T24) within the same group were performed using Friedman's test. Statistical analysis was performed using MedCalc Software for Windows version 12.5 (MedCalcSoftware, Ltd., Belgium). Significance was set at 5% for all statistical methods.
