Clinical procedure
ESPB, renal and hepatic function
BLOOD, STERILE
Preoperative visits were performed by skilled anaesthetists via the usual ambulatory path or the evening before for the urgent surgeries. Blood tests were performed following internal procedures: cell count, coagulation, renal and hepatic function were always included.Before GA induction, an intravenous catheter was placed and electrocardiography, oscillometric blood pressure cuff and pulse oximetry were monitored. GA was induced with 2% propofol 2 mg kgDue to logistic reasons, ESPB was performed after GA induction, with the patient in prone position, and exploiting the intraoperative fluoroscopy to identify the exact levels to include in the spinal fusion. At this point, in all cases, the same five trained anaesthetists performed the ESPB and the same three surgeons conducted the intervention. Ultrasound guidance (SonoSite M-Turbo, SonoSite Inc., Bothell, WA; Philips CX50, Netherlands) with a 15 − 6 MHz linear probe or a 5 − 2 MHz convex probe depending on the depth of the transverse process, and a 22 G x 80 mm needle (Echoplex +, Vygon, Ecouen-France) were used for the ESPB. The transducer was positioned in a longitudinal orientation to obtain a parasagittal view. ESPBs were performed with sterile technique and an out-of-plane approach, targeting the median transverse process of the vertebral levels instrumented (Fig. 
out-of-plane execution of ESPBN: needle tip in out-of-plane view; T: transverse process
anaesthetic spread visualisationLA: local anaesthetic spread below the erector spinae; T: transverse processPostoperative analgesia was the same for all patients: a starter bolus with morphine (4 mg) and acetaminophen (1 g) 30 min before the surgical end of the intervention, then they were connected to a patient-controlled analgesia (PCA) pump (CADD-Solis, ICU Medical Inc., San Clemente, CA, USA) filled with 50 mg of morphine and allowing 1 mg boluses at a maximum of 15 min intervals with no background infusion. GA was concluded by interrupting propofol infusion after the suture was concluded, allowing the patients to recover just after medications and supination. After extubation, the patients were kept under observation until they reached an Aldrete score ≥ 9 and then returned to the neurosurgical ward. and then returned to the neurosurgical ward. The functioning of the PCA and the NRS scale use were explained to the patients the day before intervention and repeated after awakening from GA. Paracetamol (1 g) was set every 8 h and rescue IV ketoprofen (100 mg) was allowed.