A 74-year-old patient (weight 80 kg, height 162 cm) burdened with hypertension, osteoarthritis, with varicose veins in the lower limbs, after total left hip replacement, underwent elective total right knee replacement surgery (TKR). TKR was performed under spinal anesthesia. For spinal anesthesia, a 0.5% solution of isobaric ropivacaine at a dose of 12.5 mg was used. During the procedure, music sedation was used (headphones connected to the music player) and oxygen supply via a nasal cannula were used. During the procedure, basic parameters such as non-invasive blood pressure (BP; mmHg), heart rate (HR; bpm), percutaneous saturation (Sp02; %) and body temperature were monitored. During the intraoperative period, no hypotension, episodes of bradycardia or other disturbing symptoms were observed. After the procedure, the patient was transferred to a recovery room where basic hemodynamic parameters (such as: BP, HR and Sp02) were monitored. Within the recovery room, a continuous femoral nerve block was performed under ultrasound guidance. After initial verification of the blocking side and prescanning as well as setting optimal imaging parameters, the blocking area was prepared with an antiseptic. A linear ultrasound probe with a frequency of 10–14 MHz (Sonosite M-Turbo, USA), after being covered with a sterile cover, was placed in the inguinal crease. After visualizing and identifying the basic anatomical structures (femoral vessels, femoral nerve, iliac muscle, fascia iliaca and fascia lata), the optimal needle insertion site was selected and local area infiltration was performed using 5 mL 2% lidocaine (Lignocainum Hydrochloricum WZF 2%, Polfa Warsaw, Poland). Catheter-over-the-needle set for continuous block (Contiplex C, BBraun, Germany), connected to the nerve stimulator (Stimuplex HNS 12, BBraun, Germany) with the following stimulation parameters: current 0.5 mA, pulse duration – 0.1 ms (Sequential Electrical Nerve Stimulation) mode on, under ultrasound control, was introduced by the out-of-plane method from the caudal to cranial direction. After obtaining the optimal positioning of the tip the needle under the fascia iliaca, lateral to the femoral nerve, the needle was removed from the system, leaving the catheter in place of the original position of the needle. Under ultrasound guidance, after prior aspiration, an induction dose of local anesthetic (LA; 15 mL 0.375% ropivacaine solution; Molteni, Italy + 2.5 µg/mL adrenaline; Polfa Warsaw, Poland) was injected, resulting in adequate circular distribution of LA. Then, an elastomeric pump (Easypump II, BBraun, Germany) was connected to the set filled with 270 mL of 0.2% ropivacaine solution with 2.5 µg/mL adrenaline. The rate of infusion was 5 ml/h. Patient was discharged to the orthopedic department after a 2-hour stay in the recovery room. Within 3 hours following the blocking in the orthopedic department, the patient had an episode of generalized seizures. The Early Warning System Team was called. In the ward, the upper respiratory tract was cleared, oxygen supply through the nasal cannula was turned on and 5 mg of diazepam (Relanium; Polfa Warsaw, Poland) was administered intravenously, resulting in the cessation of the seizures. The infusion of ropivacaine solution was disconnected. Due to the suspected neurotoxic effects of LA, the immediate supply of 20% fat emulsion (Intralipid®; Fresenius Kabi, Sweden) – 100 mL bolus was started. During the assessment of vital signs, bradyarrhythmias and single extrasystoles with broad QRS syndromes were observed, Sp02 97%, arterial BP 160/95 mmHg, the glucose level − 7.9 mmol/L, state of consciousness assessed with GCS − 6 points. During the initial assessment of the patient, a re-episode of generalized seizures occurred followed by sudden cardiac arrest in the ventricular fibrillation mechanism. Resuscitation was undertaken according to ERC (European Resuscitation Council Guidelines 2015) by performing a single defibrillation with a 200J current and then the return of spontaneous circulation (ROSC) was assessed. During resuscitation, a bolus of 20% fat emulsion was repeated and an Intralipid infusion (200 mL for 20 min) was started. Due to persistent impaired consciousness, the patient required intubation and assisted breathing with an Ambu bag with 100% oxygen substitution. Then, the patient was transferred to the intensive care unit (ICU). As the urgent procedure, computed tomography (CT) of the head was also performed without showing obvious pathologies. In the ICU, the infusion of 20% fat emulsion was continued, the total dose of Intralipid administered during the stay was 700 ml (10 mL/kg). The patient in the ICU was mechanically ventilated for 120 minutes. After extubation, no impaired consciousness or seizures were observed. In the first hours of the ICU stay, bradyarrhythmias and atrial fibrillation episodes were observed, and throughout the patient's stay, the administration of vasoactive and inotropic drugs was not required. Analgesia was carried out using a systemic supply of non-opioid and opioid drug. On the second day patient, without impaired consciousness and neurological deficits, fully cardiovascularly and respiratorily stable, was transferred to the orthopedics department to continue treatment. Neurologic function was intact at 2-week follow-up.