The 50:50 Nitrous Oxide vs 2% Lidocaine Challenge

• Nitrous Oxide-Oxygen (Drug), also known as Experimental Treatment: After placing the patient in the appropriate supination + Trendelenburg position, the operative field (neck and subclavian region) is prepared using antiseptic measures.Five minutes before needle insertion, a mask connected to a 50:50 Nitrous Oxide/Oxygen cylinder with a flow rate of 6 liters per minute is placed on the patient's face and held by the patient. The mask has a valve that opens only when the patient creates negative pressure during inhalation.The patient is instructed to breathe spontaneously; the mask is removed once the catheter is inserted into the target vein or after cessation of painful stimulation.
• Lidocaine 2% Injectable Solution (Drug), also known as Control: After placing the patient in the appropriate supination + Trendelenburg position, the operative field (neck and subclavian region) is prepared using antiseptic measures. Then, under ultrasound control, the most favorable central vein for puncture is determined (v. jugularis interna or v. subclavia) and a 25-gauge needle is used to infiltrate the skin and subcutaneous tissue above the selected vein with local anesthetic. Five minutes after infiltration, puncture of the selected vein is performed using the Seldinger technique. Continuous communication with the patient is maintained throughout the procedure.

Primary outcomes

• Pain Intensity During Needle Insertion [At the time of needle insertion]: Assessed using a standard Numeric Rating Scale (NRS) ranging from 0 (no pain) to 10 (worst imaginable pain). Higher scores indicate greater pain intensity.
• Pain Intensity During Catheter Placement [During catheter placement]: Assessed using a standard Numeric Rating Scale (NRS) ranging from 0 (no pain) to 10 (worst imaginable pain). Higher scores indicate greater pain intensity.
• Pain Intensity Post-Intervention [2 minutes after gas inhalation (Treatment 1) or 2 minutes post-intervention (Treatment 2)]: Assessed using a standard Numeric Rating Scale (NRS) ranging from 0 (no pain) to 10 (worst imaginable pain). Higher scores indicate greater pain intensity.

Secondary outcomes

• Patient Anxiety Score [Post-intervention (at the conclusion of the central venous catheterization procedure)]: Assessed using a standard Numeric Rating Scale (NRS) ranging from 0 (no anxiety) to 10 (worst imaginable anxiety). Higher scores indicate greater anxiety levels.
• Patient Satisfaction Score [Post-intervention (at the conclusion of the central venous catheterization procedure)]: Assessed using a Numeric Rating Scale (NRS) where patients rate their overall satisfaction with the anesthetic method. The scale ranges from 0 (completely unsatisfied) to 4 (completely satisfied). Higher scores indicate greater satisfaction.
• Incidence of Treatment-Related Adverse Events [From the start of the intervention through 30 minutes post-procedure]: Percentage of participants experiencing specific adverse events, including nausea/vomiting, unpleasant dreams, paresthesias, dizziness, and euphoria, as recorded by subjective patient reports.

Sex: All, ages: 18 Years to 65 Years, accepts healthy volunteers

Inclusion Criteria:

* Subjects who have not previously received Nitrous Oxide.
* Subjects of both sexes, aged between 18 and 65 inclusive.
* Subjects who have signed informed consent with full disclosure; no study-related procedure may begin until the subject signs the consent form.

Exclusion Criteria:

* Infection at the planned puncture site.
* Pregnancy.
* Intracranial hypertension.
* Vitamin B12 deficiency.
* Hemodynamic instability.
* BMI over 35.
* Glasgow Coma Scale (GCS) score less than 15.
* Presence of chronic pain.
* Neurological disorders.
* Hematologic disorders.
* Systemic or intrathecal methotrexate therapy
* More than two attempts at venous puncture (Subject Withdrawal Criterion).
* Subject experienced an adverse event requiring early discontinuation (Subject Withdrawal Criterion).
* Withdrawal of consent by the subject (Subject Withdrawal Criterion)

• Ana Cvetković, MD, PhD, Principal investigator, Institute for Oncology and Radiology of Serbia