The aim of the study is to compare the lateral decubitus and upright sitting positions in determining the intervertebral space measured by ultrasound in participants whose body mass index was greater than 30 and older than eighteen years old. Additionally, comparison of the sagittal and long-axis paramedian oblique planes in determining the intervertebral space measured by ultrasound in this group of volunteers. Furthermore, assesment of the reliability of measurements performed by practitioners with varying levels of ultrasound experience.

The study will be conducted in the Emergency Medicine Clinic of University of Health Sciences Kocaeli City Hospital. Participants over the age of 18 with a body mass index (BMI) of 30 or higher presenting to the emergency department will be included. After providing verbal information about the study, written informed consent will be obtained from participants who agree to take part. Participants will be evaluated by attending physicians in the emergency department, who will then inform the study investigators.

The upright sitting position is defined as the patient sitting upright on the stretcher with their legs hanging down the side, feet not touching the ground, and arms extended forward for support. The lateral decubitus position is defined as the patient lying on their side with their shoulders and hips perpendicular to the stretcher, knees flexed, and pulled toward the chest.

The ultrasound applications will be performed by two study investigators: one emergency medicine specialist with advanced ultrasound training and one with basic ultrasound training. Measurements will be taken first in the participant's preferred position (upright sitting or lateral decubitus) and then in the alternate position.

Since the traditional method for identifying the lumbar puncture (LP) site-known as Tuffier's line (the midpoint of the line connecting both anterior superior iliac spines in the lumbar region)-can be misleading in obese patients, the application site will be determined via ultrasound. An Esaote MyLab™ Gamma ultrasound device will be used, with the Esaote SL1543 linear probe as the first choice. If the linear probe does not provide adequate depth due to obesity, the Esaote SP2730 curved probe will be used.

For ultrasound application, the probe will initially be placed transversely just above the intragluteal fold, with the indicator pointing to the patient's left shoulder. Once the midline is identified, measurements will be performed in two planes: sagittal and long-axis paramedian oblique.

For the sagittal plane, the probe will be rotated 90 degrees with the indicator pointing toward the patient's head.

For the long-axis paramedian oblique plane, the probe will also be rotated 90 degrees toward the head but moved 1-2 cm laterally from the midline and angled slightly medially.

The probe will then be moved superiorly until the L4-L5 interspinal space is identified. The intervertebral space is defined as the hypoechoic region between hyperechoic spinous processes. Measurements will be taken from the L4-L5 and L3-L4 intervertebral spaces.

Each practitioner's measurements will include the distance to the ligamentum flavum, as well as data on whether the patient has undergone lumbar surgery, and their weight and height. These will be recorded in a pre-prepared standard data form. Practitioners will be blinded to each other's measurements to maintain objectivity.

Inclusion Criteria:

• participants whose body mass index was greater than 30
• participants older than eighteen years old.

Exclusion Criteria:

• participants who have a history of lumbar surgery
• participants who have an infection on the lombar region
• pregnant volunteers

Healthy volunteers who want to participate in the study will be included.