Ultrasound Mapping of the Anatomical Relationship Between the Right Internal Jugular Vein and Carotid Artery

This single-center, prospective, cross-sectional, descriptive observational study will characterize the angular relationship between the right internal jugular vein (IJV) and the right common carotid artery (CCA) using standardized transverse ultrasound images.

The primary objective is to quantify the angular position of the CCA center relative to the IJV center on a continuous 0°-360° scale and describe its circular distribution. Secondary objectives are to determine the distribution across eight 45° anatomical sectors, calculate circular descriptive statistics, assess whether the angular distribution deviates from uniformity, and explore associations between angular distribution and basic patient characteristics, including age, sex, and body mass index.

The study will consecutively enroll 200 consenting adults aged 18 years or older undergoing elective cardiac surgery at Ankara Bilkent City Hospital who are scheduled for right IJV catheterization under general anesthesia as part of routine clinical care. There will be no control group.

The sample size calculation was based on a preliminary anatomical analysis previously conducted by the research team in 68 patients, with one ultrasound image evaluated per patient. In this preliminary dataset, the right CCA center was located in the medial sector relative to the right IJV center in 24 patients and in the posteromedial sector in 41 patients. The combined proportion of these two positions was therefore 65/68 (95.6%).

The sample size was calculated to estimate the combined proportion of medial and posteromedial positions with approximately ±3 percentage points of absolute precision at a 95% confidence level. The formula for estimating a single proportion, n = Z² × p × (1-p) / d², was used. With Z = 1.96, a rounded anticipated proportion of p = 0.956, and d = 0.03, the required sample size was approximately 180 patients. Allowing for approximately 10% missing or non-evaluable images, the target enrollment was set at 200 patients. This calculation addresses the precision of estimating the combined proportion of medial and posteromedial positions; it does not establish separate precision or statistical power targets for each of the eight anatomical sectors, the continuous angular distribution, or exploratory association analyses.

Ultrasound imaging will be performed after induction of general anesthesia and endotracheal intubation, during mechanical ventilation with a positive end-expiratory pressure of 5 cmH2O. Participants will be positioned in 15° Trendelenburg with the head rotated approximately 30° to the left. A high-frequency linear ultrasound transducer will be used to visualize the right IJV and CCA in the transverse plane at the level of the cricoid cartilage. The transducer marker will face the patient's left, toward the cricoid cartilage and trachea. Minimal transducer pressure will be applied to limit venous compression.

One ultrasound image meeting the predefined standardization and image-quality criteria will be digitally recorded for each participant. Routine central venous catheterization will then proceed according to clinical practice. Research measurements will not influence the indication, site, or technique of catheterization.

De-identified images will be analyzed using custom computer-based image annotation software. The center of each vessel will be manually identified at the intersection of its anteroposterior and transverse diameters. The software will record the pixel coordinates of both centers and automatically calculate the angle of the CCA center relative to the IJV center using a two-argument arctangent function, with correction for the downward-increasing image y-axis. No manual angle measurement will be performed.

The IJV center will serve as the origin of the angular coordinate system and the center of an imaginary clock face. The position of the CCA center relative to the IJV center will be expressed as follows: 0°/360° corresponds to lateral (3 o'clock), 90° to anterior (12 o'clock), 180° to medial (9 o'clock), and 270° to posterior (6 o'clock). Angles will increase counterclockwise from the lateral direction. Each calculated angle will also be assigned to one of eight 45° anatomical sectors: lateral, anterolateral, anterior, anteromedial, medial, posteromedial, posterior, or posterolateral.

Recorded variables will include age, sex, height, weight, body mass index, planned cardiac surgical procedure, calculated angular position, and the corresponding anatomical sector. Vessel diameters, inter-center distance, and CCA-IJV overlap percentage will not be included as study variables or analyzed.

Angular data will be evaluated using circular statistical methods. Analyses will include the circular mean direction, mean resultant vector length (R), circular standard deviation, and 95% confidence interval for the mean direction. The Rayleigh test will assess deviation from circular uniformity. The number and percentage of participants in each anatomical sector will be reported. Angular distributions will be illustrated using rose diagrams and two-dimensional vessel-center distribution plots. Exploratory analyses will examine associations of age and body mass index with angular position using appropriate circular-linear methods. Directional differences between sex groups will be evaluated using appropriate circular group comparison methods.

The study involves no additional invasive procedures, medication administration, blood sampling, or ionizing radiation. Routine anesthesia and central venous catheterization management will remain under the responsibility of the clinical team.