Testing of a Non-Invasive Light Sensor to Estimate Venous Blood Oxygen Levels in Patients in the Intensive Care Unit (ICU)

Arm 1: Internal Jugular Vein (JVP) Monitoring WHY: Continuous monitoring of venous oxygen saturation (SvO2) and oxygen extraction ratio (O2ER) using a flexible, non-invasive optical sensor placed over the internal jugular vein (IJV). MATERIALS: • Custom-developed reflectance pulse oximetry sensor with red and infrared LEDs, photodetectors, and integrated temperature protection • Medical-grade adhesive (Tegaderm) • Commercial finger pulse oximeter for SaO2 reference • Blood assays PROCEDURES: • Optional ultrasound imaging to locate the IJV and optimise placement • Sensor placed on the side of the neck, preferably contralateral to the central line • Finger pulse oximeter placed on the same side as the arterial line • Sensor remains in place for up to 24 hours • Blood gas samples (arterial and central venous) are taken per routine care and aligned with sensor data • Sensor is cleaned with alcohol and reused per protocol WHO PROVIDED: Research staff with ICU experience will apply the sensor and monitor performance. HOW: • Face-to-face, bedside monitoring. • Individual use. • Sensor applied and left in place for up to 24 hours. • At least five blood draws made during 24-hour period (T=0, 2, 6, 12, 24 hours) LOCATION: Christchurch Hospital ICU, New Zealand TAILORING: • Sensor placement adjusted for neck anatomy or existing lines. • Signal quality confirmed visually at placement. Arm 2: Femoral Vein Monitoring Identical protocol to Arm 1, but the optical sensor is placed over the femoral vein rather than the jugular vein. Sensor placed over the femoral vein if central access exists and jugular placement is not feasible ADAPTATION: Only used if the internal jugular site is not available or contraindicated. Placement adjusted based Allocation to arms: Participants will initially be allocated to Arm 1 (Internal Jugular Vein monitoring). If an appropriate signal cannot be obtained or maintained from the IJV site, or no jugular catheter is present participants may be switched to Arm 2 (Femoral Vein monitoring) as an alternative site. Number and duration of sensor assessments: Each participant will have a single continuous sensor assessment lasting up to 24 hours. Re-application of the sensor for additional or extended monitoring sessions is not planned within this study. Adherence monitoring strategies: Sensor placement and signal quality will be checked every 2 hours by research staff to ensure proper positioning and data quality. Any issues with sensor adherence or signal loss will be documented.