The goal of this observational study is to investigate the occurrence of hypoxemia (an abnormally low concentration of oxygen in the blood) in trauma patients within the first 24 hours of hospital admission following arrival to a trauma center.

The main questions the study aims to answer are:

• Do trauma patients experience hypoxemia during the initial 24 hours of hospital admission following trauma?
• What is the daily distribution of potential hypoxemic episodes?

The investigators expect that hypoxemic episodes will be more frequent during the night (20.00-07.59) than during the day (08.00-19.59)

An additional pulse oximeter will be attached to the participants, which measures oxygen saturation in the blood during the first 24 hours of hospital admission after trauma.

Trauma is one of the leading causes of death and disability on a global scale. Advanced Trauma Life Support guidelines state serious concern with the appearance of hypoxemia after trauma, and consequently recommend that supplemental oxygen should be provided for all severely injured trauma patients to avoid hypoxemia in the initial phase. Oxygen is a vital part of human physiology and must be delivered to all metabolically active cells in the body. When patients receive supplemental oxygen, a series of autoregulatory mechanisms happen to ensure optimal oxygen levels and prevent hypoxemia and hyperoxemia which can have different harmful effects. The lack of oxygen resulting in hypoxemia can potentially be a reversible cause of poor outcomes and in worst case death, hence supplemental oxygen is recommended for trauma patients, although the evidence is sparse.

In other high-risk patient groups, such as surgical and chronic obstructive pulmonary disease (COPD) patients, a study monitored arterial oxygen saturation (SpO2) continuously and found that cumulative duration of desaturations with SpO2 < 85% was significantly associated with myocardial injury after both surgery and exacerbation of COPD. In stroke patients, a study found a higher mortality in patients that experienced hypoxemia, but after adjusting for National Institute of Health Stroke Scale and age, this association was not significant. In the study, hypoxemia was found at all stages during admission. Using continuous monitoring for 48 hours a study found that hypoxemia was common and prolonged in patients recovering from noncardiac surgery. Furthermore, this study showed that 90% of hypoxemic episodes SpO2 < 90% for at least one hour went undetected by standard spot checks, which were typically conducted at intervals of 4-6 hours.

Trauma patients can also be characterized as high-risk patients due to a high mortality. The incidence of prehospital hypoxemia in traumatic brain injury patients has been studied, a study found a prevalence of 37.9%,11 and another study discovered that prehospital hypoxemia was associated with higher mortality. Evidence about the incidence of hypoxemia in trauma patients after admission to a hospital is not well established. There is reason to suspect that trauma patients can experience episodes of hypoxemia after admission and that the incidence of hypoxemia may differ between day and night as shown in several other patient populations. Potential incidences of severe hypoxemia, from patients being admitted to a trauma centre to subsequent care in a ward, should be avoided, since severe hypoxemia is associated with harmful effects. Hypoxemia can be detected and potentially corrected with more advanced measuring equipment. It is relevant to conduct a study with the purpose of determining the occurrence and distribution over the first 24 hours after hospital admission of hypoxemia by continuous SpO2 monitoring in trauma patients, since this could potentially have important clinical implications and be useful in improving patient outcomes.

This study is a single centre study of trauma patients at Rigshospitalet, Denmark. Rigshospitalet holds the only major trauma centre in the eastern part of Denmark, and every year around 1000 trauma patients are treated here. Patients are admitted from the Capital Region of Denmark with around 1.9 million inhabitants and the Region of Zealand with around 850.000 inhabitants.

All patients with trauma team activation in the trauma bay at Rigshospitalet, including both direct transport and secondary transfers, will be screened for potential inclusion in the study.

The trauma bay at Rigshospitalet will be equipped with the oximetry equipment, and measurement can be started as early as possible. At the trauma bay, a doctor will conduct the screening and inclusion of patients to the study at arrival. After study inclusion, monitoring equipment will be attached by a nurse in the trauma bay. Subsequently a member of the study investigator group will obtain consent.

Continuous pulse oximetry will be used to measure SpO2. SpO2, heart rate, pulse amplitude and alarm status will be measured every second during a 24-hour period on all study participants.

The Nellcor™ Portable SpO2 Patient Monitoring System, PM10N (Medtronic, 15 Hampshire Street, Mansfield, MA 02048, USA) is used as monitoring device. The Covidien Nellcor FLEXMAX or a similar probe will be placed on the patients' index finger, alternatively another finger or toe.

Data collection will continue for up to 24 hours. If there is an untimely removal of the probe, the device will switch of, and data collection will stop at this point, unless the device is turned on again. After 24 hours of measurement, an investigator will collect the device and transfer the data by USB cable to a computer for secure storage and further analysis.

Inclusion Criteria:

• Patients aged ≥18 years
• Blunt or penetrating mechanism of trauma
• Trauma team activation
• Admission to a ward/ICU from trauma centre

Exclusion Criteria:

• Patients with a suspicion of carbon monoxide intoxication
• No applicable finger or toe for sensor attachment
• Trauma team activation was cancelled
• No available pulse oximeter at admission

All patients with trauma team activation in the trauma bay at Rigshospitalet, including both direct transport and secondary transfers, will be screened for potential inclusion in the study. Patients will be included if they comply with the above eligibility criteria

From inclusion until end of follow-up, 30 days

Adverse Events were not collected separately for participants with daytime and nighttime measurements