Effect of Preoxygenation in Sitting Position on The Oxygen Reserve Index (ORI) Alarm Time as Early Predictor of Impending Hypoxemia During Prolonged Apnea

Introduction Pre oxygenation with high fraction of inspired oxygen is not only recommended before anesthesia induction and intubation but also to delay apnea induced hypoxemia in a variety of airway management and patients with limited oxygen reserve. As well as, to prolong the duration of apneic oxygenation in some airway interventions. Several methods have been used to improve the efficiency of pre oxygenation with controversial outcome. One of these methods is the performance of pre oxygenation in head up position.

During anesthetic management, pulse oximeter has been considered as valuable noninvasive tool for monitoring of patient oxygen saturation (SpO2%).

However, the relationship between the oxygen saturation (SpO2%) and the arterial partial pressure of oxygen (PaO2) restricts the pulse oximeter monitoring of patient oxygenation due to the sigmoidal pattern of this relation, as small changes only in SpO2% occur with increase in arterial oxygen tension above 80 mmHg., Therefore, when oxygen is provided, SpO2 remains 100% over a wide range of PaO2 (>80 mmHg) So It does not reflect the degree of hyperoxia during oxygen supplementation. On the other hand, when PaO2 is falling, the SpO2 may remain 100% until the PaO2 falls below approximately 80 mm Hg. Thus, it does not predict the onset of hypoxia in an apneic patient.

Oxygen reserve index (ORI) is a non-invasive real time non-unit scale between 0.00 and 1.00, which reflects the level of venous blood oxygen saturation (SvO2). When the patient is breathing room air ORI is 0.00, With oxygen supplementation, after SpO2 reaches its plateau of 100%, SvO2 continues rising as PaO2 increasing, and at PaO2 200mmHg usually ORI reaches a plateau. Hence ORI will serve to indicate PaO2 trends (rising or falling) in the range of 100-200 mmHg), so it will reflect the range of moderate hyperoxia (PaO2 ranging from 100 to 200 mmHg) during oxygen supply and, on the other hand, it will predict impending desaturation ahead of standard pulse oximetry during safe prolonged apnea technique in surgical patients at high risk.

Our study aims at assessment of the effect of pre oxygenation in sitting position on the ORI warning time for impending de-saturation during safe prolonged apnea techniques.