The Oxygen Reserve Index (ORi™) is a non-invasive variable that reflects oxygenation continuously. The aims of this study were to examine the relationship between arterial partial pressure of oxygen (PaO 2 ) and ORi during general anesthesia, and to investigate the usefulness of ORi as an indicator to avoid hyperoxia. Twenty adult patients who were scheduled for surgery under general anesthesia with arterial catheterization were enrolled. After induction of general anesthesia, inspired oxygen concentration (FiO 2 ) was set to 0.33, and arterial blood gas analysis was performed. The PaO 2 and ORi at the time of blood collection were recorded. After that, FiO 2 was changed to achieve an ORi around 0.5, 0.2, and 0, followed by arterial blood gas analysis. The relationship between ORi and PaO 2 was then investigated using the data obtained. Eighty datasets from the 20 patients were analyzed. When PaO 2 was less than 240 mmHg (n = 69), linear regression analysis showed a relatively strong positive correlation (r 2 = 0.706). The cut-off ORi value obtained from the receiver operating characteristic curve to detect PaO 2 ≥ 150 mmHg was 0.21 (sensitivity 0.950, specificity 0.755). Four-quadrant plot analysis showed that the ORi trending of PaO 2 was good (concordance rate was 100.0%). Hyperoxemia can be detected by observing ORi of patients under general anesthesia, and thus unnecessary administration of high concentration oxygen can possibly be avoided.
KeywordsOxygen reserve index (ORi) • Arterial partial pressure of oxygen (PaO 2 ) • Hyperoxia • Hyperoxemia
The oxygen reserve index (ORi™) is a new parameter for monitoring oxygen reserve noninvasively. The aim of this study was to examine the usefulness of ORi for rapid sequence induction (RSI). Twenty adult patients who were scheduled for surgical procedures under general anesthesia were enrolled. After attaching a sensor capable of measuring ORi, oxygen (6 L/min) and fentanyl (2 μg/kg) were administered. After 3 min, propofol 2 mg/kg and rocuronium 1 mg/kg were administered without ventilation. Regardless of changes in ORi, tracheal intubation was performed either 2 min after administration of propofol or when percutaneous oxygen saturation (SpO) reached 98%. Ventilation was then provided with oxygen at 6 L/min, and trends in ORi and SpO during RSI were observed. Data from 16 of the 20 patients were analyzed. Before oxygen administration, the median SpO was 98% [interquartile range (IQR) 97-98] and ORi was 0.00 in all patients. At 3 min after starting oxygen administration, the median SpO was 100% (IQR 100-100) and the median ORi was 0.50 (IQR 0.42-0.57). There was an SpO decline of 1% or more from the peak value after propofol administration in 13 patients, and 32.5 s (IQR 18.8-51.3) before the SpO decrease, ORi began to decline in 10 of the 13 (77%) patients. The ORi trends enable us to predict oxygenation reduction approximately 30 s before SpO starts to decline. By monitoring ORi, the incidence related to hypoxemia during RSI could be reduced.
At present, there is no objective and absolute measure of nociception, although various monitoring techniques have been developed. One such technique is the Analgesia Nociception Index (ANI), which is calculated from heart rate variability that reflects the relative parasympathetic tone. ANI is expressed on a non-unit scale of 0–100 (100 indicates maximal relative parasympathetic tone). Several studies indicated that ANI-guided anesthesia may help reduce intraoperative opioid use. The usefulness of ANI in the intensive care unit (ICU) and during surgery has also been reported. However, some limitations of ANI have also been reported; for example, ANI is affected by emotions and some drugs. In 2022, a high frequency variability index (HFVI), which was renamed from ANI and uses the same algorithm as ANI, was commercialized; therefore, ANI/HFVI are currently in the spotlight. Unlike ANI, HFVI can be displayed along with other biometric information on the Root
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monitor. ANI/HFVI monitoring may affect the prognosis of not only patients in the perioperative period but those in ICU, those who receive home medical care, or outpatients. In this article, we present an updated review on ANI that has been published in the last decade, introduce HFVI, and discuss the outlooks of ANI/HFVI.
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