The oxygen reserve index (ORI): a new tool to monitor oxygen therapy

Abstract

Supplemental oxygen is administered in the vast majority of patients in the perioperative setting and in the intensive care unit to prevent the potentially deleterious effects of hypoxia. On the other hand, the administration of high concentrations of oxygen may induce hyperoxia that may also be associated with significant complications. Oxygen therapy should therefore be precisely titrated and accurately monitored. Although pulse oximetry has become an indispensable monitoring technology to detect hypoxemia, its value in assessing the oxygenation status beyond the range of maximal arterial oxygen saturation (SpO2 ≥97%) is very limited. In this hyperoxic range, we need to rely on blood gas analysis, which is intermittent, invasive and sometimes delayed. The oxygen reserve index (ORI) is a new continuous non-invasive variable that is provided by the new generation of pulse oximeters that use multi-wavelength pulse co-oximetry. The ORI is a dimensionless index that reflects oxygenation in the moderate hyperoxic range (PaO2 100-200 mmHg). The ORI may provide an early alarm when oxygenation deteriorates well before any changes in SpO2 occur, may reflect the response to oxygen administration (e.g., pre-oxygenation), and may facilitate oxygen titration and prevent unintended hyperoxia. In this review we describe this new variable, summarize available data and preliminary experience, and discuss its potential clinical utilities in the perioperative and intensive care settings.

Keywords: Critical care; Hyperoxemia; Hyperoxia; Hyperoxic acute lung injury; Hypoxemia; Hypoxia; Monitoring; Operating rooms; Oxygen therapy; Oxygenation; Perioperative period; Preoxygenation; Pulse oximetry.

Conflict of interest statement

Conflict of interest

TWLS received honoraria from and Masimo Inc. (Irvine, CA, USA) for consulting and lecturing. TWLS is an associate editor of the Journal of Clinical Monitoring and Computing, but had no role in the handling of this manuscript. FJB received honoraria from Philips, Pulsion and Masimo Inc. (Irvine, CA, USA) for lectures. AP serves as consultant for Masimo Inc. (Irvine, CA, USA) and received honoraria for lectures. AP is also a member of the medical advisory board of Pulsion/Getinge.

Informed consent

Not applicable. This is a review article not including primary research on human participants and/or animals.

Research involving human participants and/or animals

Not applicable. This is a review article not including human participants and/or animals.

Figures

Fig. 1

Arterial (red line) and venous (blue line) oxyhaemoglobin dissociation curves. In the hypoxic rage (PaO2 <100 mmHg), arterial oxygenation can be assessed by pulse oximetry (SpO2). As PaO2 increases beyond 100 mmHg, venous saturation (SvO2) at the measurement site increases even though arterial saturation (SaO2) remains maximal and unchanged. This change in SvO2 causes changes in absorption of the incident light (and hence a change in measured signals) as PaO2 changes. With Masimo’s Rainbow SET technology these signals are extractable and the system is able to detect changes in PaO2 through changes in SvO2 at the measurement site. SvO2 reaches a plateau beyond a certain level of PaO2, approximately 200 mmHg (hyperoxic range), and consequently ORI is sensitive to the changes in PaO2 in the range between 100 and 200 mmHg (orange shaded area)

Fig. 2

Example of continuous intraoperative oxygen reserve index trend (ORI; black line), continuous pulse oxygen saturation trend (SpO2; green line), and intermittent arterial partial pressure of oxygen determination (PaO2; red diamonds) obtained during surgery. ORI decreased during 30 min before aBGA documented a large decrease in PaO2. Reprinted with permission from [56]

Fig. 3

Representative example from a volunteer breathing via a tight-fitting facemask standardized oxygen concentrations ranging from normoxia (FiO2 0.21) to hyperoxia (FiO2 1.0) and mild hypoxia (FiO2 0.14) in a stepwise fashion

Fig. 4

Clinical graphic example of the ORI response in a patient receiving supplemental oxygen before tracheal intubation (pre-oxygenation) (A) and again prior to extubation (B). In both instances, the ORI sharply increases, making the effects of pre-oxygenation visible. Following intubation, the FiO2 is reduced and titrated so that the ORI remained slightly above zero, indicating sufficient oxygenation of the arterial blood and avoiding hyperoxia at the same time. With kind permission from Drammen Sykehus, Norway

Fig. 5

Representative example of simultaneous trends in Compliance (ml/cmH2O), CO2 elimination per breath (ml), airway pressure (Paw, cmH2O), transpulmonary pressure (PTP, cmH2O) and ORI values of a patient during a recruiting maneuver (RM). While Paw rises in steps at the beginning of the RM, PTP and compliance rise accordingly and ORI increases up to a maximum value. Increases of dead space due to distention reduces the CO2 elimination per breath. During the decremental PEEP trial, step-reductions in Paw and PTP do not reduce compliance or ORI values until a closing point is observed. The dotted arrow shows this point in which an abrupt drop in compliance and ORI coincide with a negative value of TPT indicating re-collapse of the lung. The optimal PEEP corresponds to the previous setting

Fig. 6

Oxyhaemoglobin dissociation curve showing the potential usefulness of the continuous and non-invasive oxygen reserve index (ORI). The ORI has its place in the mild hyperoxic range (PaO2 100–200 mmHg). For lower values, pulse oximetry for monitoring SaO2 is useful, for higher values, invasive sampling and arterial blood gas analysis for measuring PaO2 must be used