Oxygen Reserve Index as an early indicator of arterial oxygen content decrease in anaesthetized dogs

Oxygen Reserve Index (ORI) is a dimension-less index that reports real-time oxygenation status in a condition of moderate hyperoxaemia, which is defined as a partial pressure of oxygen in arterial blood (PaO₂) between 100 to 200 mmHg, and it quantifies the reserve of oxygen of a patient in that range. The aim of this study was to evaluate the changes in ORI values during apnoea in order to investigate the usefulness of the ORI as an indicator of early detection of hypoxia in anaesthetized dogs. Fifty-one client-owned dogs of various breeds, who were scheduled for surgery or diagnostic procedure under general anaesthesia, were enrolled. Dogs were placed in sternal position and maintained on mechanical ventilation. Once the ORI value reached 1.0 and the signal was stable, mechanical ventilation was interrupted and the animal disconnected from the anaesthetic breathing system. During the procedure, time was recorded at prescheduled ORI and peripheral oxygen saturation (SpO₂) values and time intervals were indicated as follow: ORIWT09 the time elapsing from an ORI of 0.9 to the lowest value of SpO₂; ORIWT05 the time elapsing from an ORI of 0.5 to the lowest value of SpO₂; and SpO₂WT the time from an ORI of 0.0 to the lowest value of SpO₂. A linear regression was used to study the relationship between those time intervals and the width of the tongue, age of the dog, body condition score (BCS), pulsatile index (PI). Variables having a statistically relevant effect were inserted in linear mixed effect models. Dogs were divided in three groups according to their BCS (normal BCS 4-5/9, overweight BCS 6-7/9 and obese BCS 8-9/9). In the statistical model the animals were inserted as random effect, while tongue width, nutritional status of dogs, time points and the interaction between time points and nutritional status were inserted as fixed effects. Results showed that the BCS does not influence the decreasing rate of oxygen using the ORI. Despite that, the mean time elapsing from an ORI of 0.9 to the lowest value of SpO₂ among normal, overweight or obese dogs were 124.5, 73.9 and 66.4 seconds, respectively, showing that ORI provided an early warning compared to traditional pulse oximetry of an impending oxygen decrease.