Reliability of calculated arterial oxygen-haemoglobin saturation in determining blood oxygenation of immobilised white rhinoceros (Ceratotherium simum)

Arterial oxygen-haemoglobin saturation (SaO 2 ) is an important biomarker of hypoxaemia, but is difficult to measure accurately in rhinoceros, especially during severe hypoxaemia. Pulse oximetry can reliably estimate SaO 2 in rhinoceros when attached to the third eyelid but only at SaO 2 ≥70%. The need for alternative methods led us to assess the reliability of the following approaches in calculating SaO 2 (cSaO 2 ) from conventional blood gas analysis: (i) Hill equation when the body mass-estimated P 50 (PO 2 when haemoglobin is 50% saturated with oxygen) is adjusted for pH alone (“pH-adjusted Hill cSaO 2 ”), (ii) Hill equation when P 50 is adjusted for pH, base excess and body temperature (“multivariable-adjusted Hill cSaO 2 ”), and (iii) the Siggaard-Andersen algorithm (“Siggaard-Andersen cSaO 2 ”). Sixteen white rhinoceroses ( Ceratotherium simum ) were immobilised with etorphine-based drug combinations and given butorphanol and, or oxygen. Arterial blood was drawn from the auricular artery and PaO 2 , PaCO 2 , and pH were measured using a portable Enterprise Point-of-Care (EPOC) blood gas analyser, and SaO 2 measured using a benchtop AVOXimeter 4000 co-oximeter (reference method). Bland-Altman and area root mean squares (ARMS) assessed the reliability of cSaO 2 values when compared to the co-oximeter SaO 2 measurements, with the reliability threshold set at ARMS ≤ 4%. The pH-adjusted Hill cSaO 2 and the multivariable-adjusted Hill cSaO 2 values were reliable within the 70–100% saturation range (bias 0% and − 2%, precision 3% and 4%, ARMS 3% and 4%, respectively). The Siggaard-Andersen cSaO 2 values were reliable only within the 90–100% range (bias 2%, precision 2%, ARMS 3%). Below 70%, all cSaO 2 methods were unreliable. However, across the entire 0-100% range, the pH-adjusted Hill cSaO 2 and multivariable-adjusted Hill cSaO 2 were only slightly above the reliability threshold (ARMS of 5%). The pH-adjusted Hill equation is best suited for calculating SaO 2 across the 70–100% saturation range in immobilised white rhinoceros. Although this equation did not meet the reliability threshold when saturations were below 70%, it nonetheless showed a marked improvement over previous pulse oximetry analyses. A simple calculator is presented to convert PaO 2 and pH into an estimate of SaO 2 , offering an additional tool to pulse oximetry especially during prolonged or complex procedures and for research applications.