Reliability of methods to determine arterial oxygen-haemoglobin saturation in immobilized southern white rhinoceros (Ceratotherium simum simum)

Abstract

The goal of monitoring blood oxygenation during chemical immobilization is to help maintain adequate tissue oxygen delivery for normal organ function. Arterial oxygen-haemoglobin saturation (SaO2), which accounts for 96-98% of total oxygen content in the arterial blood is an important and informative indicator of blood oxygenation. However, the reliability of arterial oxygen-haemoglobin saturation measured or calculated by blood oxygenation monitoring methods has never been examined in southern white rhinoceros (Ceratotherium simum simum), despite their susceptibility to hypoxaemia during chemical immobilization. In this thesis, I aimed to evaluate the reliability of 1) a Nonin PalmSAT 2500A pulse oximeter’s and a Masimo Radical-7 pulse co-oximeter’s peripheral arterial oxygen-haemoglobin saturation (SpO2) measurements, at different attachment sites (third eyelid, cheek, ear, rectum, and, or tail) (Chapters 2 & 3), and 2) arterial oxygen-haemoglobin saturation (cSaO2) calculated by four different methods (Chapters 4 & 5). In Chapter 4, I evaluated arterial oxygen-haemoglobin saturation calculated from the Enterprise Point-of-Care (EPOC) blood analyser. In Chapter 5, I evaluated arterial oxygen-haemoglobin saturation calculated from the oxygen content equation using oxygen content (CaO2) measured by a microrespirometer following the method of Tucker (1967) and those calculated from the Siggaard-Andersen algorithm, modified for the specific characteristics of rhinoceros’ blood. As part of two broader studies, a convenience sample of 16 (8×2) rhinoceroses were immobilized by darting with different etorphine-based drug combinations, followed by butorphanol or saline, administered intravenously. Thereafter, insufflated oxygen (15 L.minute-1 flow rate) was provided intranasally to improve the animals blood oxygen levels. Nonin PalmSAT 2500A pulse oximeters and Masimo Radical-7 pulse co-oximeters with their probes were secured to their attachment sites (third eyelid, cheek, ear, rectum, and tail). At pre-determined time points, arterial blood samples were drawn from a catheter inserted into the auricular artery, and near-simultaneous SpO2 measurements were recorded from the different attachment sites. Blood samples were analysed using a benchtop AVOXimeter 4000 co-oximeter (reference method), the EPOC blood analyser and the Tucker cell microrespirometer. Dissolved arterial partial pressure of oxygen and carbon dioxide (PaO2 and PaCO2) and pH, measured by the EPOC were used to calculate arterial oxygen-haemoglobin saturation from the three calculation methods (i.e., EPOC, oxygen content equation and Siggaard-Andersen algorithm). Bland-Altman (to estimate bias and precision) and area root mean squares (ARMS) analysis were used to determine the reliability of SpO2 readings from the pulse oximeter devices, at different attachment sites, and calculated arterial oxygen-haemoglobin saturation measurements (cSaO2), when compared with simultaneous co-oximeter’s SaO2 measurements. SpO2 measurements from the Nonin 2500A PalmSAT pulse oximeter and a 2000T transflectance probe attached to the third eyelid are reliable, but only at the 70-100% saturation range; the other attachment sites are unreliable across the entire 0-100% saturation range. The Masimo Radical-7 pulse oximeter with a LNCS TF-I AH transflectance probe attached to the third eyelid, and with a LNS YI AH transmission probe attached to the ear pinna are unreliable across the entire 0-100% saturation range. The EPOC’s cSaO2 readings are unreliable across the entire 0-100% saturation range. Arterial oxygen-haemoglobin saturation calculated by the oxygen content equation using Tucker-derived CaO2 is also unreliable across the entire 0-100% saturation range. Arterial oxygen-haemoglobin saturation calculated by the Siggaard-Andersen algorithm, modified for rhinoceros is reliable but only at saturations above 90%; below 90% it is unreliable. I conclude that blood oxygenation and hypoxaemia can be monitored reliably in immobilized southern white rhinoceros using a Nonin PalmSAT 2500A pulse oximeter with a 2000T transflectance probe, attached in the space between the third eyelid and the sclera, facing the third eyelid, and when saturation levels are between 70 and 100%; and below 70%, a benchtop co-oximeter should rather be used.