Short CommunicationTwo methods to adapt the human haemoglobin–oxygen dissociation algorithm to the blood of white rhinoceros (Ceratotherium simum) and to determine the accuracy of pulse oximetry
Introduction
Achieving general anaesthesia in large quadrupeds, such as the white rhinoceros (Ceratotherium simum simum and cottoni), is a challenging task for veterinarians but is indispensable in conservation management and medical interventions. Close monitoring of arterial oxygen saturation by blood gas analysis and pulse oximetry during anaesthesia is essential as hypercapnia and, especially, hypoxemia are reported during general anaesthesia in white rhinoceros. Blood gas analysis and pulse oximetry are two methods of ensuring the adequate monitoring of oxygen status in the arterial blood. Its continuous monitoring, low cost and simple application makes pulse oximetry very popular, whereas blood gas analysis is more costly and is discontinuous. Both methods have one big disadvantage: they are validated for use in humans or, in the best case, in domestic animals, but not in wildlife. Today, pulse oximetry shows good accuracy in pets such as dogs (Burns et al. 2006) and farm animals. Pulse oximetry devices generally do not overestimate oxygen saturation significantly (Matthews et al. 2003; Burns et al. 2006). A pulse oximeter estimates oxygen saturation (SpO2) based on light absorbance of defined wavelengths of human oxy- and deoxy-haemoglobin. Variant human haemoglobins are responsible for low SpO2 measurements (Verhovsek et al. 2010). Measuring the arterial pressure of oxygen (PaO2), the unbound oxygen content in the blood, with a blood gas analyser is, by contrast, species-independent. Generally SpO2 and oxygen saturation by the blood gas analyser (SaO2) should correlate closely. In the majority of animals the standard measurements are satisfactory, but in species with extreme p50 values (partial pressure at which haemoglobin is 50% saturated with oxygen), such as the white rhinoceros (Baumann et al. 1984), or extreme adaptations to physiological changes in pH or 2,3-diphosphoglycerate (2,3-DPG), such as in camels, this may not be the case. The algorithm for calculating oxygen saturation in blood gas machines requires haemoglobin-specific validation. Humans show a p50 of about 3.99 kPa (30 mmHg), whereas the p50 at a pH of 7.4 and temperature of 37 °C in rhinoceros is extremely low at 2.66 kPa (20 mmHg). With decreasing pH, oxygen binding affinity decreases more than in humans because of the strong Bohr effect of −0.62, whereas oxygen affinity changes little with 2,3-DPG and adenosine triphosphate (ATP) (Baumann et al. 1984). The reason for this special characteristic of rhinoceros haemoglobin may reflect a specific glutamic acid residue at position β2 in haemoglobin (Mazur et al. 1982). However, other than in Ball et al. (2011), this fact has rarely been considered in evaluations of the oxygen status of the anaesthetized white rhinoceros.
The algorithm to calculate oxygen saturation (sO2) can be adapted to calculate species-specific sO2 values, if data on the p50, Bohr effect, effect of 2,3-DPG and ATP (Mazur et al. 1982; Baumann et al. 1984) and physiological blood chemistry (Citino & Bush 2007) are available, as they are for the white rhinoceros. Herein, we present two methods to adapt the algorithm for species-specific validation by empirical fitting throughout the entire Hill curve.
Section snippets
Materials and methods
Captive white rhinoceros (C. s. simum and cottoni) of both sexes at reproductive age were anaesthetized during the years 1999–2003 within the European Endangered Species Programme (EEP) for serial clinical reproductive monitoring to elucidate the causes of female and male reproductive failure. All procedures were conducted according to the guidelines of the ethics committee of the University of Veterinary Medicine Vienna and the respective legislation in the countries in which the procedures
Results
The adapted algorithms for oxygen saturation take into account the low p50 of rhino haemoglobin and therefore lead to a left shift of the ODC compared with the algorithm used by the commercial blood gas analyser (Fig. 1). Oxygen saturation calculated by Method 1 is on average 16.38% higher, and by Method 2 14.12% higher than the values derived from the commercial human-adapted blood gas analyser [analysis of variance (anova): Methods 1 and 2, p < 0.001]. Methods 1 and 2 demonstrate that oxygen
Discussion
The underestimation of oxygen saturation by blood gas analysis and pulse oximetry originates from two different methodological factors. Firstly, the blood gas analyser does not consider species-specific shifts of the ODC and the pulse oximeter measures the light absorption at two fixed wavelengths. The calculation by Method 1 allows the inclusion of the important physiological parameters, including the p50 value, the Bohr effect (Baumann et al. 1984), and the physiological PaCO2, pH and body
Acknowledgements
The authors are grateful to Ole Siggaard-Andersen, (Professor emeritus, Clinical Biochemistry, University of Copenhagen, DK), for providing them with the oxygen status algorithm (OSA) program and for his helpful comments. The authors declare that they have no conflict of interest.
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