Retrospective comparison of equine hemoglobin oxygen saturation measured by a human-specific co-oximeter, or derived from an algorithm using temperature-corrected and -uncorrected oxygen tension.Vet Anaesth Analg. 2019 Sep; 46(5):636-642.VA
Hemoglobin oxygen saturation (SO2) is measured by co-oximetry (SO2cox) or calculated from partial pressure of oxygen (PO2) using algorithms (SO2alg). To compare venous and systemic arterial blood sample data retrospectively and to examine whether temperature correction of PO2 is important.
A group of 21 healthy, adult, sedated or anesthetized horses.
Mixed-venous and systemic arterial blood samples (1 mL) were anaerobically collected using commercial preheparinized syringes from the right ventricle and facial artery, respectively. Blood was analyzed using a commercial gas analyzer and human-specific co-oximeter within 10 minutes of collection or stored on ice and analyzed within 30 minutes. PO2 was measured at 37 °C and corrected using body temperature (Tcore) from a pulmonary artery catheter thermistor. SO2cox and hemoglobin subtypes were measured by co-oximetry (37 °C). An algorithm developed for Thoroughbred horse blood was used to calculate SO2alg using PO2 at 37 °C and SO2algcorr with PO2 corrected to Tcore. SO2alg and SO2algcorr were each paired with SO2cox using Bland-Altman (repeated measures) ratio of SO2alg/SO2cox (204 samples).
SO2alg overestimated SO2cox when PO2 was <80 mmHg (10.7 kPa); ratio and limits of agreement: 1.2 (0.9-1.6) but became accurate when PO2 was ≥80 mmHg: 1.0 (1.0-1.0). With all data, SO2algcorr did not differ from SO2alg:1.1 (0.8-1.4). Methemoglobin (FMetHb) and carboxyhemoglobin (FCOHb) were significantly higher in venous [FMetHb: median (range): 1.8 (0-2.9)%; FCOHb: 0.1 (0-2)%] than in arterial blood [FMetHb: 0.5 (0-2.2)%; FCOHb: 0 (0-0.3)%].
The algorithm appeared robust when PO2 was ≥80 mmHg (10.7 kPa) but overestimated when PO2 was lower. Temperature correction was not important within 34.9-37.3 °C.
SO2alg overestimation in venous blood can result in calculation of higher intrapulmonary shunt fraction than SO2cox values.