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.

OBJECTIVE

Hemoglobin oxygen saturation (SO) is measured by co-oximetry (SO) or calculated from partial pressure of oxygen (PO) using algorithms (SO). To compare venous and systemic arterial blood sample data retrospectively and to examine whether temperature correction of PO is important.

STUDY DESIGN

Retrospective study.

ANIMALS

A group of 21 healthy, adult, sedated or anesthetized horses.

METHODS

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. PO was measured at 37 °C and corrected using body temperature (T) from a pulmonary artery catheter thermistor. SO and hemoglobin subtypes were measured by co-oximetry (37 °C). An algorithm developed for Thoroughbred horse blood was used to calculate SO using PO at 37 °C and SO with PO corrected to T. SO and SO were each paired with SO using Bland-Altman (repeated measures) ratio of SO/SO (204 samples).

RESULTS

SO overestimated SO when PO was <80 mmHg (10.7 kPa); ratio and limits of agreement: 1.2 (0.9-1.6) but became accurate when PO was ≥80 mmHg: 1.0 (1.0-1.0). With all data, SO did not differ from SO: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)%].

CONCLUSIONS

The algorithm appeared robust when PO was ≥80 mmHg (10.7 kPa) but overestimated when PO was lower. Temperature correction was not important within 34.9-37.3 °C.

CLINICAL RELEVANCE

SO overestimation in venous blood can result in calculation of higher intrapulmonary shunt fraction than SO values.