Co-oximetry in clinically healthy dogs and effects of time of post sampling on measurements

CO-oximetry measurements and antioxidant effects of ascorbic acid and methylene blue in equine methemoglobinemic blood

OBJECTIVES

To determine the effects of time after sampling on CO-oximetry measurements of equine blood samples and the effects of adding ascorbic acid (AscAc) and methylene blue (MetBlue) to samples with methemoglobinemia.

DESIGN

Experimental study.

SETTING

University teaching hospital.

ANIMALS

Thirty healthy adult horses assigned to 5 groups.

INTERVENTIONS

Repeated CO-oximetry determinations were performed on venous (n = 6) and arterial blood samples (n = 7) stored at 0°C for 48 hours. Methemoglobinemia was induced in vitro in 17 additional blood samples. Six were used as untreated controls, 6 had AscAc added, and 5 had MetBlue added. Total hemoglobin, oxyhemoglobin, carboxyhemoglobin, methemoglobin (MetHb), and oxygen saturation of hemoglobin (SO₂ ) were measured.

MEASUREMENTS AND MAIN RESULTS

Oxyhemoglobin and SO₂ increased from 69.8% ± 10.2% and 90% ± 3% to 82.8% ± 7.9% and 99% ± 3%, respectively, after 8 hours in venous blood (mean ± SD, P < 0.001). There was an effect of treatment (P = 0.032) and of time (interaction P = 0.003) on MetHb% in methemoglobinemic samples. The difference in absolute MetHb% from time 0 was as follows: 7.0% (interquartile range [IQR] = 21.2), -0.2% (IQR = 3.5), and -4.4% (IQR = 5.2) at 48 hours in control, AscAc, and MetBlue groups, respectively (P < 0.05). There was no effect of time on MetHb% in the AscAc group (23% [IQR = 52.6] at time 0 to 23.2% [IQR = 56.9] after 48 h).

CONCLUSIONS

Storage of blood in ice water to determine O₂ Hb and SO₂ using a CO-oximeter should not exceed 4 hours. Measurement of MetHb% could be delayed by up to 48 hours if AscAc is added to the sample. MetBlue significantly decreased MetHb% over time. The limitations of this study include the fact that the antioxidant effects of AscAc and MetBlue were evaluated in vitro and not in vivo. Further studies are needed to evaluate different storage temperatures and syringe types.

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 (SO₂) is measured by co-oximetry (SO_2cox) or calculated from partial pressure of oxygen (PO₂) using algorithms (SO_2alg). To compare venous and systemic arterial blood sample data retrospectively and to examine whether temperature correction of PO₂ 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. PO₂ was measured at 37 °C and corrected using body temperature (T_core) from a pulmonary artery catheter thermistor. SO_2cox and hemoglobin subtypes were measured by co-oximetry (37 °C). An algorithm developed for Thoroughbred horse blood was used to calculate SO_2alg using PO₂ at 37 °C and SO_2algcorr with PO₂ corrected to T_core. SO_2alg and SO_2algcorr were each paired with SO_2cox using Bland-Altman (repeated measures) ratio of SO_2alg/SO_2cox (204 samples).

RESULTS

SO_2alg overestimated SO_2cox 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, SO_2algcorr did not differ from SO_2alg: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 PO₂ was ≥80 mmHg (10.7 kPa) but overestimated when PO₂ was lower. Temperature correction was not important within 34.9-37.3 °C.

CLINICAL RELEVANCE

SO_2alg overestimation in venous blood can result in calculation of higher intrapulmonary shunt fraction than SO_2cox values.