Effects of syringe material and temperature and duration of storage on the stability of equine arterial blood gas variables

Objectives: To evaluate the consistency of partial pressures (P) of arterial oxygen (aO2), arterial carbon dioxide (aCO2) and pH measurements in equine carotid arterial blood samples taken into syringes made from three different materials and stored at room temperature or placed in iced water for measurement at three different times. Study design: Prospective observational study over 19 days. Animals: Four clinically normal Thoroughbred or Thoroughbred-cross horses (three geldings, one mare, mean age 6.25 years, range 5–7 years). Methods: Identical blood samples were taken on two separate occasions from the carotid arteries of the four horses into syringes made of glass, plastic and polypropylene. PaO2, PaCO2 and pH determinations were performed on blood from each syringe type at 10, 60 and 120 minutes post-sampling with samples stored at room temperature (approximately 20 °C) or in iced water (approximately 0 °C). Data were analysed by ANOVA and a split plot model fitting syringe within horse X pair and time within temperature within syringe. Results: Syringe material, storage temperature and time before analysis all had significant effects on PaO2 (p <0.001). PaCO2 was unaffected by syringe material or storage temperature. However, over 120 minutes, storage duration significantly (p = 0.002) affected values. Temperature of storage and duration prior to analysis both significantly affected pH values (p = 0.005 and p <0.001, respectively), but syringe material did not. Several significant interactions between these variables were noted. Conclusions: Equine arterial blood gas determination has a different sensitivity to storage conditions compared to other veterinary species. Clinical relevance: For accurate equine arterial blood analysis, PaO2 samples need to be analysed within 10 minutes or taken into glass syringes, stored on ice and analysed at 2 hours post-sampling. PaCO2 and pH measurements can be performed on samples stored in glass, plastic or polypropylene syringes at room temperature for up to 1 hour post-sampling