Evaluation of a Portable Blood-Gas Analyzer in Horses

Overall, the handheld analyzer underestimated pCO2, pO2, and HCO3, but values seemed to be accurate enough to evaluate patient oxygenation and guide ventilatory management of clinical patients.

1. Introduction

Blood gas analysis provides valuable information about ventilation and acid-base status in many equine patients. Until recently, the technology for blood gas evaluation was very expensive and required significant technician time for upkeep of the equipment. Newer human analyzers use disposable microelectrodes to facilitate "point-of-care" or portable analyzers. Although these analyzers meet human laboratory standards for accuracy and reliability, there is little information available about their performance in equines. The purpose of this project was to compare blood gas values from a portable analyzer to a standard laboratory analyzer in equine patients.

2. Materials and Methods

Ninety-seven blood samples were collected from horses and one mule presented to the Teaching Hospital for anesthesia and surgery. A variety of breeds and ages were represented. Arterial blood samples were collected (from a catheter placed for blood pressure monitoring) into heparinized syringes, air bubbles were removed, and the samples were capped and maintained in ice water until analysis. Samples were sent to the clinical pathology laboratory for immediate analysis on a bench top blood gas analyzer [a], and all were also analyzed immediately on a portable analyzer [b] using a single use blood gas cartridge. Each cartridge underwent internal calibration before analysis. All horses were anesthetized; most were on an inhalant (100% oxygen), but 31 samples were from patients maintained on injectables not receiving supplemental oxygen (21% oxygen).

Values for pH, pCO2, pO2, and HCO3 were compared by calculation of bias (difference between the bench top and handheld analyzer) and precision (SD of the difference) for all samples. In addition, bias and precision were separately calculated for the 31 samples where the horses were on room air. Pearson correlation coefficients were also calculated for pH, pCO2, pO2, and HCO3.

3. Results

For all samples, measured values for pH ranged from 7.04 to 7.46, pCO2 ranged from 31 to 88 mm Hg, and pO2 ranged from 37 to 534 mm Hg. Calculated values for HCO3 ranged from 13.5 to 31.5 mM. For pH, the mean values (±SD) for the bench top and handheld analyzers were 7.33 ± 0.09 and 7.33 ± 0.09, respectively, while bias and precision were 0.004 and 0.02, respectively. For pCO2, mean values (±SD) were 51.2 ± 10.2 and 50.1 ± 10.9, respectively, while bias and precision were -1.1 and 2.7, respectively. For pO2, mean values (±SD) were 262 ± 168 and 240 ± 151, respectively, while bias and precision were -22.5 and 34.7, respectively. Mean values for HCO3 were 26.5 ± 4.3 and 24.9 ± 5.1, respectively, while bias and precision were -0.5 and 1.4, respectively. Negative values indicate that the handheld unit underestimated the bench top analyzer. For the 31 samples where pO2 was <100 mm Hg (room air), bias and precision for pH was 0.004 and 0.01, for pCO2 was -0.7 and 3.2, for pO2 was -3.7 and 4.2, and for HCO3 was -0.13 and 1.4, respectively.

Correlation coefficients for pH, pCO2, pO2, and HCO3 were 0.98, 0.97, 0.98, and 0.95, respectively.

4. Discussion

Although we were able to find three references [1-3] comparing blood gas data from another manufacturer's handheld analyzer, we were unable to find equine data for this analyzer. The most variable results were for pO2, where the handheld unit tended to underestimate, especially when pO2 was very high (as occurred when horses were maintained on 100% oxygen). We felt that the more clinically relevant data were from horses maintained on room air; therefore, those data were separately analyzed and were less variable.

As with all blood gas analyzers, HCO3 was not measured; it is a calculated value (based on the Severinghaus/NCCLS or Siggaard-Anderson formulas) [4]. This should be kept in mind when evaluating the use of a blood gas analyzer to detect bicarbonate supplementation (milkshaking) of performance horses.

Footnotes

[a] Corning 280 Blood Gas Analyzer, Ciba-Corning Diagnostics Corp., Medfield, MA 02052-2308.
[b] IRMA Blood Analysis System, Diametrics Medical, Inc., St. Paul, MN 55113.