Abstract

In zoo and wildlife medicine, patient physical examinations, diagnostics and treatment often take place simultaneously while the patient is under anesthesia. Portable point-of-care clinical analyzers provide immediate “bedside” blood gas and blood chemistry analysis, which can rapidly guide anesthetic adjustments, facilitate early diagnosis of biochemical and electrolyte disturbances, and promote timely and appropriate administration of fluids and other therapeutics. This is of particular importance when working with patients that are difficult to treat once recovered from anesthesia. Immediate, patient-side analysis of ionized calcium eliminates the effects of storage on sample temperature and pH, which strongly influence calcium equilibrium. Knowledge of true calcium status is particularly helpful in evaluating renal, reproductive, metabolic, and neurologic conditions in reptiles and birds. Use of a portable clinical analyzer during field anesthesia allows for instantaneous information on the animal’s ventilation, perfusion and acid/base status. In-the-field analysis of blood samples obviates the need to store and transport the samples and eliminates the effects of prolonged storage at possible suboptimal conditions. The i-STAT portable clinical analyzer (Heska Corp, Fort Collins, CO) has been used in field immobilization of free-ranging giraffes (Giraffa camelopardalis),¹ and sika deer (Cervus nippon),²⁰ and during anesthetic studies in captive red wolves (Canis rufus)¹⁹ and alpine goats.¹⁷

The i-STAT is a compact portable clinical analyzer system comprised of a handheld device and disposable, self-contained cartridges. The handheld analyzer weighs 539 g and runs on two 9-V batteries.⁷ Over ten different cartridge configurations offer a choice of different biochemical panels. The cartridges contain a series of thin film electrodes, or biosensors, that connect with the blood sample and send signals to the handheld analyzer. The i-STAT requires 0.06–0.20 ml of sample and produces results within 2 minutes. Ideally, whole blood stored in lithium heparin should be used. Whole blood without added anticoagulant will also produce accurate results if used in a timely manner. Blood stored in a sodium heparin tube is not recommended, due to anticoagulant effects on sodium analysis. The blood sample is contained within the cartridge and does not come into contact with the analyzer at any time.⁷

The i-STAT has been used extensively in human medicine at emergency centers, during anesthesia, and during cardiac bypass surgery.^7,14,23 Several studies have investigated its use in domestic animal species.^2,9,16 The i-STAT has been used to evaluate ionized calcium in green iguanas (Iguana iguana),³ and blood gases in African grey parrots (Psittacus erithacus timnus).⁶ To our knowledge, validation or investigation into the accuracy of the i-STAT in non-mammalian patients has not been published.

Measurements of hematocrit (HCT), ionized calcium (iCa), glucose, and potassium have been shown to be divergent to the point of clinical significance when i-STAT results are compared to a standard laboratory analyzer.⁹ Potassium will not be addressed in this literature review.

The i-STAT measures HCT via conductivity.¹³ Erythrocytes act as insulators and decrease conductivity, therefore measured conductivity is inversely related to HCT.⁵ I-STAT HCT values of equine, feline, and canine blood samples were consistently lower than values obtained by the microhematocrit method or by an automated cell counter.^9,16 Total serum protein can affect conductance and measurement of HCT. In people, the i-STAT measurement of HCT can be affected when HCT<40% and protein is over 8.0 g/dl or under 6.5 g/dl.⁹ Avian and reptile HCT can fall below 40% in healthy animals, and total serum protein is commonly lower than 6.5 g/dl. One potential reason behind the difference in i-STAT HCT values and those of microhematocrit tubes or automated cell counters could be related to the anticoagulant used. Tri-potassium ethylenediaminetetra-acetic acid (K₃EDTA) causes human RBCs to shrink due to an osmotic pressure gradient, artificially reducing the HCT below its true in vivo value.¹³ Because K₃EDTA is the anticoagulant of choice in human laboratories, automated cell counters are calibrated to match microhematocrit determinations based on samples anticoagulated with K₃EDTA. The i-STAT is also calibrated to match the measurements of microhematocrit tubes containing human whole blood with K₃EDTA, making it less reflective of the in vivo HCT. This “under-calibration” may carry over to non-human mammals, and potentially to avian and reptile results as well. Equine HCT values from the i-STAT measured consistently lower than HCT values from microhematocrit tubes containing whole blood and lithium heparin.¹⁶ Canine, feline, and equine HCT values from the i-STAT measured consistently lower than HCT values determined by an automated cell counter.⁹ In a pilot study conducted at the Sacramento Zoo (Howard, unpublished data), HCT was measured in nine Caribbean flamingos (Phoenicopterus ruber) on the i-STAT and by conventional centrifugation. The i-STAT-measured HCT ranged 30–43% (Table 1), and the centrifuged samples ranged 44–51%. Concurrent total protein, measured in a standard veterinary diagnostic laboratory, ranged 3.8–5.6 g/dl. Fortunately, the i-STAT does appear to be consistent in the underestimation of HCT, usually resulting in a 4–5% reduction from other methods of measurement.⁹ Although the absolute values obtained by the i-STAT may not be accurate, the relative consistency of HCT results by the i-STAT allows the clinician to follow trends within a patient or population.

Table 1. i-STAT-derived venous blood results of Caribbean
flamingos (Phoenicopterus ruber) at the Sacramento Zoo

^aNumber of samples
^bValues are calculated

The portion of total serum calcium that is ionized decreases with increasing pH.²¹ In humans, a 0.1–0.2 increase in pH will result in a 10% decrease in iCa.¹⁰ The pH of blood changes dramatically with temperature and duration of storage. Once blood is collected, pH decreases due to in vitro cell metabolism by 0.01 pH units every 10 minutes, and pH decreases as sample temperature increases.¹⁵ If the blood sample cannot be analyzed immediately, it is recommended to store the sample in an ice water bath at 0°C, which decreases the metabolic reaction rate and slows the alterations in pH.¹⁵ Immediate analysis of ionized calcium with a portable clinical analyzer such as the i-STAT has the potential to eliminate variables associated with sample storage. In equine, canine and feline blood samples, the i-STAT underestimated iCa when iCa>1.3 mmol/L.⁹ The range of ionized calcium in avian and reptile serum may be affected by the i-STAT’s tendency to underestimate iCa greater than 1.3 mmol/L. Ionized calcium in blue and gold macaws has been measured between 1.15–1.55 mmol/L.¹⁸ Ionized calcium in green iguanas has been measured on the i-STAT as 1.22–1.62 mmol/L.³ In a pilot study conducted at the Sacramento Zoo (Howard, unpublished data), iCa in sixteen thick-billed parrots (TBP, Rhynchopsitta pachyrhyncha) and nine Caribbean flamingos was measured on the i-STAT. In the TBP, iCa ranged 1.03–1.4 mmol/L, with a concurrent pH of 7.201–7.446. In the flamingos, iCa ranged from 1.3–1.4 mmol/L, with a concurrent pH of 7.168–7.465 (Table 1). Fortunately, the tendency to underestimate iCa at high concentrations is less likely to lead to clinical misinterpretations and initiation of inappropriate, and potentially fatal, calcium therapy. In avian and reptile patients, however, it has the potential to affect clinical evaluation of reproductive and renal disorders.

The i-STAT measures glucose amperometrically via the product of the glucose oxidase reaction.⁷ In equine, canine and feline samples, the i-STAT was accurate at physiologic mammalian glucose levels, 60–120 mg/dl.⁹ At low concentrations (<50 mg/dl), the i-STAT overestimated glucose concentration. At high concentrations (>120 mg/dl), the i-STAT underestimated glucose concentrations. Overestimation of glucose at low concentrations has the potential to affect interpretation of reptile clinical chemistries, as glucose concentrations that may be considered low for mammals can be within normal clinical range for reptiles. The range of serum glucose in reptiles is highly variable, from 10–60 mg/dl in pythons²² to 150–280 mg/dl in green iguanas.⁴ Plasma glucose concentrations from 207–334 mg/dl were obtained in healthy green iguanas using the i-STAT.³ Underestimation of glucose at high concentrations could be problematic in avian patients. Glucose concentrations in clinically healthy birds range from 180–350 mg/dl, and avian patients with diabetes mellitus can have glucose concentrations >1,000 mg/dl.⁸

The i-STAT, with its low sample requirement and rapid production of results, has the potential to facilitate anesthetic monitoring in avian patients via arterial or venous blood gas analysis. It has been used in anesthetized African grey parrots with favorable results.⁶ Blood gas values have been established for a few avian species, including budgerigars: HCO₃ (21–26 mmol/L), pH (7.334 to 7.489), and PCO₂ (30.6–43.2 mm Hg).¹¹ Venous blood gas values obtained with the i-STAT on nine Caribbean flamingos at the Sacramento Zoo (Howard, unpublished data) are included in Table 1.

The i-STAT is relatively simple to use, and can be operated by non-traditionally trained, non-laboratory personnel.¹⁴ Proper storage of the single-use cartridges is essential to obtaining optimal results. Individual cartridge expiration dates vary with test combination and are printed on cartridge packages. For long-term storage, cartridges should be refrigerated at 2–8°C (35.6–46.4°F).¹² Once removed from the refrigerator, if stored at 18–30°C (64.4–86.0°F), cartridges are viable for 14 days. It is important to note that once removed from the cold storage at 2–8°C, the cartridges cannot be placed back into cold storage for effective long-term storage. The cartridge should be removed from cold storage at least 5 minutes before intended use.

Product Information

• Cost
• Handheld analyzer: $5,490.00 (10% discount to nonprofit institutions)
• Cartridges: $3 to $10 each
• Tests available
• EC 8+: Sodium (Na), HCT, potassium (K), hemoglobin (Hb), chloride (Cl), TCO₂, pH, HCO₃, pCO₂, base excess (BE), blood urea nitrogen (BUN), anion gap (AG), glucose (BG)
• EG 7+: Na, pCO₂, K, HCT, iCa, pO₂, sO₂, TCO₂, HCO₃, Hb, pH, BE
• EG 6+: Na, HCT, K, pCO₂, HCO₃, TCO₂, sO₂, pO₂, pH, Hb, BE
• 6+: Na, K, Cl, BUN, BG, HCT, Hb
• CG 4+: pH, pCO₂, pO₂, HCO₃, TCO₂, sO₂, BE, lactate
• EG 4+: Na, K, BG, HCT, Hb
• ACT: Celite-activated coagulation time
• Crea: Creatinine (Cr)
• G: BG

Distributor Information

Heska Corporation; web address: www.heska.com; phone number: 1-800-GO-HESKA (1-800-464-3752), Mon–Fri, 7 am–5 pm MST.

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