Abstract

With the widespread availability of point-of-care analyzers, blood gas analysis can easily be performed in zoo collections and in the field. Deleterious changes in blood pH, oxygenation and ventilation can be rapidly diagnosed and treated. This presentation will discuss commonly available equipment and interpretation of arterial and venous blood gases. We will cover common sampling errors and artifacts, as well as species specific nuances in blood gas interpretation for non-mammalian species.

Introduction

Point-of-care blood gas analyzers are ideally suited for use in field and zoological settings. The most commonly used analyzers are battery operated and require only a small volume of blood (0.2–0.5 mL). This allows them to be used in remote settings and with small patients. Additionally, results are generated in 2-3 min, which can satisfy the needs of many impatient zoo clinicians. Most analyzers can provide immediate information about blood oxygenation, ventilation, blood pH, glucose and electrolytes. Immediate information also allows rapid adjustment of anesthetic management. This lecture will focus on clinical scenarios and how the use of a blood gas analyzer can affect management.

Maintenance of normal blood pH (approximately 7.4 for most mammals) is crucial for homeostatic function. As such, respiratory and metabolic processes have evolved with multiple redundancies to preserve physiologic pH. Most ectothermic species and some mammals and birds (especially diving animals) have adaptations which allow for a much wider range of acceptable blood pH. While this provides an additional safety buffer for these animals, it does complicate interpretation of blood gasses and determining what is normal and what requires correction. Hypoxemia, hypoventilation and poor perfusion are some of the most significant anesthetic complications experienced in the anesthesia of captive and free ranging wildlife. With routine use of blood gas analyzers many of these complications can be detected early and potentially corrected. In addition to perianesthetic management, there is a role for blood gas analyzers in the management of many intensive medical cases.

What can a blood gas tell you that you did not already know? Both pulse oximetery and capnography provide useful, non-invasive second to second, assessments of ventilation and oxygenation. But both methodologies can have some drawbacks that compromise their accuracy. Direct blood gas analysis can identify abnormalities in species for which non-invasive methodology is not validated. A capnograph can provide a non-invasive assessment of expired CO₂, which should be similar to arterial CO₂. Without blood gas analysis, it is possible that an increase in physiologic dead space (causes listed below) could compromise the accuracy of the capnograph. In species capable of pulmonary shunting, such as reptiles, the expired CO₂ may not represent the arterial CO₂ at all.

Technical Aspects of Blood Gas Analysis

Sample Collection

Blood should be collected anaerobically into a heparinized syringe. Excessive heparin can affect parameters including hematocrit and ionized calcium.¹ Samples should be analyzed as quickly as possible and care should be taken to not introduce air bubbles into the sample. In reality, small air bubbles may not affect the accuracy of the sample significantly, but can cause the analyzer to malfunction. Excessive room air contamination can cause a decrease in the measured CO₂ and an increase or decrease in PO₂ depending on the percentage of inspired oxygen.

Arterial Collection Sites

Auricular artery: elephants, rhino, most ruminants

Facial, transverse facial artery: equids, some ruminants.

Radial artery: Apes

Dorsal pedal artery: large carnivores

Femoral artery: small carnivores, small primates

Superficial ulnar artery: birds.

Venous Collection Sites

Jugular, auricular or lingual veins: Jugular and auricular sites provide the easiest sampling and jugular sampling also provides the closest approximation of a true mixed venous sample. Lingual samples provide a close approximation of arterial gas tensions.

Arterial vs. Venous Comparison

Arterial samples are crucial for assessing pulmonary performance, especially oxygenation. A venous blood gas can provide pH, CO₂ and a crude estimate of body oxygen demand. The oxygen content of venous blood varies greatly depending on the sampling site, and level of metabolism and should not be used to approximate arterial oxygen content. A central mixed venous sample can provide very useful information about oxygen consumption and anaerobic metabolism, but requires an arterial sample for comparison.

Reported Values

A blood gas report contains multiple values, some of which are directly measured and some are calculated from the measured parameters based on algorithms validated for human use. Measured parameters include pH, partial pressure of CO₂ (PCO₂), Partial pressure of oxygen (PO₂), hematocrit and lactate. Most analyzers will also measure certain electrolytes, including Na, Cl, K and ionized Ca. From these measurements, the machine will calculate hemoglobin, bicarbonate (HCO₃), base excess and oxygen saturation (SO₂). Formulae for calculated parameters are given in the appendix. Oxygen saturation is calculated assuming a “normal” adult human hemoglobin oxygen dissociation curve. Some machines will correct for changes electrolyte concentrations based on changes in blood pH.

Correcting Blood Gases for Temperature

All blood gas analyzers measure dissolved gasses at a standard temperature (usually, 37°C). When the patient’s body temperature differs significantly from 37°C it may be useful to “correct” the sample data to the body temperature. Temperature correction of blood gasses is fairly controversial, even in human medicine. In short, the concern is that in a hypothermic patient, reported values (measured at 37°C) may not represent the true values in the patient. At the same time, corrected values are not applicable to any known reference ranges. Human reference ranges are designed to be used with a blood temperature of 37°C. For near normothermic mammals, temperature correction does not make a significant difference and the difference is not usually a reason to change the course of treatment. For ectotherms and severely hypothermic mammals, temperature correction may be more valuable. Most analyzers use a built-in algorithm for temperature correction. There are multiple published references detailing correction formulae for ectotherms.²

The i-STAT® portable analyzer only operates at an ambient temperature of 16 to 30°C. When working outside of these temperatures, it is critical to control the temperature of the analyzer with extra heat packs or ice packs in an insulated cooler. The analyzer has an internal thermometer and will report its temperature and if there is an ambient temperature error.

Blood Gas Interpretation

Basic interpretation should be a straightforward, step-by-step process taking 30–60 seconds. While there are more advanced ways of interpreting acid-base changes in human and veterinary critical care, they will not be covered here.

6-Step Interpretation

1. What is the pH? Normal pH for most mammals is 7.35–7.45. Is the patient’s pH low (acidemia) or high (alkalemia)?

pH: pH only gives us the direction and extent of the derangement, but does not tell us the source of the problem. It does help narrow down the differentials for the primary problem and the list of actions that need to be done to correct the problem. Carnivores tend to have slightly more acidic pH, while herbivores and omnivores with high carbohydrate diets tend to have more alkaline blood pH.

2. What is the PO₂ (arterial sample)? Normal arterial PO₂ 100 mm Hg, while breathing room air. Is the patient hypoxemic? Is the patient’s oxygen tension appropriate for the fraction of oxygen that it is breathing?

PaO₂: Normal PaO₂ (arterial partial pressure of oxygen) is 100 mm Hg when breathing room air and 400–500 mm Hg when breathing 100% oxygen. Hypoxemia is defined as a PaO₂ <80 mm Hg. Calculating an Alveolar-to-arterial oxygen gradient (A-a gradient) provides useful information about the cause of the hypoxemia. In addition, calculating a PaO₂/FiO₂ (partial pressure to fraction of inspired oxygen) ratio provides a very easy means of assessing pulmonary function. The A-a gradient is most accurate when the patient is breathing room air (21% oxygen) while the PaO₂/FiO₂ ratio can be done with any FiO₂.³

A measured hypoxemia is typically the result of one of five problems:

• Hypoventilation: i.e., Patient is not breathing frequently or deeply enough. In cases of hypoventilation, the PaO₂ is low but there is a normal A-a gradient.
• Low FiO₂: The inspired percentage of oxygen is too low. This is rare as most anesthetized animals are breathing an enriched oxygen mixture. The animal is hypoxemic with a normal P/F ratio.
• Ventilation/perfusion mismatching: This is quite common and is likely the main source of hypoxemia in anesthetized large animals. It is often associated with atelectasis and can be exacerbated by poor cardiac output and poor pulmonary perfusion.
• Diffusion impairment: Rare, will not be discussed.
• Anatomic right to left shunt: Rare, will not be discussed.

3. What is the PCO₂? Normal PCO₂ is 35–45 mm Hg. PCO₂ represents the respiratory component of the acid base derangement. Changes in PCO₂ result in respiratory acidosis and alkalosis.

PCO₂: Carbon dioxide tension quantifies the balance between cellular metabolism and alveolar ventilation. Hypercapnea typically results from a decrease in ventilation, but can be a result of increased metabolism (exertion). Hypocapnea could be from hyperventilation or decreased metabolic activity. PCO₂ can also be compared to end-tidal CO₂ to determine if there is an increase in physiologic dead space. End-tidal CO₂ should slightly underestimate arterial CO₂ by 5 mm Hg. An increase in this difference indicates that there are areas of lung that are ventilated, but not perfused. This occurs with pulmonary thromboembolism and decreased pulmonary perfusion secondary to decreased cardiac output.

4. What is the metabolic component (HCO₃ and base excess)? Are the changes appropriate for the changes in PCO₂ or is there a metabolic acidosis or alkalosis?

Bicarbonate and base excess: Both of these calculated parameters provide information about metabolic alkalosis or acidosis. These are indirect measures as both are derived from the measured CO₂ on the blood gas (formulas given above). A typical reference range for HCO₃ is 19 to 24 mEq/L. To account for the effect of CO₂ on HCO₃ calculation, base excess (BE) can be used. Base excess determines the amount of bicarbonate that needs to be added to blood to bring the pH back to 7.4, when PCO₂ is set at 40. Essentially, base excess factors in the effect of body buffer systems and factors out the effect of CO₂ on bicarbonate to determine the metabolic contribution to acid-base balance. Reference interval for bicarbonate is -4 to 4 mEq/L. While not a perfect system, BE provides a rapid way of determining the metabolic disturbance. If BE is high, there is a metabolic alkalosis and if it is low there is a metabolic acidosis, regardless of the respiratory disturbance.

5. What is the level of compensation? Derangements of either respiratory or metabolic acid-base balance often result in compensatory change from the other system, i.e. metabolic acidosis often results in a compensatory respiratory alkalosis (hyperventilation).

Compensation: Before evaluating compensation, look back at the pH. If the pH is low, the primary process is an acidosis and the compensatory process (if present) is an alkalosis. Compensation rarely brings the patient back to a normal pH and never overcompensates. A primary chronic respiratory acidosis (hypoventilation) will lead to a compensatory metabolic alkalosis, but pH will not return to normal and will not become alkalotic. Methods of compensation include chemical buffers (few seconds), respiratory (few minutes) and metabolic compensation (few days).³

6. Are there abnormalities in electrolytes or lactate?

Lactate and electrolytes: Blood lactate is a by-product and indicator of anaerobic metabolism. Increases in blood lactate typically accompany decreases in tissue perfusion. This could include ischemic muscle from a positional or exertional myopathy in which metabolic oxygen demand has outstripped the available oxygen delivery. Focal ischemia (strangulated intestine, compromised blood flow to a limb after trauma) can also increase lactate production, and in some cases the hyperlactatemia will only be seen after perfusion is reestablished. Electrolyte interpretation is similar to routine chemistry interpretation.

This step-by-step process should lead to an assessment of oxygenation and acid-base disturbance. Acid-base disturbances are described by the pH change (acidosis or alkalosis) and the source of the disturbance (metabolic or respiratory). In many cases, there is a mixed metabolic and respiratory disturbance. A few causes of the four main acid-base disturbances in animals are listed:

Metabolic acidosis: Gastrointestinal bicarbonate loss (diarrhea), renal bicarbonate loss, Lactic acidosis secondary to hypoperfusion.

Metabolic alkalosis: Pyloric outflow obstruction, excessive exogenous bicarbonate therapy.

Respiratory acidosis: Hypoventilation due to anesthesia, muscle relaxation, central nervous system (especially medullary or cervical) disease, airway obstruction, excessive dead space ventilation or hyperthermia.

Respiratory alkalosis: Hyperventilation due to hypoxemia, pain, anxiety, inappropriate ventilator settings.

Effect of Capture on Blood Gases

Typically changes seen during capture include hypoxemia, hypercapnea, lactic acidosis and hyperkalemia from acidosis and myocyte rupture. Strenuous capture can result in metabolic acidosis from increased production of lactate. Hypoxemia can exacerbate lactate production and hypercapnea from increased metabolic production of CO₂ during exertion can lead to worsening of acidosis. Similar changes in blood oxygenation, CO₂ and lactate production can be result from body positioning during anesthesia. For example, moving a rhinoceros from lateral to sternal recumbency may improve its ventilation and oxygenation while compromising muscle perfusion and increasing lactate build-up.⁴

Ectotherm Blood Gases

There are a number of published reports on the use of blood gases for evaluating reptiles, fish and invertebrates.² Important aspects to keep under consideration include the role of temperature compensation and the wide range of acceptable blood pH in most ectotherms. The majority of the referenced studies use a taxon-specific formula for correcting pH and dissolved gases for body temperature. In many cases, body temperature is assumed to be the ambient temperature. In some instances, clinically significant abnormalities were not noted unless temperature correction was performed. While the correction formulas may be valid, it is important to remember that corrected values should not be compared to standard reference ranges. Species and temperature specific ranges should be established.

Appendix

Formulae used by blood gas analyzers for calculated parameters:

1.  HCO₃: log HCO₃ = pH+log (PCO₂ -7.608)

2.  Base excess = HCO₃ -24.8+16.2 (pH -7.4)

3.  Anion Gap: ([Na⁺]+[K⁺])-([Cl^-]+[HCO₃^-])

4.  A-a gradient=PAO₂-PaO₂= (FiO₂x(P_bar-PH₂O) -PaCO₂/0.8)-PaO₂
If performed at standard atmospheric pressure (760 mmHg) and room air (21% oxygen) the formula is simplified to: 150-1.2(PaCO₂)-PaO₂. Normal A-a gradient is 10–15 mm Hg

5.  Physiologic dead space: Vd/Vt=(Pa CO₂-Et CO₂)/Pa CO₂. Normal dead space is 0.3-0.5.

Literature Cited

1.  Hopper, K., M. Rezende and S. C. Haskins. 2005. Assessment of the effect of dilution of blood samples with sodium heparin on blood gas, electrolyte, and lactate measurements in dogs. Am J Vet Res. 66(4): 656–660.

2.  Keller, K.A., C.J. Innis, M.F. Tlusty A. Kennedy, S. Bean, J. Cavin and C. Merigo. 2012. Metabolic and respiratory derangements associated with death in cold-stunned Kemp’s Ridley turtles (Lepidochelys kempii):32 cases (2005–2009). J Am Vet Med Assoc. 240:317–323

3.  Martin, L. 1999. All You Need to Know to Interpret Arterial Blood Gases, 2^nd Ed. Lippincott, Williams and Wilkins, Baltimore, Maryland.

4.  Morkel, P., R. W. Radcliffe, M. Jago, P. du Preez, M. Flaminio, D. V. Nydam, A. Taft, D. Lain, M. Miller and R. D. Gleed. 2010. Acid-base balance and ventilation during sternal and lateral recumbency in field immobilized black rhinoceros (Diceros bicornis) receiving oxygen insufflation: a preliminary report. J Wildl Dis. 46(1): 236–245.