Accurate measurement of arterial blood gases helps clinicians understand patients’ underlying condition and provide appropriate care.
The results of arterial blood gas (ABG) reports are considered almost vital signs in many institutions. To analyze ABGs, a small amount of arterial blood is drawn either through an arterial line or via what is commonly known as an “art stick.” This is an intermittent needle withdrawal from, most commonly, the radial artery. This does present some pain to the patient and possible risk to the artery. What is becoming more common is the placement of an arterial line if multiple arterial blood gas results will be needed. A small amount of blood is drawn. Approximately 2 mL in adults or youngsters in their late teens and smaller amounts for younger patients are needed if transport of blood occurs in numerous manners, depending on facility protocols. The results have multiple purposes.
An ABG can determine many factors: the pH of the blood, oxygen content, and carbon dioxide levels, as well as a host of other factors, such as bicarbonate levels. These numbers tell us effectiveness of ventilation, oxygenation status of patients, as well as possible metabolic activities affecting the patients’ homeostatic levels. This will indicate the appropriateness of the settings of the mechanical ventilator if the patients are intubated, or whether the patients can maintain their current survival through spontaneous breathing. It allows us, as clinicians, to see into the patients and determine how they are handling their current disease states.
We must always be aware of the correct ranges of these factors as this gives us insight into the direction the patient is heading. For example, if the pH starts to climb outside of the normal range, cell metabolism can be affected. This in turn can cause multiple issues regarding oxygen consumption, carbon dioxide production, etc. With knowledge, the clinician can head off problems before the situation becomes dire, thus saving the patient from harm or long-term effects.
One of the more critical aspects of the arterial blood gas sample is how long it takes to measure the sample from the time it is drawn. In a perfect world, the measurement should be instantaneous; however, the goal should be to run the sample within 10 minutes from the time of withdrawal. To prevent hemolysis, the sample should be handled with care. The sample should be mixed gently with a twirling action. Some studies have indicated that it is appropriate to ice the sample, while others have shown no such need.1,2
What information should you have when running a sample of the arterial blood? Oxygen concentration the patient is on, as this will affect the total tension of oxygen measurements. The patient’s body temperature should be noted as this also will have an effect on blood gas samples; however, caution should be noted as accurate measurements should be assured before entering the information into your measuring device. A misconstruction of the values could lead to inappropriate care. Many facilities have thus gone away from temperature corrected values due to these ramifications.
Who Is a Candidate for the Test?
Any time there is a question about breathing problems, oxygen and carbon dioxide exchange, or acid and alkali levels in the body, this test may be of help. Examples of patient conditions often include: shortness of breath due to acute or chronic lung conditions, heart diseases or blood conditions, or unconsciousness due to unknown etiology. The test should be run on patients who are suspected of having too much or too little acid in the body, such as someone with kidney failure, and patients who are mechanically ventilated. In these cases, ABGs are done regularly to ensure that the machine settings are correct for the individual’s condition.
What Do the Test Results Mean?
There are several values that are measured in an ABG (Table 1). Each of the values has a set range that is considered to be within healthy limits. If any of the main values becomes severely abnormal, the person may die.
The pH level is an important part of this test. This is a measure of the level of acid in the blood. High acid levels may be the result of:
- kidney failure and/or damage
- certain cases of uncontrolled diabetes
- exposure to certain toxic substances, such as a drug overdose
- shock, which may occur from heart failure, serious infections, or massive blood or fluid loss
- difficulty breathing, such as in the case of lung infections, asthma, emphysema, or not breathing fast enough
- certain medications
Low acid levels may be the result of:
- certain types of kidney problems
- hyperventilation syndromes
- excessive vomiting/diarrhea
- sodium imbalances, which may result from various issues happening in the body
- certain medications
If the pH is abnormal, the other parts of the test can help find out potential causes. For example, if the acid level in the body is too high, it could be from respiratory or metabolic-related processes. It is crucial to know what is causing the high acid level so that the best treatment can be chosen. If the acid level is too high because of a respiratory issue, the patient may need extra oxygen or even a mechanical ventilator. If the acid level is too high from metabolic conditions, the patient may need to be assessed for possible kidney dialysis or may need antibiotics or other medications to help alleviate the underlying cause. The kidney and the liver are the two main organs responsible for the metabolic homeostasis of pH, so this will allow us to narrow down some of the possible causes or disease originations of the patient’s condition.
Oxygen and Carbon Dioxide Levels
Other parts of the test are the oxygen and carbon dioxide levels in the blood. The job of the lungs is to take in oxygen and get rid of carbon dioxide. If some type of breathing or respiratory problem is present, these values will be abnormal. The oxygen level also can be used to check if a person is getting enough oxygen or whether they need extra oxygen.
The level of bicarbonate (alkali) in the blood is calculated by the laboratory equipment from the pH and carbon dioxide levels. This level indicates if there is a metabolism problem. Health care professionals must look at the pH, breathing, and metabolic parts of the test as a whole. This allows them to sort out different medical conditions that the person may have.
Information Provided by an ABG
Depending on the type of analyzer, health care practitioners may evaluate some of the information presented below:
pH and hydrogen ion (H+) activity. The pH scale ranges from 1 to 14, 1 being the strongest level of acid to be measured and 14 being the strongest level of alkalinity measured. Although the pH may vary slightly within a range and still be considered normal, for the purpose of ABG analysis, 7.35-7.45 is the generally acceptable range, with 7.4 being the absolute norm.2
The pH of a solution gives information on the potential for it to become acidic or alkalotic. When acids are dissolved in water, they release free hydrogen ions (H+). The concentration of H+ is far greater in an acid than it is in an alkali, and the stronger the acid, the more H+ it contains. It is therefore slightly confusing that something with a high potential to become acidic has a smaller number on the pH scale. The reason for this is that the scale is a negative logarithm designed to help understand very small numbers with many decimal places. A pH of 1 means that the concentration of H+ in that solution is 0.1. A pH of 6 denotes that the concentration of H+ in that solution is 0.000001 and a pH of 14 denotes an H+ concentration of 0.00000000000001. It would be impractical to keep writing down all these decimal places, so we tend to refer to the number of zeros as the pH.
For every decrease in the pH by 1, there is a 10-fold increase in the number of hydrogen ions. Small changes in pH can create huge problems within the body.
PaCO2. This is the partial pressure of carbon dioxide dissolved within the arterial blood. It is used to assess the effectiveness of ventilation. The normal range for a healthy person is 35 to 45 mm Hg, although in certain chronic disease processes, it may be elevated above the normal range.2-4
PaO2. This is the partial pressure of oxygen dissolved within the arterial blood and will determine oxygen binding to hemoglobin (SaO2). It is of vital importance to indicate the amount of oxygen in the blood allowing for tissue oxygenation. The normal range for a healthy person is approximately 80 to 100 mm Hg at sea level. Progressing elevations decrease the “normal” PaO2 levels. Measurements of PaO2 that are higher than normal are usually associated with unnecessarily high levels of supplementary oxygen, while low readings indicate hypoxemia.2-5
SaO2. The arterial saturation depends on the PaO2 but also the hemoglobin affinity to oxygen. Although similar to SpO2 (measured by a pulse oximeter), it is more accurate. The normal levels are 97% and above, although levels above 90% are often acceptable in critically ill patients.2-4
HCO3- . HCO3- is the chemical formula for bicarbonate, an alkali. It is the main chemical buffer in plasma and alludes to the body’s metabolic status. It is not directly measured, but calculated from other values such as the PaCO2, hemoglobin (Hb), and pH using complicated formulas. Again this is not measured, but is calculated and slightly more accurate as it takes into account bicarbonate produced as a result of respiratory failure.5
Several textbooks have published many different normal ranges for HCO3-. It is easy to remember the normal range as being in the 20s (mmol/L). If the HCO3- is in the low 20s, it might be heading out of range, and if it is in the high 20s, this also indicates it is heading out of range. The ideal number is somewhere in the middle, such as 24, 25, or 26 mmol/L.
Base excess. Base excess (BE) is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a PCO2 of 40 mm Hg (5.3 kPa). The value is usually reported as a concentration in units of mEq/L. For many practitioners, this is perhaps the most confusing element of the ABG report. Base excess is a surplus amount of base (alkali) within the blood. When there is no surplus base within the blood, it is reported as a negative base excess (base deficit). This, too can be normal. The normal range can be anywhere on the linear scale between –2 mEq/L and +2 mEq/L of blood.
What does the “base” do? The body stores this “base” for times of need. If the body becomes too acidic in hydrogen ion levels, it releases this “base” to help reduce the acidic levels in the body. If this occurs, this storage for the base goes into the negative zone—the base has been used. If the base excess were reported as +1 mEq/L, this means that in theory 1 mEq/L of base needs to be removed to return the pH to 7.4. Conversely, a base excess of -1 mEq/L means that the blood would have a pH of 7.4 if an extra 1 mEq/L of base were added.
Practitioners who find the base excess difficult to comprehend may prefer to use the normal bicarbonate range when analyzing ABGs and refer to the BE as a control measure. This is because HCO3 and BE work in conjunction—they are effectively measuring the same thing. If the HCO3 is out of range and reading high, the BE also will be out of range and reading high (in this case a positive number, greater than 2). If the HCO3 is out of range and reading low, the BE also will be out of range and reading low (in this case a negative number, less than –2).
The more sophisticated blood gas analyzers also are able to measure lactate, glucose, urea, electrolyte levels, Hb, and the anion gap. Lactate measurement is a particularly important marker in conditions such as sepsis.6 The measurement of arterial blood gas has become essential in the treatment of a patient’s condition; however, it should be used with clinical relevance.
Arterial blood gases are a major qualifier in patient care if used judiciously. In this cost-conscious health care environment, we must always endeavor to bring high-quality patient care as efficiently as possible. Patient care and safety should be our top priority, however, and using the values mentioned throughout this article can help the practitioner determine the patient’s underlying condition and lead to accurate and improved care. As we look at the values as a whole instead of individually, we can assess and improve our care.
- Knowles TP, Mullin RA, Hunter JA, Douce FH. Effects of syringe material, sample storage time, and temperature on blood gases and oxygen saturation in arterialized human blood samples. Respir Care. 2006;51:732-6.
- Pagana KD, Pagana TJ. Mosby’s Manual of Diagnostic and Laboratory Tests. 4th ed. St Louis: Mosby Elsevier; 2010.
- Chernecky CC, Berger BJ. Laboratory Tests and Diagnostic Procedures. 5th ed. St Louis: WB Saunders; 2008.
- Fischbach FT, Dunning MB III, eds. Manual of Laboratory and Diagnostic Tests. 8th ed. Philadelphia: Lippincott Williams and Wilkins; 2009.
- Simpson H. Interpretation of arterial blood gases: a clinical guide for nurses. Br J Nurs. 2004;13:522–8.
- Dellinger RP, Levy MM, Carlet JM, et al. Surviving Sepsis Campaign: international guidelines for management of severe sepsis and septic shock. Intensive Care Med. 2008;34:17-60.