Arterial blood gases (ABGs) provide valuable information to help inform patient care decisions and to monitor the effectiveness of treatment.

By Bill Pruitt, MBA, RRT, CPFT, FAARC


Arterial blood gases (ABGs) are important in knowing the condition of the cardiopulmonary system and in seeing the effects of various interventions. Adult patients are often continuously monitored by several noninvasive devices including pulse oximetry for oxygen status (SpO2) and capnography for carbon dioxide (ETCO2) or transcutaneous CO2 (TcCO2). Newer technology can provide some non-invasive details about hemoglobin (by total hemoglobin- SpHb, methemoglobin – SpMet, and carboxyhemoglobin – SpCO). However, ABGs will give a “snapshot” of a bigger picture and provides a look at oxygenation (by SaO2 and PaO2), ventilation (by PaCO2), as well as acid-base status (by the balance between pH, PaCO2, and bicarbonate or HCO3-) and is more accurate than the non-invasive monitors. When co-oximetry is included in analyzing an arterial blood sample we can also get accurate details on hemoglobin (by HHb, O2Hb, COHb and MetHb).1  Further expansion in analysis can provide a results on electrolytes, metabolites, or check pleural fluid for pH. ABGs provide valuable information to help inform decisions in care and knowing if actions are bringing about desired outcomes. This article will discuss sampling and analysis, and practical interpretation of ABGs in adults.

Arterial Blood Gas Sampling and Analysis

An arterial sample for ABG analysis can be taken from an existing arterial line, obtained by a transcutaneous puncture of an artery (often the radial, brachial, or femoral artery), or may be a capillary sample (often used in neonates). For useful videos on these three sampling procedures see the Siemens Healthineers website.2 ABG analysis may be done at a centralized station (ie, in the lab or respiratory care department) or at bedside using a point-of-care (POC) testing device. Companies that provide ABG analyzers include Abbott Laboratories, Nova Biomedical, Radiometer, Roche Diagnostics, Siemens, and Werfen. POC devices are becoming a standard across many facilities and are constantly being improved.

ABG analyzers can obtain results from very small samples (for example with a volume of 65 µL) but for adults a 1 to 2 cm3 sample is the usual volume obtained. Heparin is used in the collection syringe or capillary tube to prevent the blood from clotting. Blood gas testing has federal regulations in place to ensure that test results are accurate, reliable, and timely. These regulations are found in the Clinical Laboratory Improvement Amendments of 1988 (referred to as CLIA88) and provide the requirements for ABG machine calibration and quality control.3 Results from ABG analysis are ready in just a minute or two but may take several more minutes to be officially recorded and communicated. 

Arterial Blood Gas Interpretation

ABG interpretation takes practice and careful thinking to arrive at the right result. Knowing the “absolute” normal values and normal ranges for the major components is foundational. (See Table 1 for these details.) There are four steps to ABG interpretation. The first three steps are concerning the acid-base status of the results (and are the most difficult to figure out); the fourth is concerned with oxygenation and is fairly easy to determine.4

Step 1. Does the pH reflect an acidosis or alkalosis?

If the pH is below the lower limit of normal (7.35), there is an acidosis. If above the upper limit of 7.45, there is an acidosis. If the pH is between 7.35 and 7.45 but either the PaCO2 or the HCO3– is outside their normal limits, there is a problem — look for the cause. If all three are within their respective normal limits, there is no acidosis or alkalosis present. If two or three of these measurements are outside the normal limits, either an alkalosis or acidosis is occurring and there may (or may not) be compensation.

Step 2: For either an acidosis or alkalosis, what is the cause?

Respiratory issues link to the PaCO2, which is regulated by the lungs and ventilation. Think of the PaCO2 as an acidic substance. If too much is in the blood, it causes a respiratory acidosis and if too low, it causes a respiratory alkalosis. Metabolic issues link to the HCO3-, which is regulated by the kidneys. Think of HCO3- as a base or alkaline substance. If too much is in the blood, it causes a metabolic alkalosis and if too low, it causes a metabolic acidosis. With an abnormal pH, either the PaCO2 or the HCO3- will match the pH in being an acidosis or an alkalosis. The exception to this occurs if both the PaCO2 and the HCO3- match the pH. In this case, there is a combined respiratory and metabolic problem (described as either a combined acidosis or alkalosis depending on the pH).

Step 3: Is there compensation occurring to correct the problem?

Compensation that occurs with abnormal ABGs are labeled as one of three possibilities – either uncompensated, partially compensated, or fully compensated. When an acidosis or alkalosis is occurring, the pH and the system causing the problem (either respiratory or metabolic) will match in labeling the issue. Over time, the other system will start working on compensation for the pH imbalance and will move in the opposite direction (ie, if a respiratory acidosis is the problem with an elevated PaCO2 the metabolic system will begin to compensate by increasing the HCO3–  to add more alkaline to the blood). If only partial compensation has occurred, all three values (pH, PaCO2 and the HCO3-) will be outside their normal limits with one system matching the pH movement and the other system moving in the opposite direction. When compensation has fully occurred, the pH will move back into its normal limit but it will not be absolutely normal (7.40). If full compensation has occurred the ABG will have a pH that is not absolutely normal (7.40) but the pH has moved one way or the other (toward either an acidosis) and is still within the normal limits (7.35 -7.45). Linked to this will be either a respiratory cause or a metabolic cause that matches the movement of the pH, and an abnormal value in the other system that is opposite the pH move. In an uncompensated acidosis or alkalosis, the pH and the system causing the problem will be abnormal and will match up in the labeling of acidosis or alkalosis, and the other system will have values that are still within its normal limits. This situation occurs early in a respiratory or metabolic problem and as the condition persists over time, compensation will begin to appear unless the issue involves both the respiratory and the metabolic systems (a combined acidosis or alkalosis). In this situation, no compensation is occurring since both systems are involved.

Step 4: Is there a problem with oxygenation?

This is the easy part in interpreting an ABG. PaO2 and SaO2 values are used to evaluate oxygenation. If these are too low, hypoxemia is present and can lead to tissue hypoxia. Keep in mind that hemoglobin is the major carrier for oxygen and the circulatory system provides transportation. Tissue hypoxia can occur if there is a problem with hemoglobin (ie, anemia or carbon monoxide poisoning) or in circulation (ie, very poor or absent circulation) or other issues. Three general categories of hypoxemia are defined by the PaO2 values: mild hypoxemia 60-79 mmHg, moderate hypoxemia 40-59 mmHg, and severe hypoxemia <40 mmHg.7

Table 1. Normal ranges and absolute normal values for ABGs

Measured itemNormal rangeAbsolute normal
pH7.35-7.457.40
paO280-100 mmHg>80 mmHg
PaCO235-45 mmHg40 mmHg
HCO3–22-26 mEq/L24 mEq/L
For a better understanding compare the ABG ranges and absolutes to what is seen in measuring blood pressure. The normal ranges for systolic pressure is 90-129 mmHg and diastolic 60-80 mmHg with an absolute normal blood pressure of systolic/diastolic being 120/80 mmHg.5,6


Conclusion

ABGs provide a valuable look at many factors regarding ventilation, oxygenation, acid-base status, and give guidance to making decisions for managing patients in need of help. Blood gas analyzers are constantly being upgraded, automated, and more accessible to the bedside caregiver. Obtaining blood samples involves specific careful steps while analysis involves paying attention to proper steps, machine maintenance, and timely, accurate results. Interpretation involves understanding the interplay between breathing, circulation, renal function, and other systems/conditions. Respiratory therapists play a major role in all aspects of ABG testing and in the care offered to address possible problems uncovered in the ABG results.

RT

Bill Pruitt, MBA, RRT, CPFT, FAARC, is a writer, lecturer, and consultant. He has over 40 years of experience in respiratory care, and has over 20 years teaching at the University of South Alabama in Cardiorespiratory Care. Now retired from teaching, he continues to provide guest lectures and write. For more info, contact [email protected].

References

  1. From AcuteCareTesting.org website: https://acutecaretesting.org/en/articles/to-coox-or-not-to-coox
  2. Siemens Healthineers website on POC testing:  https://www.siemens-healthineers.com/en-us/point-of-care-testing/featured-topics-in-poct/blood-gas-featured-topics/blood-gas-proper-sample-handling#06237626
  3. From AcuteCareTesting.org website: https://acutecaretesting.org/en/articles/the-new-clia-quality-control-regulations-and-blood-gas-testing
  4. From the Level Up RN website: https://www.leveluprn.com/blogs/abg-interpretation/3-steps-for-interpretation
  5. From the NIH website: https://www.nhlbi.nih.gov/health/low-blood-pressure
  6. From the Healthline website: https://www.healthline.com/health/high-blood-pressure-hypertension/blood-pressure-reading-explained#stage-1
  7. From RT for Decision Makers in Respiratory Care website: https://respiratory-therapy.com/department-management/clinical/the-abcs-of-abgs-blood-gas-analysis/