Arterial Blood Gas (ABG) Interpretation: Components, Disorders, Examples, Risks, and Complications

As a nurse, interpreting arterial blood gases (ABGs) is a critical skill to possess. It can reveal key information about a patient's health and give insight into how to proceed with treatment.

Mastering the art of ABG interpretation is a complex task. It involves an understanding of human biology, including blood, blood components, oxygenation, and acid-base imbalances. When reviewing ABG results, the nurse can work with a healthcare provider to make an appropriate treatment plan.

ABG Overview

Before diving into mastering the art of ABG interpretation, we need to discuss a few basics.

Arterial blood gases are the gold standard for assessing acid-base (pH) imbalances. Also, it assesses how well the body is oxygenating and removing excess carbon dioxide. This is measured by the amount of oxygen (PaO2) and carbon dioxide (PaCO2) in the blood. If the body is not using oxygen as it should, this results in an acid-base imbalance and an abnormal ABG.

Depending on the healthcare facility guidelines and training, ABGs are collected by nurses, respiratory therapists, providers, or other healthcare team members, requiring additional training when asked to do so.

Collect or draw a small blood sample through an arterial blood stick or an arterial line (also known as an a-line or art-line). The radial artery is the preferred site to collect an arterial blood gas sample. Alternative sites are the brachial or the femoral artery. However, these may be hard to locate and have more surrounding structures that can be damaged.

When Is an ABG ordered?

Healthcare providers may order ABGs on patients who have symptoms of acid-base disorders. Some of these symptoms include:

•  Persistent nausea or vomiting
•  Confusion
•  Fatigue
•  Rapid breathing
•  Heart rate or rhythm issues

The above list is not all-inclusive. Several other symptoms may warrant an ABG order. Healthcare providers may also order an ABG for certain health conditions. Some health conditions or problems that can cause acid-base imbalances are:

•  Sedation or coma
•  Chest wall injuries
•  Head or neck injuries leading to central nervous system (CNS) depression
•  Sepsis or lactic acidosis
•  Diabetic ketoacidosis
•  Respiratory disorders or obstructions
•  Renal failure
•  Carbon monoxide or another chemical poisoning
•  Drug overdose
•  Cardiac arrest or other circulation issues

Patients may also require arterial blood gases for pre-existing lung conditions such as asthma, chronic obstructive pulmonary disorder (COPD), or cystic fibrosis.

Accurately interpreting and understanding ABG results help the clinician decide on the patient's diagnosis and treatment plan. This can include oxygen therapy, medications, electrolyte replacements, and other treatments to improve a patient's acid-base imbalance.

Essential Components of an ABG Interpretation

As a nurse or any healthcare professional, it's vital to understand the essential components of an ABG interpretation. The components of an ABG reading and interpretation include:

•  pH: the body's blood acid-base balance (acidosis or alkalosis)
•  PaO2:  partial pressure of oxygen
•  PaCO2: partial pressure of carbon dioxide
•  HCO3-: concentration of bicarbonate
•  SaO2: arterial oxygen saturation
•  Base excess (BE): excess or deficit of base in the blood

The normal range for each of these components is:

• pH: 7.35-7.45
• PaO2: 80-100 mmHg
• PaCO2: 35-45 mmHg
• HCO3-: 22-26 mEq/L
• SaO2: 95-100%
• BE: -2 to +2 mmol/L

Blood pH is the amount of hydrogen ion (H+) concentration in blood circulation. Blood pH refers to how acidic or alkalotic the blood is, depending on the value outside of the standard measurement. The body is in acidosis when the blood pH is less than 7.35. When the blood pH is greater than 7.45, this is referred to as the body in alkalosis.

Related: Neuro Breath: Neuro ICU Mystery Explained

Two mechanisms can affect pH, which are metabolic and respiratory. The respiratory component is carbon dioxide, an acid, and the metabolic component is bicarbonate, the base. As the body's pH falls below 7.35 (acidosis), CO2 is exhaled, and HCO3- is retained. Alternatively, as the body's pH rises above 7.45 (alkalosis), CO2 is retained, and HCO3- is excreted (through urine).

Each component must be evaluated to assess the patient's acid-base status accurately. Additionally, the healthcare professional must consider any other relevant clinical information affecting the patient's ABG results and the body's ability to achieve homeostasis.

Types of Acid-Base Disorders

There are four main acid-base disorders. These include:

•  Respiratory acidosis
•  Respiratory alkalosis
•  Metabolic acidosis
•  Metabolic alkalosis

Respiratory acidosis occurs with hypoventilation. When the CO2 is greater than 45 mmHg, this is called hypercapnia. Respiratory alkalosis occurs with hyperventilation. 

Hypocapnia is when the CO2 is less than 35 mmHg. In addition, respiratory acid-base disturbances can be further broken down into acute or chronic.

Metabolic imbalances are related to bicarbonate (HCO3-) levels. The normal range for bicarbonate in the blood is 22-26 mEq/L. When serum HCO3- decreases below 22 mEq/L, this is metabolic acidosis. When serum HCO3- increases above 26 mEq/L, this is metabolic alkalosis.

When the body is not in homeostasis, it uses compensatory mechanisms to restore itself to a normal pH. Acid-base disorders can be simple or complex (also called mixed). A simple acid-base disturbance has one acid-base imbalance and compensatory mechanism. A complex acid-base disturbance has two or more primary imbalances.

Steps to Interpreting an ABG

After running your ABG sample, you must take several steps to interpret the results accurately. First, look at the pH and see if it's outside the normal range of 7.35 to 7.45. Remember, the higher the pH, the higher the base in the body. However, don't be fooled by a normal pH. Sometimes if the patient's body is temporarily compensating, the pH will be within the normal range.

Next, you must determine if the issue is respiratory or metabolic. To determine this, look at the carbon dioxide  (PaCO2) and the bicarbonate (HCO3-) levels. If the PaCO2 reading is outside the normal range and the bicarbonate is normal, then respiratory is the primary mechanism. The primary mechanism is metabolic if the HCO3- is outside the normal range and the carbon dioxide is normal. An easy way to remember this is by using the acronym ROME. ROME stands for respiratory opposite and metabolic equal (same direction). To further detail this, see below:

• Respiratory Opposite: As pH decreases, the CO2 (respiratory) will increase, and vice versa.
  • Respiratory Acidosis: pH <7.35, PaCO2 >45 mmHg
  • Respiratory Alkalsosis: pH >7.45, PaCO2 <35 mmHg
• Metabolic Equal: As pH decrease, the HCO3- (bicarbonate) will decrease, and vice versa. So they move in the same direction.
  • Metabolic Acidosis: pH <7.35, HCO3- <22 mEq/L
  • Metabolic Alkalsosi: pH >7.45, HCO3- >26 mEq/L

The final, and probably most complex step, is determining whether it's compensated or uncompensated. Meaning that your body will compensate (or not compensate) for an imbalance with either a change in a metabolic or respiratory mechanism. With this, there are two things you need to remember:

• If the primary imbalance is metabolic, then the body will compensate with carbon dioxide
• If the primary imbalance is respiratory, then the body will compensate with bicarbonate

Look at the opposite component level. If the opposite component is within normal limits, then it is uncompensated. For example, if there is respiratory alkalosis, look at the bicarbonate level. If the bicarbonate level is normal, then it's uncompensated respiratory alkalosis.

In addition, you will need to look back at the pH to determine if there is compensation present. The cutoff point for compensation is a pH of 7.4. This falls within the normal range, which can get confusing.

The cutoff point means that if the pH is greater than 7.4, this is alkalosis. A pH of less than 7.4 means it's acidosis. Looking back at our example of respiratory alkalosis, if the pH is 7.41 (normal range) and the bicarbonate is abnormal, then this is compensated respiratory alkalosis. Another way to look at it is if the pH is slightly above the cutoff point of 7.4 yet still within normal range, and the PaCO2 is abnormal, this is compensated respiratory alkalosis.

As a quick overview of the steps:

1. Determine if the pH is alkalotic or acidotic. If the pH is within the normal range, use the 7.4 cutoff point.
2. Determine if the issue is respiratory or metabolic by looking at PaCO2 and HCO3-
3. Look at the opposite component. If uncompensated, the opposite component will be normal.
4. If potentially compensated, look back at the pH cutoff point of 7.4. It is compensated if it's near the cutoff point but pH is within normal range.

Examples of ABG Interpretation

Mastering the art of ABG interpretation takes practice and repetition. You can find an ABG interpretation quiz or tool through a quick Google search. Let's review some examples below. As a reminder, here are the normal ranges for each component:

• pH: 7.35-7.45
• PaO2: 80-100 mmHg
• PaCO2: 35-45 mmHg
• HCO3-: 22-26 mEq/L
• SaO2: 95-100%
• BE: -2 to +2 mmol/L

Example 1

• pH: 7.38
• Partial pressure of carbon dioxide (PaCO2): 51 mmHg
• Partial pressure of oxygen (PaO2): 60 mmHg
• Bicarbonate (HCO3):  29 mEq/L
• Oxygenation (SaO2): 92% room air

Steps:

1. pH is within normal range. However, the pH cutoff point is less than 7.4. So this is acidosis.
2. The PaCO2 is greater than 45 mm Hg. From our ROME acronym above, respiratory is the opposite of pH. The pH is decreasing, and the PaCO2 is increasing. This matches the respiratory opposite. So the primary mechanism is respiratory acidosis. If you look at HCO3-, you notice that it's increased. This does not match the metabolic equal or same direction statement. Therefore, the primary mechanism cannot be metabolic.
3. Looking back at the bicarbonate (HCO3-), it is abnormal. Since it's abnormal, this means there is compensation present.
4. Thus, the answer is compensated respiratory acidosis.

Related: Student Registered Nurse Anesthetist: What Is an SRNA, Salary, and How to Become One

Example 2

• pH: 7.47
• Partial pressure of carbon dioxide (PaCO2): 33 mmHg,
• Partial pressure of oxygen (PaO2): 135 mmHg
• Bicarbonate (HCO3): 23 mEq/L
• Oxygenation (SaO2): 93% room air

Steps:

1. pH is above the normal range of 7.35 to 7.45. Thus, this example is alkalosis.
2. The PaCO2 is less than 35 mm Hg. From our ROME acronym above, respiratory has an opposite relationship of pH. The pH is increasing, and the PaCO2 is decreasing. This matches the respiratory opposite. So the primary mechanism is respiratory alkalosis. If you look at HCO3-, you notice it is within normal limits. This does not match the metabolic equal or same direction statement. Therefore, the primary mechanism cannot be metabolic.
3. Looking back at the bicarbonate (HCO3-), it is normal. Since it's normal, this means the body is uncompensated.
4. Thus, the answer is uncompensated respiratory alkalosis.

Risks and Complications Associated with ABG Interpretation?

Mastering the art of ABG interpretation helps nurses and healthcare professionals identify underlying conditions and provide insights to make informed decisions about proper treatment. However, interpreting ABG results also comes with potential risks and complications. Inaccurate interpretation can lead to an incorrect diagnosis or treatment regimen and complications. The complication rate after performing an ABG procedure is 0.14%. Some complications of performing an ABG procedure include:

• Nerve damage
• Hematoma
• Aneurysm
• Vasovagal response

Many other complications can arise after performing an ABG. Inaccurate collection of an ABG can lead to inaccurate results. Inaccurate results can lead to misdiagnoses or other serious medical complications. Therefore, it's imperative to receive proper training before performing an ABG. Also, interpreting results inaccurately can lead to the same misdiagnoses or improper treatment. Thus, mastering the art of ABG interpretation is a crucial skill.

Key Considerations When Interpreting Abnormal ABG Results

Before mastering the art of ABG interpretation, a few considerations must be considered. These include:

• Before performing an ABG procedure, you must be trained.
• Practice ABG interpretation by searching Google for an ABG interpretation quiz or ABG interpretation tool.
• When interpreting ABGs, ask yourself which is the predominant disorder.
• Use the ROME (Respiratory Opposite, Metabolic Equal) acronym as an easy trick.
• Look at the patient's overall condition, as specific factors can influence results.
• Acute and chronic respiratory disorders can give varying results based on the patient's baseline.

Mastering the art of ABG interpretation is an essential skill to have as a nurse. ABGs can help healthcare professionals diagnose patients and can guide a patient's treatment plan. You, as a nurse, are a valuable member of your patient's healthcare team. So, practice ABG interpretation as much as possible.