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5+ Arterial Blood Gas (ABG) Tips for the TMC Exam (2025)

by | Updated: Jan 22, 2025

Arterial blood gas (ABG) analysis is a vital component of respiratory therapy and a key topic on the TMC Exam. ABGs provide critical information about a patient’s oxygenation, ventilation, and acid-base status, making it essential to understand how to interpret these results accurately.

Mastering ABG analysis not only helps you pass the exam but also enables you to make informed decisions when managing patient care.

In this guide, we’ll share essential tips and strategies to help you confidently tackle the ABG questions on the TMC Exam, bringing you one step closer to earning your RRT credentials.

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ABG Tips for the TMC Exam

  1. Master ABG interpretation
  2. Remember ABG quality control
  3. Understand capillary blood gases
  4. Understand V/Q mismatch
  5. Know how to optimize a patient’s PaO2

Watch this video or keep reading to learn essential tips and tricks for mastering the ABG questions that you’ll see on the TMC Exam.

1. Master ABG Interpretation

If you’ve made it to this point, it’s safe to say you’ve either graduated from respiratory therapy school or are nearing the finish line. This means you’re already familiar with interpreting arterial blood gases (ABGs)—a skill that’s not only vital for clinical practice but also for passing the TMC Exam.

ABG interpretation is a core competency for respiratory therapists, and you can expect a significant portion of the exam questions to include a patient’s ABG results. In these scenarios, you’ll be required to analyze the ABG values and make appropriate recommendations based on the patient’s condition and ventilator settings.

Here’s a real-world example to show why this skill is so important:

Example Question:

A 66-year-old male patient weighing 80 kg is receiving volume-controlled ventilation in SIMV mode. The ventilator settings are as follows: tidal volume of 350 mL, rate of 10/min, FiO2 of 30%, and PEEP of 5 cmH2O. An ABG sample was collected with the following results:

pH: 7.28
PaCO2: 52 torr
HCO3-: 25 mEq/L
PaO2: 87 torr
SaO2: 95%

Which of the following changes would you recommend?

A. Increase the set rate
B. Increase the tidal volume
C. Increase the level of PEEP
D. Add mechanical dead space

Explanation:

To choose the right answer, you first need to interpret the ABG results. Let’s analyze:

  • pH 7.28 indicates acidosis.
  • PaCO2 52 torr shows elevated CO2, confirming that the acidosis is respiratory in nature (respiratory acidosis).
  • HCO3- 25 mEq/L is within the normal range, indicating no metabolic compensation.
  • PaO2 87 torr and SaO2 95% show normal oxygenation, so no changes to FiO2 or PEEP are necessary.

The primary issue is acute respiratory acidosis, which means the patient’s ventilation is inadequate. To correct this, the goal is to lower the PaCO2 by increasing minute ventilation. There are two main ways to achieve this:

  • Increase the Tidal Volume: Raising the tidal volume increases the volume of air delivered with each breath, thus lowering PaCO2.
  • Increase the Respiratory Rate: Increasing the rate will increase the number of breaths per minute, thereby eliminating more CO2.

Now let’s apply it to this scenario:

  • The patient’s set rate is 10 breaths/min, which is on the lower end of the normal range (typically 10–20/min). Increasing the rate is an option, but not the best one in this case.
  • The set tidal volume is 350 mL, which is significantly below the recommended range for this patient. Given the patient’s weight of 80 kg, the tidal volume should be set between 480–640 mL (6–8 mL/kg of IBW).

Correct Answer: B. Increase the tidal volume.

Increasing the tidal volume to fall within the appropriate range (480–640 mL) will effectively lower the patient’s PaCO2 and correct the respiratory acidosis.

This example highlights the importance of ABG interpretation and applying it to ventilator management. You’ll see several similar questions on the exam that require you to analyze ABG results and make appropriate ventilator adjustments. Make sure to master this skill so you can confidently answer these questions and earn a passing score.

Note: By understanding ABG interpretation and its impact on ventilator settings, you’ll be equipped to handle a wide range of scenarios on the TMC Exam with confidence.

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2. Remember ABG Quality Control

The Levey-Jennings chart is a quality control tool used to monitor the performance of ABG analyzers and ensure that the results they produce are accurate and reliable. As a respiratory therapist, being able to interpret this chart is essential because it helps identify potential issues with the equipment, such as calibration errors or faulty electrodes.

For the TMC Exam, you will likely encounter a question that requires you to interpret a Levey-Jennings chart and determine whether the analyzer is functioning properly. Here’s what you need to know to ace those questions.

The Levey-Jennings chart displays quality control values plotted over time, using a horizontal line to represent the mean and two additional lines to mark the ±2 standard deviation (SD) limits. Each plotted point indicates a quality control measurement, and deviations from the mean help identify potential errors.

Levey-Jennings chart

Types of Patterns and What They Mean

  • In Control: All values are within two standard deviations of the mean. This indicates that the analyzer is functioning correctly and producing accurate results. No action is needed.
  • Random Error: One value falls outside of the two-standard deviation range, but all others are within limits. This may be due to a one-time error such as a sample bubble or transient issue. Monitor for recurring errors, but typically no immediate action is required.
  • Trend: Values are progressively increasing or decreasing but still remain within the two-standard deviation range. This pattern suggests a gradual drift in the analyzer, indicating that it may soon fall out of calibration. Action should be taken to recalibrate the machine before it goes out of control.
  • Shift: All values are within the two-standard deviation range, but the mean has shifted noticeably higher or lower. This sudden change may indicate a new baseline, suggesting a calibration adjustment or maintenance issue that needs attention.
  • Out of Control: Multiple values fall outside of the two-standard deviation range. This is a red flag that the analyzer is no longer reliable and requires immediate troubleshooting. Possible causes include a malfunctioning electrode, improper calibration, or equipment failure. Replace or recalibrate the electrode as needed.

For the TMC Exam, make sure you can recognize these patterns quickly. When you see a Levey-Jennings chart:

  • Determine the Pattern: Look at the distribution of the points relative to the mean and the standard deviation lines.
  • Identify the Type of Error (if any): Determine if the error is a random error, trend, shift, or out of control scenario.
  • Select the Correct Response: Choose the option that correctly identifies the issue and suggests the appropriate corrective action.

Pay close attention to whether values fall inside or outside the two-standard deviation range. This detail often determines whether the analyzer is functioning properly or needs intervention. Also, remember that a trend or shift may still be in control but indicates a potential issue that requires monitoring.

Note: By mastering the Levey-Jennings chart and understanding these patterns, you’ll be well-prepared to tackle quality control questions on the TMC Exam and ensure ABG results are accurate and dependable.

3. Understand Capillary Blood Gases

A capillary blood gas is a diagnostic test that involves collecting a blood sample from an infant to evaluate their ventilatory status. This test measures the pH and PaCO2 (carbon dioxide concentration) in the blood, providing insight into how effectively the neonate is breathing and removing CO2. While similar to an arterial blood gas (ABG), there are important differences you need to understand for the TMC Exam.

A capillary blood gas sample is obtained by pricking the infant’s skin and collecting a small drop of blood from the capillaries. The lateral area of the heel is the preferred puncture site due to its rich vascularization.

If the heel is not accessible, other recommended sites include the fingertip, big toe, or earlobe, but these should only be used if a heel stick attempt is unsuccessful.

Here’s what you need to know about capillary blood gas values:

  • pH and PaCO2: Capillary blood samples correlate well with arterial blood samples for pH and PaCO2 levels, making them effective for assessing the infant’s ventilation and CO2 removal.
  • PaO2 (Oxygenation): The PaO2 from a capillary sample does not provide an accurate measure of oxygenation. Capillary blood gases cannot reliably assess the infant’s oxygenation status because the oxygen levels in capillary blood do not reflect true arterial oxygenation.

The TMC Exam will likely include questions that test your understanding of when to use a capillary blood gas versus an arterial sample. Remember:

  • Use capillary blood gases to evaluate ventilation and CO2 removal (pH and PaCO2).
  • Do not rely on a capillary sample to assess oxygenation (PaO2).

Preferred Puncture Sites for Capillary Blood Gases

  • Primary Site: Lateral area of the heel
  • Alternative Sites: Fingertip, big toe, or earlobe (only if a heel puncture is not feasible)

Always choose the lateral heel for the most effective and accurate sample, as it is highly vascularized and minimizes the risk of injury to bones and nerves.

Capillary blood gases are a valuable tool for evaluating an infant’s ventilatory status, but they should never be used to assess oxygenation. Understanding these differences will help you confidently answer related questions on the TMC Exam and make accurate clinical decisions in real-world settings.

4. Understand V/Q Mismatch

A ventilation/perfusion (V/Q) mismatch occurs when the balance between airflow (ventilation) and blood flow (perfusion) in the lungs is disrupted. This imbalance can result in inadequate gas exchange and oxygenation issues.

The normal V/Q ratio is approximately 0.8, meaning there is slightly less ventilation than perfusion in a healthy lung. Any deviation from this ratio—either high or low—can lead to complications.

High V/Q Ratio: More Ventilation, Less Perfusion

A high V/Q ratio indicates that there is adequate or increased ventilation but reduced perfusion. This occurs when blood flow to a specific area of the lung is decreased or blocked, creating areas of ventilation without corresponding perfusion, also known as dead space.

  • Example: Pulmonary embolism, where a blood clot obstructs blood flow to a portion of the lung. In this scenario, the alveoli are ventilated, but the lack of blood flow results in poor gas exchange.
  • Key Point: High V/Q ratios can lead to inefficient oxygen delivery and CO2 elimination because there’s plenty of air moving in and out of the alveoli, but not enough blood flow to pick up and transport the gases.

Low V/Q Ratio: Less Ventilation, More Perfusion

A low V/Q ratio occurs when there is reduced ventilation but normal or increased perfusion. This happens when airflow into the alveoli is restricted, creating areas of perfusion without effective ventilation. This condition is also referred to as a shunt.

  • Example: Atelectasis, where alveolar collapse leads to inadequate ventilation despite normal blood flow. The blood bypasses the collapsed alveoli without being oxygenated, resulting in hypoxemia.
  • Key Point: Low V/Q ratios are problematic because they result in poorly oxygenated blood being distributed throughout the body, leading to systemic hypoxemia.

Both high and low V/Q ratios can cause refractory hypoxemia—a severe condition in which adequate oxygenation cannot be achieved even with an FiO2 greater than 50%.

In cases of refractory hypoxemia:

  • Problem: Increasing FiO2 is ineffective. The underlying problem is a mismatch between ventilation and perfusion, not a lack of oxygen in the air being inhaled.
  • Solution: Increase PEEP. Positive End-Expiratory Pressure (PEEP) helps by recruiting collapsed alveoli (in low V/Q situations) or redistributing blood flow (in high V/Q situations), improving the overall V/Q balance.

Understanding how to differentiate and respond to high and low V/Q ratios is essential for managing oxygenation issues effectively and will likely appear on the TMC Exam.

Note: By focusing on the primary causes and appropriate treatments, you’ll be well-prepared to handle V/Q mismatch scenarios both on the exam and in clinical practice.

5. Know How to Optimize a Patient’s PaO2

One of the core concepts on the TMC Exam is understanding how to adjust a patient’s PaO2 levels effectively. This requires knowing when to modify the FiO2 versus the PEEP settings on a mechanical ventilator to achieve safe and optimal oxygenation.

Example Scenario:

Imagine a patient was recently intubated and placed on the ventilator with an FiO2 of 100% and a PEEP of 5 cmH2O. After 30 minutes, an ABG is obtained, and you notice the PaO2 is significantly elevated. This means the patient is receiving too much oxygen, and adjustments are needed to lower the PaO2 to safe levels.

How to Decrease PaO2

There are two primary ways to reduce the patient’s PaO2:

  • Decrease the FiO2: This should be your first step when PaO2 is too high. Begin by lowering the FiO2 until it reaches a safe threshold of 60% or below. Reducing FiO2 first helps prevent oxygen toxicity, which can occur with prolonged exposure to high levels of oxygen.
  • Decrease the PEEP: Once the FiO2 is at or below 60%, only then should you consider reducing the PEEP. Decreasing PEEP too early can lead to alveolar collapse and worsening oxygenation.

How to Increase PaO2

When a patient’s PaO2 is too low, the goal is to improve oxygenation by increasing one or both of these parameters:

  • Increase the FiO2: Raise the FiO2 to 60% before making changes to PEEP. This is the fastest way to improve oxygenation and should be your first step when dealing with hypoxemia.
  • Increase the PEEP: After the FiO2 reaches 60%, increase the PEEP level to recruit more alveoli and improve oxygenation. Adding PEEP helps enhance the surface area for gas exchange, stabilizes alveoli, and reduces intrapulmonary shunting.

Always remember this sequence for the TMC Exam: FiO2 adjustments come first for fine-tuning oxygenation, followed by PEEP adjustments to provide longer-term stability.

This knowledge will help you confidently manage questions related to ventilator settings and patient oxygenation, ensuring that you select the safest and most effective option for each clinical scenario.

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Final Thoughts

The ABG section of the TMC Exam can be challenging, but with a solid grasp of the key principles, you’ll be equipped to approach it with confidence.

Focus on understanding normal values, identifying imbalances, and applying clinical reasoning to determine the best course of action for each patient scenario.

If you found these tips helpful, check out our TMC Exam Hacks video course for more expert advice, insider strategies, and proven insights to help you succeed. Best of luck on your journey to becoming a registered respiratory therapist (RRT).

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.

References

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  • Egan’s Fundamentals of Respiratory Care. 12th Edition. Kacmarek, RM, Stoller, JK, Heur, AH. Elsevier. 2020.
  • Mosby’s Respiratory Care Equipment. Cairo, JM. 11th Edition. Elsevier. 2021.
  • Pilbeam’s Mechanical Ventilation. Cairo, JM. Physiological and Clinical Applications. 8th Edition. Saunders, Elsevier. 2023.
  • Rau’s Respiratory Care Pharmacology. Gardenhire, DS. 11th Edition. Elsevier. 2023.
  • Wilkins’ Clinical Assessment in Respiratory Care; Heuer, Al. 9th Edition. Saunders. Elsevier. 2021.
  • Clinical Manifestations and Assessment of Respiratory Disease. Des Jardins, T, & Burton, GG. 9th edition. Elsevier. 2023.

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