Are you ready to learn about Arterial Blood Gases and ABG interpretation? If so, you’re in the right place, because that is what this study guide is all about.
As a Respiratory Therapist (or student), is it critically important to know and understand the ins and outs ABGs. With that said, it’s a topic that many students struggle with — especially when it comes to ABG interpretation.
If that’s you, not to worry. This article is designed to help make that learning process much easier for you.
So if you’re ready, let’s go ahead and dive right in. But first, what exactly is an ABG?
You can now get instant access to this study guide for FREE — no strings attached.
What is an Arterial Blood Gas?
An Arterial Blood Gas, or ABG for short, is a test that measures the blood levels of oxygen (PaO2), carbon dioxide (PaCO2), and acid-base balance (pH) in the body. It’s a test that is used to assess how well oxygen is being distributed throughout the body and how well carbon dioxide is being removed.
Normally, healthy lungs move oxygen into the blood and push carbon dioxide out efficiently during inhalation and exhalation.
This process is referred to as Gas Exchange.
The body is receiving fuel in the form of oxygen while eliminating waste in the form of carbon dioxide. If a patient has breathing problems or a disease that affects the lungs, their ABG results would be abnormal.
This means that their ABG results would fall outside of the normal ranges. So, in order to interpret an arterial blood gas properly, you first must know the normal values.
What are the ABG Normal Values?
For you to better understand the key elements of an ABG test, it is important for you to know the definition of the values that are being measured.
- pH – Refers to the acid-base balance of the blood.
- Partial Pressure of Oxygen (PaO2) – Refers to the amount of oxygen that is in
- Partial Pressure of Carbon Dioxide (PaCO2) – Refers to the amount of carbon dioxide that is in arterial blood.
- Bicarbonate (HCO3-) – Refers to the total amount of CO2 that is transported in the blood.
- Oxygen Saturation (SpO2) – Refers to the amount of hemoglobin in the blood that is saturated with oxygen.
ABG Normal Value Ranges:
- pH: 7.35-7.45
- Partial Pressure of Oxygen (PaO2): 75-100 mmHg
- Partial Pressure of Carbon Dioxide (PaCO2): 35-45 mmHg
- Bicarbonate (HCO3-): 22-26 mEq/L
- Oxygen Saturation (SpO2): 94-100%
Keep in mind that the normal value ranges may vary slightly from one publication to another. With that said, the values that are listed here are the ones we recommend specifically for Respiratory Therapy Students.
ABG Interpretation Made Easy (Step by Step)
Being able to interpret the results of an ABG sample is extremely important for Respiratory Therapists. Your interpretation of the patient’s acid-base status will help determine the best course of action to take when it comes to treating the patient.
Now that you’ve learned the ABG normal values, we can get move on to learn exactly how to interpret ABG results.
Here are the Basic Steps for ABG Interpretation:
Now let’s go through each step and break it all down. If you’re a visual or audile learner, you can watch the video.
Otherwise, just keep scrolling and we’ll walk you through each step.
Step 1: Obtain and Run the ABG Sample
First things first, in order to be able to interpret an ABG, you must collect the actual arterial blood sample from the patient. We discuss how to stick an ABG below, but for now, let’s focus on interpretation.
After you have collected and analyzed the sample, now we can take a look at the results to figure out what’s going on with the patient.
Step 2: Determine if the pH is Alkalosis or Acidosis
The first thing we need to determine if the pH is acidotic or alkalotic. To do this, again, you needed to know the normal value for pH which is 7.35-7.45.
- Acidosis = pH < 7.35
- Alkalosis = pH > 7.45
Examples of pH Interpretation:
- If the pH is 7.26, this is less than 7.35, so the pH is Acidosis.
- If the pH is 7.49, this is greater than 7.45, so the pH is Alkalosis.
- If the pH is 7.39, this falls within the normal range, so the pH is Normal.
Step 3: Determine if the Issue is Respiratory or Metabolic
In this step, we will look at the PaCO2 (carbon dioxide) and the HCO3- (bicarb) to determine if the issue is a respiratory issue or a metabolic issue.
- Carbon Dioxide (PaCO2) is being regulated by the lungs.
- Bicarb (HCO3-) is being regulated by the kidneys.
If the PaCO2 value is abnormal, meaning that it falls outside of the normal range (35-45 mmHg), while the Bicarb value is normal, this would mean that a Respiratory issue is present.
If the PaCO2 value is normal, meaning that it is within the normal range (35-45 mmHg), while the Bicarb value is abnormal, this would mean that a Metabolic issue is present.
- Carbon Dioxide (PaCO2) = Acid
- Bicarb (HCO3-) = Base
- Looking at the pH, we can interpret it as Acidosis since 7.26 is less than 7.35.
- Looking at the PaCO2, we can see that it is increased above the normal range, which is abnormal. This indicates that there is a Respiratory issue.
- Looking at the HCO3-, we can see that it falls within the normal range. This also helps confirm that there is a Respiratory issue.
So now we have confirmed that the pH is Acidosis and we looked a the PaCO2 and HCO3- to determine there is a Respiratory issue.
This ABG can be interpreted as: Respiratory Acidosis
- Looking at the pH, we can interpret it as Acidosis since 7.26 is less than 7.35.
- Looking at the PaCO2, we can see that it falls within the normal range, so this tells us that it is not a Respiratory issue.
- Looking at the HCO3, we can see that it falls below the range, which tells us that a Metabolic issue is present.
So now we have confirmed that the pH is Acidosis and we looked a the PaCO2 and HCO3- to determine there is a Metabolic issue.
This ABG can be interpreted as: Metabolic Acidosis
Step 4: Determine if it’s Compensated or Uncompensated
After identifying whether the blood gas is Acidosis or Alkalosis and whether it’s a Respiratory or Metabolic issue, now we must observe the compensatory component of the ABG results.
Here are two things that you should remember:
- When there is a Respiratory Problem (PaCO2), our body will compensate with Bicarbonate.
- When there is a Metabolic Problem (HCO3-), our body will compensate with Carbon Dioxide.
For example, when we have Respiratory Acidosis the body will try to compensate by increasing the amount of Bicarb in our system.
Bicarbonate is a base, so one of its functions is to try to neutralize the acid that is causing the problem. When we have Respiratory Alkalosis, it’s going to do the opposite by decreasing the amount of Bicarb.
For us to conclude that there is compensation, the increase or decrease in HCO3- has to go outside the normal range. In other words, it has to be lower than 22 or higher than 26.
If the Bicarb value is still within normal limits, you can conclude that there is no compensation going on.
As we have already learned using the previous steps, we can conclude that the pH is Acidosis because it is less than 7.35.
Now we need to identify if there is a Respiratory or Metabolic problem. The PaCO2 is increased above the normal range which indicates that there is a Respiratory issue.
This ABG can be interpreted as: Respiratory Acidosis
Now we need to look at the Bicarb to determine if it’s compensated or uncompensated.
The Bicarb value is 47 which means that the body detected that there was acidosis so it tried to compensate by increasing the amount of base in the system. This tells us that there is definitely is some compensation going on.
Is this a Full Compensation or Partial Compensation?
To answer this question, we need to look back at the pH. Since the pH of 7.29 is outside of the normal range, this means that the compensation was not enough to bring the pH back to normal.
This ABG can be interpreted as: Partially Compensated Respiratory Acidosis
Remember, a partial compensation only occurs in an abnormal pH, because the compensation is not enough to bring the pH back to normal.
For there to be full compensation, the pH would need to be within the normal range.
When we have a metabolic problem, always remember that our Respiratory system will compensate by regulating the amount of carbon dioxide in the blood.
For example, when we have Metabolic Acidosis the body will compensate by decreasing the amount of carbon dioxide. Carbon Dioxide is associated with acidity, so when the body detects that there is acidosis, it will try to compensate by decreasing the amount of carbon dioxide in our system.
When we have Metabolic Alkalosis, our body will do the opposite. It will try to compensate by increasing the amount of carbon dioxide in our system.
For us to conclude that there is compensation, the increase or decrease of carbon dioxide has to go outside the normal range. In other words, it has to be lower than 35 or higher than 45.
If the carbon dioxide level is still within the normal range, you can conclude that there is no compensation going on.
As we have already learned using the previous steps, we can conclude that the pH is Alkalosis because it is greater than 7.45.
Now we need to identify if there is a Respiratory or Metabolic problem. The PaCO2 is increased above the normal range which would typically indicate that there is a Respiratory issue. However, the HCO3- value is also increased.
Remember that the PaCO2 value represents acidity and the HCO3- represents a base. We’ve already decided that the pH is Alkalotic which indicates that there are more bases (HCO3-) in the blood.
This ABG can be interpreted as: Metabolic Alkalosis
Since we have a Metabolic problem, the next step is to look at the respiratory system. In this case, we see that the carbon dioxide (PaCO2) is increased above the normal range which tells us that there is some compensation going on.
Is this a Full Compensation or Partial Compensation?
To answer this question, we need to look back at the pH. Was the compensation enough to bring the pH back to normal?
The answer is no, so this indicates that there is only partial compensation. If the pH would have been within the normal range, then it would be a full compensation.
This ABG can be interpreted as: Partially Compensated Metabolic Alkalosis
Oxygen Saturation and Hypoxemia Levels
The final step of ABG Interpretation is to look at the patient’s oxygenation status to determine if hypoxemia is present. For this, you need to look at the PaO2 value, or Partial Pressure of Arterial Oxygen.
Here are the levels of hypoxemia when classified into categories of either normal, mild, moderate, or severe.
80 – 100 mmHg
60 – 79 mmHg
90 – 94%
40 – 59 mmHg
75 – 89%
< 40 mmHg
ABG Interpretation Calculator:
Use this handy ABG Interpretation Calculator as an easy way to practice interpreting ABG results. This calculator was built for practicing and is not intended for medical use.
ABG Calculator (Arterial Blood Gas Values)
ABG Calculator (Arterial Blood Gas Values)
Medical Disclaimer: This calculator is not intended to be used for professional medical advice, diagnosis, or treatment. The authors make no claims of the accuracy of the information contained herein. Please consult with a physician with any questions that you may have regarding a medical condition. This calculator is for educational purposes only — not for medical use. All calculations must be confirmed before use.
Why are ABGs Important?
An ABG test is routinely used in the diagnosis and monitoring of patients who are suffering from critical conditions. Because this test provides a precise measurement of the levels of oxygen and carbon dioxide in the blood, it can help medical professionals determine the status of the patient’s lung and kidney function.
In most cases, the physician may order an Arterial Blood Gas for the following symptoms:
- Breathing difficulties
- Changes in mental status
- Nausea or vomiting
In addition, an ABG test can help with the following:
- To assess whether treatments for lung conditions are effective
- To check for chemical poisoning
- To check the acid-base balance in patients with kidney disease, diabetes, and those recovering from a drug overdose
- To determine the presence of a ruptured blood vessel or metabolic disease.
How to Stick an ABG?
An ABG test requires collecting a small sample of blood from an artery. The sample must be obtained by a Respiratory Therapist, doctor, or skilled technician.
ABG Sample Sites:
- Wrist (radial artery)
- Upper arm (brachial artery)
- Groin (femoral artery)
In addition, a blood sample can also be obtained from a pre-existing arterial line. An ABG blood sample cannot be obtained from a vein, as this would instead be a VBG or venous blood gas.
Once the site is determined, the healthcare provider will sterilize the injection site using an antiseptic or antimicrobial solution. The patient will be then positioned either lying down or sitting with the arm well supported.
The healthcare provider may use a rolled towel positioned under the patient’s wrist in order to provide comfort and hyperextend the site of injection. This position also makes it easier to palpate the pulse.
After the radial artery is located, the
In some cases, the syringe needs to be repositioned in order to locate and puncture the artery. When doing this, the healthcare provider will withdraw the tip of the syringe to the subcutaneous tissue to prevent severing the artery or tendons and avoiding damage to the nearby tissues.
The blood sample will be immediately sent to the laboratory for analysis. The specimen must be analyzed within 10 minutes after extraction in order to ensure accurate ABG results.
It is important to keep in mind that an ABG test may be difficult to perform in uncooperative patients, those with hard-to-find pulses, and patients with cognitive impairment, tremors, and a significant amount of body fat.
In some cases, multiple attempts are needed in order to draw a blood sample. With that said, repeated puncture of a single site increases the prevalence of a hematoma (swelling of clotted blood within the tissues) and scarring.
In severe cases, it can also cut the artery and cause a significant amount of bleeding. In cases such as this, the healthcare provider may need to use alternate sites in order to draw a blood sample. Therefore, collecting a blood sample for an ABG test can be quite challenging for some Respiratory Therapists.
But as I always say, practice makes perfect. The more you do it, the easier it will become and the better you’ll get at sticking ABGs.
What are the Causes of Respiratory Acidosis?
Respiratory Acidosis occurs when your lungs cannot remove all of the carbon dioxide that has formed in your body. As a result, the blood and other body fluids become too acidic.
When carbon dioxide mixes with water in the body, it produces carbonic acid. If left untreated, long-term Respiratory Acidosis causes the body to compensate by increasing the excretion of carbonic acid while retaining bicarbonate base in the kidneys.
The acidifying effect of long term elevated carbon dioxide levels can be lessened in the blood. However, this effect is not lessened in the brain. As a result, the patient can suffer from sleeping difficulties, headaches, memory problems, anxiety, and mood changes.
Causes of Respiratory Acidosis:
- Breathing problems
- Cardiac arrest
- Lung disorders such as chronic obstructive pulmonary disease (COPD), emphysema, asthma, or pneumonia.
- Neuromuscular disorders that affect the muscles of the airways (e.g. Multiple sclerosis, muscular dystrophy, or Guillain–Barré syndrome).
- Obstruction of the airways
- Sedative overdose
- Severe obesity (affects lung expansion)
What are the Potential Errors When Running an ABG?
There are several factors that can affect the results of an ABG test. Here are some of the primary examples:
Drawing the blood sample from the incorrect patient
This can significantly alter the course of treatment of a critical patient. This can be caused by posting the ABG results on the incorrect patient record, or mislabeling the blood sample.
Failure to obtain a blood sample from an artery or vein
In some cases, inexperienced healthcare providers might stick the vein instead of the artery. In this case, the sample will be filled with venous blood instead of arterial blood, which will show vastly different results.
It is highly recommended to analyze the blood sample 10 minutes after extraction in order to avoid clotting. Analyzing a blood sample that has already clotted will yield inaccurate results and will basically render the specimen useless.
Obtaining a blood sample on incorrect settings or support
This can significantly affect the course of treatment for the patient and the medical team’s assessment of their health needs.
For instance, if a Respiratory Therapist obtained a blood sample when the patient is still on supplemental oxygen instead of room air, the results would be incorrect. This can yield falsely elevated PaO2 levels.
Air contamination of the blood sample
Air contamination can alter the results of an ABG by causing the measured PaO2 of the patient to drop towards the PaO2 of room air.
Contamination caused by too much heparin
Too much liquid heparin dilutes the blood sample and causes changes in pH levels and can significantly affect the oxygen and carbon dioxide values.
Inappropriate mixing of the blood sample
Depending on the hospital or laboratory protocol, healthcare providers thoroughly mix the blood sample with heparin immediately upon collection in order to avoid clotting. It’s also remixed before it goes into the analyzer.
The best way to mix the sample is to roll it between your palms. The most common error that healthcare providers commit when mixing the blood sample is vigorously shaking the vial or container. Another error is not mixing iced samples for a long enough amount of time. It is recommended to mix iced samples longer in order to promote mobilization and mixing of all the components of the blood sample.
Prolonged delays in blood sample analysis
The blood sample must be sent to the laboratory for analysis no longer than 10-15 minutes after the blood was drawn. Any delay in blood sample analysis causes changes in the PaO2 and PaCO2 levels due to continuous red blood cell metabolism.
What are the Indications for Sticking an ABG?
- To assess the patient’s response to treatment, such as mechanical ventilation
- To determine a patient’s oxygen-carrying capacity
- To determine the need for supplemental oxygen
- For the diagnosis of respiratory, metabolic, and mixed acid-base disorders
- To monitor the patient’s acid-base status
- To collect a blood sample in emergency situations when access to the vein is not possible
- For the quantification of hemoglobin levels
What are the Contraindications for Sticking an ABG?
Not all patients are potential candidates for an ABG test. Here are the contraindications for when you would not want to stick an arterial blood gas:
- An abnormal Modified Allen Test
- Blood clotting problems
- Local infection or damage at the injection site
- Patients who are receiving anticoagulation therapy
- Patients who are taking thrombolytic agents
- The presence of a disease that affects the blood vessels
- The presence of arteriovenous fistulas or vascular grafts
What is the Modified Allen Test?
The Allen Test was originally developed by Edgar V. Allen in 1929 as a non-invasive method of assessing the patency of a patient’s arteries. Since then, it has now been adopted as the Modified Allen Test.
The difference between the original Allen Test and the modified version is that the Modified Allen Test efficiently evaluates the adequacy of blood circulation in one hand at a time. In contrast, the original Allen test compresses one artery of each hand at the same time.
The Modified Allen Test measures the competency and quality of the artery and should be performed prior to performing an arterial blood gas stick.
Here are the Steps for Performing a Modified Allen Test:
1. Have the Patient Make a Fist
Instruct the patient to clench his or her fist in order to enhance the circulation within the arteries. If the patient lacks the ability to do so, you can close his or her hand manually.
2. Locate the Radial and Ulnar Arteries
Face the patient and locate the radial and ulnar arteries. The radial artery is located on the thumb side of the wrist on the underside of the forearm, while the ulnar artery is on the pinky side of the wrist. Make sure to locate both the radial and ulnar pulses.
3. Grab the Patient’s Hand
Using your right hand, slowly grab your patient’s left hand. You can also use your left hand to grab your patient’s right hand, depending on your preference.
4. Locate the Pulse
Place your middle finger on top of the radial pulse and your pointer finger on the ulnar pulse of the patient.
5. Apply Pressure to Both Arteries
When the pulses can be felt, apply occlusive pressure to both the ulnar and radial arteries to temporarily stop blood circulation of the hand. Be sure to tell the patient to relax his or her hand while performing this step.
6. Have the Patient Open Their Hand
This is done to check whether the palm and fingers have blanched. Blanching means that you have completely occluded the radial and ulnar arteries with your fingers. The hand should now have a white-ish appearance in color.
7. Slowly Release the Pressure on the Ulnar Artery
You can release pressure on the ulnar artery while keeping the radial artery occluded. If the patient’s hand flushes (meaning that it turns pink) within 5-15 seconds, this means that the ulnar artery is patent or has a good blood flow.
This indicates that collateral circulation is present and you may proceed to stick the radial artery of that hand.
Now that we’ve covered pretty much everything there is to know about arterial blood gases and ABG interpretation, let’s take it a step further by going through some practice questions on this topic.
Doing so helps to reinforce the information into your brain which makes it easier to learn and memorize for future use. First, we’ll break down a sample TMC Practice Question on the topic of arterial blood gases.
Example TMC Practice Question:
An arterial blood gas sample was obtained from a 38-year-old female patient:
PaCO2 22 torr
HCO3 12 mEq/L
PaO2 111 torr
The results display which of the following?
A. Acute metabolic alkalosis
B. Partially compensated metabolic acidosis
C. Partially compensated respiratory alkalosis
D. Acute respiratory acidosis
This is just a classic ABG interpretation question. You likely won’t see many of these on the TMC Exam because at this point, the NBRC will assume that you already know how to interpret ABGs. Otherwise, you wouldn’t have made it this far.
With that said, you still absolutely MUST know how to interpret them because you will be required to do so for several questions on the exam.
So now let’s go ahead and interpret this one. The pH is decreased which means acidosis. The Bicarb and Base Excess levels are severely decreased, which tells us that there is a metabolic issue.
The PaCO2 is decreased which tells us that the patient is trying to compensate by hyperventilating. But, since the pH is still outside of the normal range, there is only partial compensation.
The correct answer is: B. Partially compensated metabolic acidosis
This practice question was actually taken from our TMC Test Bank, which is a massive bank of premium practice questions that cover every topic that students must know in order to pass the TMC Exam.
As you can see, each question comes with a detailed rationale that explains exactly why the answer is correct. This is critical when it comes to learning the information that’s required for you to know.
If you’re interested in getting access to more practice questions like this one, definitely consider checking it out.
ABG Practice Questions and Problems:
1. What are the causes of Respiratory Acidosis?
Some of the causes include the build-up of CO2 in the blood, hypoventilation, and increased dead space.
2. How can you correct Respiratory Acidosis?
You can increase the number of breaths per minute, increase the size of the breaths, or decrease dead space.
3. What are the causes of Respiratory alkalosis?
A low amount of CO2 in the blood, hyperventilation, pain, and anxiety.
4. How can you correct respiratory alkalosis?
You can decrease the number of breaths per minute, give medication for pain, and treat the anxiety.
5. What are the causes of metabolic acidosis?
A low amount of Bicarb in the blood, diarrhea, aspirin toxicity, diabetes, and renal failure.
6. How can you correct metabolic acidosis?
You can correct metabolic acidosis by stopping or correcting whatever is causing the issue. You can also provide medication for diarrhea, and treat the renal failure.
7. What are the causes of metabolic alkalosis?
An increase in Bicarb in the blood, vomiting, NG suctioning, and the ingestion of sodium Bicarb.
8. How can you correct metabolic alkalosis?
You correct it by stopping or correcting the vomiting, discontinue NG suctioning, and stop the sodium Bicarb ingestion.
9. What is oxygenation?
Oxygenation is represented by the PaO2 values and it is measured only of the oxygen dissolved in plasma.
10. What is the first step before doing an ABG?
The first step is to check the patient’s chart to confirm the doctor’s order.
11. What is the preferred site for an ABG in adults?
The Radial artery the preferred site.
12. What is the longest time an ABG sample could go (without ice) without being analyzed?
13. What test is performed to confirm collateral circulation before doing an ABG stick?
The Modified Allen Test.
14. What is an adequate amount of blood for an ABG sample?
2 –4 milliliters of blood is enough for a sample.
15. What are some causes of metabolic acidosis?
Some causes include Diarrhea, starvation, and diabetic ketoacidosis.
16. What are the 3 major hazards of an arterial puncture?
The 3 major hazards include Bleeding, obstruction of the vessel, and infection.
17. What are the three major criteria for the selection of the arterial puncture site?
Collateral blood flow, vessel accessibility, and peripheral structures.
18. What is collateral blood flow?
Collateral blood flow helps to prevent loss of distal blood flow in the event of an arterial obstruction.
19. What does vessel accessibility mean?
The best vessel for an arterial puncture is one that is easy to palpate, relatively superficial, and somewhat stable.
20. What are peripheral structures?
Peripheral structures are referring to something that is easy to get to. In this case, we’re referring to the radial artery in regards to sticking an ABG.
21. How to perform the Modified Allen test?
To perform this test, you need to Elevate the patient’s hand and make a fist for 20 seconds, hold firm pressure against the radial and ulnar arteries, the patient then opens the hand and it should blanch white, the examiner releases only the ulnar compression, and you should see the hand pinken up which indicates that collateral circulation is present.
22. What are alternative methods of assessing for collateral circulation?
Doppler ultrasound and pulse oximetry.
23. What would you look at if you wanted to determine the oxygenation status of a patient?
You would look at their PaO2.
24. An increase in CO2 causes the pH levels to become what?
The pH becomes acidic.
25. A patient comes in with a pH of 7.52, a PaCO2 of 25, a Bicarb of 25, and a base excess of +1. What would be the interpretation of this blood gas?
The interpretation is Respiratory Alkalosis.
26. A patient has the following ABG results: A pH of 7.1, a PaCO2 of 20, Bicarb of 10, and a Base Excess of -20. What is your interpretation of this ABG?
The interpretation is Metabolic Acidosis.
27. What do you look at in a blood gas to determine ventilation?
You would look at the PaCO2.
28. Can a blood gas be considered normal if the Base Excess is NOT within the normal limits?
No; everything must be within normal limits for the blood gas to be considered normal.
29. If you get a gas and the pH is within normal range, and the CO2 and Bicarb are moving in the same direction, how would you first classify the gas?
It can be classified as being Fully compensated.
30. If you get an ABG and it reads: pH is 7.56, CO2 is 42, Bicarb is 34, and Base Excess is +5. How would you interpret this gas?
The interpretation of this ABG is Acute or Uncompensated metabolic alkalosis.
31. At what pH should we intubate the patient?
We would generally want to intubate at a pH of 7.2 or below.
32. Severe hypoxemia is classified as a PaO2 of less than what?
Less than 40.
33. What is a normal range for PaO2 on room air?
The normal range is 80 –100.
34. What is the range of moderate hypoxemia?
Moderate hypoxemia is the range of 40 – 59.
35. You collect an ABG and it reads: pH is 7.42, CO2 is 43, Bicarb is 25, and Base Excess is +2. How would you classify this gas?
This is a normal ABG.
36. If the pH decreases below 7.35, then it is considered to be what?
It is Acidotic.
37. If the pH is above 7.45, it is considered to be what?
It is Alkalotic.
38. If the patient is hypoventilating, their CO2 will do what?
It will increase.
39. If the patient is hyperventilating, their CO2 would do what?
It would decrease.
40. What are the drugs that can cause a low pH?
Narcotics, barbiturates, acetazolamide, ammonium chloride, and paraldehyde.
41. What are the drugs that can cause an elevated pH?
Sodium bicarbonate, sodium oxalate, and potassium oxalate.
42. The primary goal of acid-base homeostasis is to maintain what?
To maintain a normal pH.
43. What are some potential causes of Respiratory Alkalosis?
The potential causes are Anxiety, Hypoxemia, and Pain.
44. Which organ system maintains the normal level of Bicarb at 24 mEq/L?
The renal system.
45. What is the limiting factor for hydrogen excretion in the renal tubules?
46. What acts as the “first-line” or immediate defense against the accumulation of Hydrogen ions?
Blood buffer systems.
47. A primary respiratory problem is determined by what?
It is determined if the PaCO2 is less than 35 or greater than 45.
48. When does a primary metabolic problem occur?
It occurs when the Bicarb is less than 22 or greater than 26.
49. If a patient has a pH of 7.49, what would this define?
This would define Alkalosis.
50. What are the common sites for a transcutaneous blood gas electrode?
Chest, abdomen, and lower back.
51. What are the sites used for Arterial Blood sampling by percutaneous needle puncture?
The sites include the Femoral, radial, and brachial arteries.
52. Before a sample of capillary blood is taken, what should you do to the site?
You should Warm the site to 42 degrees Celsius and clean it with an antiseptic solution.
53. A mechanically ventilated patient exhibits a sudden decrease in end-tidal CO2 levels. What are possible causes of this change?
Massive pulmonary embolism, disconnection of the ventilator, and a sudden drop in cardiac output.
54. Factors to determine the volume need for an arterial blood sample include?
They include the ABG analyzer’s requirements, specific anticoagulant used, and the other tests that will be done.
55. After obtaining an arterial blood sample, what should you do?
You should apply pressure to the site until bleeding stops, then place the sample in a transport container with an ice slush, then mix the sample by rolling and inverting the syringe.
56. When is transcutaneous blood gas monitoring indicated?
It is indicated when the need to continuously analyze gas exchange in infants and children, to quantify the real-time responses to bedside interventions, and to continuously monitor for hyperoxia in newborn infants.
57. What size needle would you recommend to obtain an ABG sample on an infant?
You would recommend a size 25 gauge needle for an infant.
58. What are the indications for arterial blood sampling by percutaneous needle puncture?
Monitor the severity of a disease process, evaluate ventilation and acid-base status, and evaluate a patient’s response to therapy.
59. After obtaining an arterial blood sample from an arterial Line, what should you do?
You should flush the line and stopcock with heparinized intravenous solution, confirm that the stopcock port is open to the intravenous bag solution and catheter, and then confirm an undamped pulse pressure waveform on the monitor.
60. What patient parameters should be assessed as part of arterial blood sampling?
The temperature, position and activity level, and clinical appearance should be assessed.
61. What are the clinical signs of acute respiratory alkalosis?
The signs of acute respiratory alkalosis include Convulsions, dizziness, and paresthesia.
62. A low PaCO2 best describes what?
It best describes Respiratory Alkalosis.
63. With partially compensated respiratory alkalosis, which of the following blood gas abnormalities would you expect to encounter?
You would expect a decreased Bicarb, decreased PCO2, and increased pH.
64. What are the causes of Respiratory Acidosis in patients with normal lungs?
Neuromuscular disorders, spinal cord trauma, anesthesia, and central nervous system depression.
65. Before connecting the sample syringe to an adult arterial line stopcock, what should you do?
You should first aspirate at least 5 mL of fluid or blood using a wasted syringe.
66. What is the equipment needed for capillary blood sampling?
For capillary blood sampling, you need a Lancet, capillary tubes, and a warming pad.
67. When is capillary blood gas sampling indicated?
It is indicated when an ABG analysis is needed but arterial access is not available.
68. The compensation for metabolic acidosis occurs through what?
It occurs through a decrease in blood CO2 levels.
69. What are the causes of Metabolic Alkalosis?
The causes of metabolic acidosis include: Diuretics, hypochloremia, and vomiting.
70. What clinical findings would you expect in a fully Compensated Respiratory Acidosis patient?
An elevated Bicarb and a pH between 7.35 and 7.39.
71. What is the normal pH range?
The normal pH is 7.35-7.45
72. What is the normal range for carbon dioxide?
The normal range for PaCO2 is 35-45.
73. What is the normal range for Bicarb?
The normal range for Bicarb is 22-26.
74. What is the normal range for base excess?
The normal range for base excess is -2 – +2.
75. What type of issues are we looking for when we look at the Bicarb and base excess values?
Looking at these values means that we’re looking for Metabolic issues.
76. What type of issues are we looking for when we look at the PaCO2 values?
This would mean that we are looking at the patient’s Ventilation status.
77. What does the PaO2 measure?
It measures Oxygenation status.
78. What range of PaO2 is considered normal on room air?
A PaO2 of 80–100 is considered normal on room air.
79. What ABG value would we look for in patients that currently smoke or have smoked heavily in the past?
We would want to look at the percentage of Methemoglobin.
80. What ABG value would we look for in patients that have carbon monoxide poisoning or have been in a fire?
We would want to look at the percentage of Carboxyhemoglobin.
81. In a given ABG, if the pH and CO2 values are going in DIFFERENT directions, what is this ABG considered to be?
In this case, we would know that the ABG issue is Respiratory-related.
82. In a given ABG, if the pH and Bicarb values are going in the SAME direction, what is this ABG considered to be?
In this case, we would know that the ABG issue is Metabolic-related.
83. When interpreting a given ABG, what values must be abnormal for it to be considered “partial”?
For is to be considered partial, all values must be abnormal.
84. When interpreting a given ABG, what values must be normal for it to be considered “uncompensated”?
For it to be considered uncompensated, either the CO2 or HCO3 must be normal.
85. When interpreting a given ABG, what value must be normal for it to be considered “compensated”?
For it to be considered compensated, the pH must be normal.
86. How do you prevent pre-analytical errors in ABG samples?
To prevent pre-analytical errors, make sure the sample is obtained anaerobically, properly anticoagulated, bubbles removed, and analyzed as quickly as possible.
87. How is CO2 transported?
45-55 milliliters of CO2 per 1 deciliter of blood is transported by either ionized bicarb, dissolved in plasma, or plasma protein transport.
88. How much blood is needed for an adequate ABG sample?
2 –4 milliliters of blood is enough for a sample.
89. For accurate ABG results, what are the components of quality control?
The components are recordkeeping, performance validation, preventative maintenance and function checks, automated calibration and verification, internal statistical quality control, and external quality control.
90. What are the reasons for drawing an ABG?
The reasons for drawing an ABG are: Sudden unexplained dyspnea, acute shortness of breath or tachypnea, abnormal breath sounds, cyanosis, heavy use of accessory muscles, changes in ventilator settings, CPR, diffuse infiltrates in the chest x-ray, sudden cardiac arrhythmias, and acute hypotension.
91. What does the blood gas machine accuracy depend on?
It depends on accurately measuring barometric pressure, properly calibrating machine-running measurements against known values, maintaining electrodes, and running quality control procedures.
92. Inadequate warming and squeezing of the puncture site does what?
Squeezing the puncture site may result in venous and lymphatic contamination of the sample.
93. What are secondary values to ABGs that need to be calculated?
The secondary values include: Bicarbonate or HCO3, Base Excess, and Hemoglobin saturation.
94. What are the benefits of indwelling catheters?
Indwelling catheters can provide ready access for blood sampling. Also, they allow for continuous monitoring of vascular pressures.
95. What are the site locations for indwelling catheters?
The answer is: The normal routes are the peripheral arteries, the femoral artery, the central vein, and the pulmonary arteries.
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96. What can a good capillary blood gas sample provide and reflect?
The answer is: It can provide the estimated arterial oxygenation and PCO2.
97. What can be used if frequent blood sampling is needed?
98. What is hemoximetry?
Hemoximetry is the laboratory analytical procedure of monitoring hemoglobin and oxygen saturation that requires invasive sampling of arterial blood.
99. What determines the ventilation status of a patient?
The PaCO2 determines the patient’s ventilatory status.
100. What is the rule of thumb in regards to the PaCO2 and FiO2?
The answer is: The PaCO2 should be about 5 times the FiO2.
101. Does oxygenation decrease with age?
Yes, oxygenation does decrease with age.
102. What happens when an ABG is partially compensated?
In this case, The pH is out of range and the CO2 and Bicarb are going to the same direction.
103. Which vessel some sometimes accidentally stuck when attempting an arterial stick?
Sometimes inexperienced practitioners will accidentally stick the vein instead of the artery.
104. What is the normal value for Carboxyhemoglobin?
The normal value is less than 3 percent.
105. What does the Carboxyhemoglobin indicate?
When Carboxyhemoglobin is present, it indicates that the patient was exposed to carbon monoxide or a house fire.
106. What is the normal value for Methemoglobin?
The normal value for Methemoglobin is less than 2 percent.
107. What does the Methemoglobin indicate?
It indicates if the patient is a smoker or not.
108. Why do we analyze ABGs?
ABG analysis gives important information to assist in the clinical management of patients with respiratory and metabolic problems.
109. What does the pH represent?
The pH represents a measurement of the overall acid-base balance and is used to assess the overall status of the blood.
110. What does the PaCO2 represent?
The PaCO2 represents the arterial carbon dioxide level and is used to assess the patient’s ventilatory status.
111. What does the PaO2 represent?
The PaO2 represents the oxygen tension level in the arterial blood and is used to evaluate the patient’s oxygenation status.
112. What does the HCO3 represent?
The HCO3 represents the bicarbonate level which is an important buffer in the blood. It is used to evaluate the metabolic aspect of acid-base balance.
113. What does BE represent?
BE represents the Base Excess level of the blood and is used to indicate the metabolic aspect of acid-base balance.
114. What does the SaO2 represent?
The SaO2 represents the level of saturation of hemoglobin with oxygen and also provides a measure of arterial oxygenation.
115. What is compensation?
Compensation is the altering of function of the respiratory or metabolic system in an attempt to correct for an acid-base disorder.
116. What is Hypoxemia?
Hypoxemia means that there are low levels of oxygen in the blood.
117. What is the relationship between minute ventilation and ABG interpretation?
As the minute ventilation increases, the PaCO2 will decrease and pH will increase. This is Alkalosis. As the minute ventilation decreases, the PaCO2 will increase and the pH will decrease. This is Acidosis.
118. What are the ABG indications?
To monitor ABG values, to evaluate response to therapeutic or diagnostic procedures, and to monitor disease progression or severity.
119. Describe the femoral artery in regards to an ABG stick?
The femoral artery is a very large artery. It is a risky stick because of the surrounding veins and arteries.
120. Describe the radial artery in regards to an ABG stick?
The radial artery is the preferred site to stick an ABG. It has good collateral circulation, it is superficial and easy to palpate which is why it is the usually the best artery for sticking. Also, it’s not near any large veins, and the stick is relatively pain-free . But, I mean, you won’t see me volunteering to have my radial artery stuck any time soon.
121. Describe the brachial artery in regards to an ABG stick?
The brachial artery is a risky stick because it is near nerves and large veins. Also, there is no collateral circulation with this stick, and there is an increased risk for accidentally obtaining a venous sample.
122. What are the contraindications for an ABG stick?
The patient had blood clotting problems. The patient has a local infection or damage at the injection site. The patient is on anticoagulation therapy. The patient is taking thrombolytic agents. The patient has a disease affecting the blood vessels. And the patient has arteriovenous fistulas or vascular grafts.
123. What are the indications for an arterial line?
The indications for an arterial line include continuous arterial blood pressure monitoring, and/or the need for repeated ABGs.
124. The capillary blood gas is an alternative to what?
A capillary blood gas is sometimes an alternative to the ABG procedure.
125. The capillary blood gas gives what in regards to an arterial blood gas?
It gives a rough estimate of the pH and PaCO2. However, the PO2 is of no value for estimating oxygenation because venous blood does not carry oxygen.
126. How is the ABG procedure done?
An ABG is done by sticking an artery with a syringe in order to collect a sample of arterial blood.
127. What are some hazards of an ABG?
Infection, bleeding, and obstruction of the vessel.
128. What are the sites for a capillary blood gas?
The site include the heel of the foot, fingertip, and earlobe.
129. ABG samples provide what?
They provide precise measurements of Acid-Base balance and the lung’s ability to oxygenate the blood and remove carbon dioxide.
130. An accurate interpretation of an ABG requires what?
It requires knowledge of patient’s total clinical picture including any treatment that they are receiving.
131. Where are mixed venous blood samples drawn?
They are drawn from the right atrium or from the pulmonary artery.
132. What is a mixed venous blood sample used for?
It is used to evaluate the overall tissue oxygenation.
133. Prior to an ABG draw, what should the Respiratory Therapist review in the patient’s chart?
The RT should look for a low platelet count or increased bleeding time.
134. What must be evaluated prior to a radial stick?
You should check for collateral circulation of the hand via the modified Allen’s test.
135. How is the modified Allen’s test performed?
Have the patient make a tight fist, then the Respiratory Therapist compresses both the radial and ulnar arteries, then instruct the patient to open their hand and relax; the Respiratory Therapist then releases the ulnar artery to check to see if the hand turns pink. If so, this indicates good collateral circulation because blood is flowing back into the hand through the ulnar artery.
136. What is a positive Modified Allen’s test?
For a positive Modified Allen’s Test, the hand turns pink within 10-15 seconds after you release the ulnar artery. This means circulation is adequate for the puncture site.
137. What should the Respiratory Therapist do if the Modified Allen test is negative?
In this case, the RT should try the other arm. If it’s negative as well, you can then try the brachial artery.
138. What should the Respiratory Therapist do for a patient who needs frequent ABG’s?
In this case, they should recommend the insertion of an indwelling arterial catheter.
139. What do bubbles in the ABG sample do?
Bubbles in the sample can cause the oxygen reading in the results read inaccurately.
140. How should the Respiratory Therapist handle an ABG sample after it has been drawn?
The answer is: They should remove any air bubbles, store in ice water to stop metabolism, and analyze as soon as possible.
141. Room temperature samples must be analyzed how soon?
Room temperature samples must be analyzed within 10-15 minutes.
142. How long should pressure be applied to a stick wound?
Pressure should be applied for at least 3-5 minutes, or longer if the patient has a clotting problem.
143. What is the kidney’s role in acid-base balance?
The kidney’s role is to remove small quantities of acid and restore the buffer capacity of fluids by replenishing Bicarb.
144. What is the description of Base Excess values?
A positive value indicates either a base has been added or a buffer removed; the larger the number, the more severe the metabolic component.
145. What is the importance of Base Excess?
Base Excess allows for the analysis of pure metabolic components of acid-base balance, changes in metabolic components alter acid-base, whereas respiratory components do not.
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146. Do changes in CO2 effect the Base Excess?
The answer is No. Only metabolic changes alter the Base Excess.
147. What are the common causes of respiratory acidosis?
The causes of respiratory acidosis include: Acute upper airway obstruction, severe diffuse airway obstruction, and massive pulmonary edema.
148. What are the common non-respiratory problems that can cause respiratory acidosis?
A drug overdose, spinal cord injury, neuromuscular diseases, head trauma, and trauma to thoracic cage.
149. How can you describe fully compensated respiratory acidosis?
There is enough Bicarb to bring the pH within the normal range.
150. If the expected level of Bicarb compensation is not occurring for acute or chronic acidosis, what should the Respiratory Therapist suspect?
In this case, the Respiratory Therapist should suspect that a complicating metabolic disorder is also present.
151. In acute respiratory acidosis, how high does the CO2 have to get for the patient to reach a comatose state?
Once the CO2 levels reach 70, the patient may lose consciousness.
152. Because CO2 causes systemic vasodilation, what cardiac manifestations should be expected?
Warm flush skin, a bounding pulse, and arrhythmias should be expected in this case.
153. How do you identify respiratory alkalosis in an ABG?
The PaCO2 is below the expected level indicating that the ventilation is exceeding the normal level.
154. What are the common causes of respiratory alkalosis?
The common causes include Hyperventilation caused by pain, hypoxemia, and anxiety.
155. How do the kidneys compensate for respiratory alkalosis?
They excrete Bicarb.
156. What are the clinical signs and symptoms associated with respiratory alkalosis?
Tachypnea, dizziness, sweating, tingling in fingers and toes, muscle weakness, and spasms.
157. When does a Respiratory Therapist need to be cautious not to induce respiratory alkalosis?
The RT needs to be especially cautious during IPPB and mechanical ventilation.
158. How does the body compensate for metabolic acidosis?
It compensates with Hyperventilation.
159. What is the most common and obvious sign of metabolic acidosis?
160. What are the most common causes of metabolic alkalosis?
Hyperkalemia, hypochloremia, NG suctioning, vomiting, post-hypercapnic disorder, diuretics, steroids, or too much Bicarb.
161. How does the body compensate for metabolic alkalosis?
The body compensates hypoventilation to retain carbon dioxide.
162. What do ABG results determine?
They are used to determine the oxygen level, acid-base balance, and are very useful in the management of mechanical ventilation.
163. What does the pH measure?
The pH measures the state of the blood; either acidic or basic.
164. What does the PaCO2 measure?
The PaCO2 measures the partial pressure of carbon dioxide in the blood.
165. What does the PaO2 measure?
The PaO2 measures the partial pressure of oxygen in the blood.
166. What does the HCO3 measure?
The HCO3 measures the concentration of bicarbonate in the blood.
167. What three ways do we classify an ABG?
The ABG is either Normal, acidosis, or alkalosis.
168. What two types do we classify as the primary problem of an ABG?
The ABG problem is either Respiratory related or metabolic related.
169. Which of the parameters is the respiratory component?
The PaCO2 in the respiratory component.
170. Which of the parameters is the metabolic component?
The Bicarb is the metabolic component.
171. What happens to the pH when there is an increase in hydrogen ions?
In this case, the pH will decrease and become more acidotic.
172. What happens to the pH when there is a decrease in hydrogen ions?
In this case, the pH will increase and become alkalotic.
173. Under what range of the pH will a patient have to be intubated?
The patient will need to be intubated when their pH is less than 7.2
174. If the pH and PaCO2 are going in opposite directions, what does this indicate?
This indicates that there is a respiratory problem.
175. If the pH and Bicarb are going in the same direction, what does this indicate?
This indicates that there is a metabolic problem.
176. What type of compensation is indicated when the pH, PaCO2, and Bicarb are all out of range?
This indicates that the ABG is Partially compensated.
177. What type of compensation is indicated when either the pH or PaCO2 is out of range?
This indicates that the ABG is Uncompensated.
178. What type of compensation is indicated when the pH is normal and the PaCO2 and Bicarb are both out of range?
This indicates that the ABG is Fully compensated.
179. What does the SaO2 measure?
It measures the percentage of oxygen saturation of arterial blood.
180. What two things are used to determine the accurate percentage of the methemoglobin and the carboxyhemoglobin?
An ABG analyzer and co-oximeter are used.
181. What are the most important value to examine when looking at ABG’s?
The most important value to examine is the patient’s Ventilation and Oxygenation.
182. How is ventilation measured?
Ventilation is measured by looking at the PaCO2 levels.
183. How is oxygenation measured?
Oxygenation is measured by looking at the PaO2 levels.
184. What two electrochemical oxygen analyzers are good for basic FiO2 monitoring?
The Clark electrode and galvanic cell.
185. Where can blood gas samples be taken from?
Blood gas samples can be taken from Peripheral arteries, indwelling catheters, or via capillary sampling.
186. What is considered the gold standard of gas exchange analysis?
Of course, an ABG.
187. Why is the radial artery the preferred site for arterial blood sampling?
Because it is near the surface, it is easy to palpate and stabilize, the ulnar artery gives good collateral circulation, it is not near any large veins, and the stick is relatively pain-free.
188. What are the indications for ABGs?
The need to evaluate ventilation, acid base, oxygenation, status and oxygen carrying capacity of blood; the need to assess the patient’s response to therapy and/or diagnostic tests; and the need to monitor the severity and progression of a documented disease process.
189. Blood errors in a sample can be caused by what?
They can be caused by air in the sample, venous admixture, excess anticoagulant, and metabolic effects.
190. The ABG sample should be analyzed within how many minutes?
It should be analyzed within 15 minutes.
191. What are some hazards and complications of ABGs?
Bleeding, hematoma, infection, air or blood embolism, arterial spasm, occlusion, vessel damage, ischemia distal to the sample site, and necrosis.
192. What do you want to obtain for a patient who just survived a house fire?
In this case, you will want to get an ABG to check for carbon monoxide, and be sure to run the blood through a co-oximeter.
193. What are the four main values you look at while trying to name a disorder based off the results of an ABG?
You will want to look at, of course, the pH, PaCO2, Bicarb and Base Excess.
194. What are the normal results for an Modified Allen test?
The hand color should flush or turn pink within 5-7 seconds.
195. What are some alternative methods for checking for collateral blood flow?
Use a Doppler ultrasound or a pulse oximeter.
196. What should you label on the syringe of an ABG sample?
You should label the Date, time, patient name, oxygen percentage, temperature, and your initials.
197. What should you document in the chart after obtaining an ABG?
You should document the Date, time, puncture information, and verify that you sent the sample to the lab.
198. What does blood gas analyzer directly measure?
It directly measures the pH, PaCO2, and the PaO2.
199. What does a blood gas analyzer indirectly measure?
It indirectly measure the Bicarb and Oxygen saturation. This is done by calculating from the direct measurements.
200. What does a co-oximeter measure?
It measures the hemoglobin content and values related to hemoglobin binding. These include the SaO2, carboxyhemoglobin levels, and the methemoglobin levels.
So there you have it. That wraps up our study guide on arterial blood gases and ABG Interpretation. I truly hope that this information was helpful for you.
Understanding ABGs is definitely one of the most important skills that is required of Respiratory Therapy Students. That is why I cannot stress to you enough how crucial it is for you to go through this information until you truly know and understand it.
I have faith that you can do it and I wish you the best of luck on your journey to becoming a Respiratory Therapist. Thank you so much for reading and as always, breathe easy my friend.
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The following are the sources that were used while doing research for this article:
- “Interpretation of Arterial Blood Gas.” PubMed Central (PMC), Apr. 2010, www.ncbi.nlm.nih.gov/pmc/articles/PMC2936733.
- “Diagnosing Metabolic Acidosis in the Critically Ill: Bridging the Anion Gap, Stewart, and Base Excess Methods.” PubMed, 1 Mar. 2009, pubmed.ncbi.nlm.nih.gov/19247746.
- Rose, Burton, and Theodore Post. Clinical Physiology of Acid-Base and Electrolyte Disorders (Clinical Physiology of Acid Base & Electrolyte Disorders). 5th ed., McGraw-Hill Education / Medical, 2001. [Link]
- Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 11th ed., Mosby, 2016. [Link]
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