Are you ready to learn about arterial blood gases and ABG interpretation? I sure hope so because that is what this study guide is all about.

As a Respiratory Therapist (or even an RT student), is it crucially important that you know and understand everything there is to know about ABGs. And the goal of this post is to help you do just that. The problem is, it can be very difficult at first to learn how to interpret arterial blood gas results. An ABG can serve as one of the most accurate ways to assess a patient’s clinical condition, which will help make decisions for the plan of care.

So if you’re ready to get started learning how to interpret ABGs, let’s go ahead and dive right in. But before you learn how to interpret ABGs, you must first know exactly what is an Arterial Blood Gas?

What is an ABG?

An arterial blood gas (ABG) is a test that measures the blood levels of oxygen and carbon dioxide as well as the level of acid-base (pH) in your body. An ABG test is used to check how well your lungs move oxygen into different body parts and how efficient it eliminates carbon dioxide.

Normally, healthy lungs move oxygen into the blood and push carbon dioxide out efficiently during inhalation and exhalation (called gas exchange). With this process, your body receives energy while making sure to eliminate waste. However, if you have breathing problems or a disease that affects your lung function, your ABG result can be abnormal that’s why your doctor may order this test.

What are the normal ABG Values?

For you to better understand the key elements in an ABG test, it is important for you to know the definition of each:
  • pH – This is used to measure the acidity or basicity of an aqueous solution.
  • Partial pressure of oxygen (PaO2) – This refers to the amount of oxygen in arterial blood and it shows how efficiently oxygen is transported from the lungs to the blood.
  • Partial pressure of carbon dioxide (PaCO2) – This measures how efficiently carbon dioxide is transported from the lungs to the blood.
  • Bicarbonate (HCO3) – This measures the amount of a form of carbon dioxide known as bicarbonate, in the blood. Normally, bicarbonate is transported into your lungs through your blood, and then eliminated upon exhalation in the form of carbon dioxide.
  • Oxygen saturation (SpO2) – This measures the degree to which the hemoglobin contained in your red blood cells is saturated with oxygen.
The key elements above all have different normal values:
  • 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%
It is important to keep in mind that normal value ranges may vary slightly in different publications, but these are typically the values that you will need to remember.

How to interpret an ABG?

Interpreting an arterial blood gas (ABG) is a crucially important skill for respiratory therapists. ABG interpretation is especially important in critically ill patients because it helps the healthcare team determine the best course of action in regards to treating the patient.

In order to best truly learn ABG interpretation, you must first learn and understand the normal values. They are listed above, so if you need to take a minute to review them, you can do that now. If you know the normal ABG values, then let’s move right along. 

Step 1: Obtain and run the ABG Sample

First things first, in order to be able to interpret ABG results, you must obtain an actual arterial blood sample from the patient. We discuss how to stick an ABG below, but for now, let’s focus on the interpretation.

So after you have obtained an arterial sample, ran the sample, and have the results, now we can figure out what the results mean.

Step 2: Look at the pH to determine if it is Acidosis or Alkalosis

First and foremost, we need to determine if the pH is acidotic or alkalotic. Again, the normal value for pH is 7.35 – 7.45.
  • Acidosis = pH less than 7.35
  • Alkalosis = pH greater than 7.45

Let’s look at some examples:

  • If the pH is 7.26, this is less than 7.35, so it’s Acidosis.
  • If the pH is 7.49, this is greater than 7.45, so it’s Alkalosis.
  • If the pH is 7.39, this falls within the normal range, so the pH is Normal.

Step 3: Indentify if it 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.

To make this simple, remember these two tips:

  • Carbon Dioxide (PaCO2) is being regulated by your lungs.
  • Bicarb (HCO3) is being regulated by your kidneys. 

If the PaCO2 value is abnormal, meaning that it falls outside of the normal range (35 – 45 mmHg), while the HCO3 value is normal, this would mean you have a Respiratory issue.

If the PaCO2 value is normal, meaning that it is within the normal range (35 – 45 mmHg), while the HCO3 value is abnormal, this would mean that you have a Metabolic issue.

  • Carbon Dioxide (PaCO2) = Acid
  • Bicarb (HCO3) = Base

Let’s look at an example:

  • Looking at the pH, we can see that this is Acidosis since 7.26 is less than 7.35.
  • Looking at the PaCO2, we can see that it is elevated 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

Let’s look at another example:

  • Looking at the pH, we can see that this is Acidosis since 7.26 is less than 7.35.
  • Looking at the PaCO2, we can see that it falls in 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 we have a Metabolic issue. 

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: Identify if it is Compensated or Uncompensated

After identifying whether it’s Acidosis or Alkalosis and whether it’s a Respiratory or Metabolic issue, now we must observe the compensatory component of the ABG results.

Be sure to remember these two tips:

  • When we have a Respiratory problem (PaCO2), our body will compensate with Bicarbonate.
  • When we have a Metabolic problem (HCO3), our body will compensate with Carbon Dioxide.

Metabolic Compensation:

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 of 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 is still within normal limits, you can conclude that there is no compensation going on. 

Let’s look at an example:

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 it is a Respiratory or Metabolic problem. The PaCO2 is elevated above the normal range which indicates that there is a Respiratory issue. 

The ABG interpretation would be:  Respiratory Acidosis.

Now we need to look at the Bicarb to determine if it’s compensated or uncompensated.

The HCO3 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. So this tells us that there is definitely is compensation.

Is this a full compensation or a 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. 

Respiratory Compensation:

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. 

So let’s look at an example:

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 it is a Respiratory or Metabolic problem. The PaCO2 is elevated above the normal range which would typically indicate that there is a Respiratory issue. However, the HCO3 value is also elevated.

Remember that PaCO2 is acidic and HCO3 is basic. We’ve already decided that the pH is Alkalotic which indicates that there are more bases (HCO3) in the blood.

So this ABG interpretation would be:  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 elevated above the normal range which tells us that there is some compensation.

Is this a full compensation or a partial compensation?

To answer this question, we need to look back at the pH. What the compensation enough to bring the pH back to normal? The answer is no, so this indicates that there is only a 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

Why are ABGs important?

An ABG test is routinely used in the diagnosis and monitoring of patients suffering from critical conditions. Because this test provides a precise measurement of the levels of oxygen and carbon dioxide in your body, it can help your doctor determine your lung and kidney function.

In most cases, your doctor may order an ABG if you have the following symptoms:

  • Breathing difficulties
  • Changes in mental status
  • Nausea and vomiting
In addition, an ABG test can help your doctor:
  • Assess whether treatments for lung conditions are effective.
  • Check for chemical poisoning.
  • Check the acid-base balances in patients with kidney disease, diabetes, and those recovering from drug overdoses.
  • 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. First and foremost, your healthcare provider will determine the best site for collecting the blood sample.

ABG Sample Sites:

  • Wrist (radial artery)
  • Upper arm (brachial artery)
  • Groin (femoral artery)

In addition, a blood sample can also be obtained in 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 healthcare provider will then insert a sterile needle into the artery to draw blood.

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 (fat tissue) to prevent severing the artery or tendons and avoiding damage to the nearby tissues.

Once the blood sample is obtained, a sterile bandage will be placed over the punctured wound in order to stop bleeding and avoid infection.

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 an accurate ABG result.

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.

However, repeated puncture of a single site increases the prevalence of 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 this case, 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; and the more you do it, the easier it gets and the better you get at sticking ABGs.

What causes Respiratory Acidosis?

Respiratory acidosis occurs when your lungs cannot remove all of the carbon dioxide formed in your body. As a result, your 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 elevation in carbon dioxide levels can be lessened in the blood. However, this effect is not lessened in your brain. As a result, you can suffer from sleeping difficulties, headaches, memory problems, anxiety, and mood changes.

Respiratory acidosis can be caused by the following:

  • 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
  • Scoliosis
  • Sedative overdose
  • Severe obesity (affects lung expansion)

What are the potential errors when running an ABG?

There are several factors that can affect the result of an ABG test. These are the following:
  • 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 mislabelling the blood sample.
  • Failure to obtain a blood sample from an artery or vein. In some cases, inexperienced healthcare providers might not hit an artery or vein, therefore obtaining blood sample from the surrounding tissues. Also, obtaining blood sample from an existing IV line increases the chance of aspirating blood mixed with intravenous fluid.
  • Blood clotting. 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 result and will render the specimen useless.
  • Obtaining a blood sample on incorrect settings or support. This can significantly affect the course of the treatment of the patient and the medical team’s assessment of health needs. For instance, if a nurse obtained a blood sample when the patient is still on supplemental oxygen instead of room air, the results can be incorrect. This can yield falsely elevated PaO2 levels.
  • Air contamination of the blood sample. Air contamination can alter the result of an ABG by causing the measured PaO2 of the patient to drop toward room air PaO2.
  • Contamination caused by too much heparin. Too much liquid heparin dilutes the blood sample and causes changes in pH levels. Moreover, it can decrease the measurement of hemoglobin/hematocrit available on modern instruments and it can cause liquid bubble. These mechanisms can significantly affect PaO2 and PaCO2 values.
  • Inappropriate mixing of the blood sample. Depending on hospital or laboratory protocol, healthcare providers thoroughly mixed the blood sample with heparin immediately upon collection to avoid clotting. It’s also remixed before introduction to the instrument. 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 mixing iced samples for a shorter period of time. It is recommended to mix iced samples longer 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 not longer than 10 minutes after extraction. 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 an ABG?

Indications for an ABG test include the following:
  • Assessment of the patient’s response to treatment strategies such as mechanical ventilation.
  • Determination of a patient’s oxygen-carrying capacity.
  • Determination of the need for supplemental oxygen.
  • Diagnosis of respiratory, metabolic, and mixed acid-base disorders.
  • Monitoring of acid-base status.
  • Procurement of a blood sample in emergency situations when access to the vein is not possible.
  • Quantification of hemoglobin levels.

What are the Contraindications for an ABG?

Not all patients are potential candidates for an ABG test. The following are the contraindications for the test:
  • An abnormal modified Allen test.
  • Blood clotting problems
  • Local infection or damage at the injection site.
  • Patients who are anticoagulation therapy.
  • Patients who are taking thrombolytic agents.
  • Presence of a disease affecting the blood vessels.
  • The presence of arteriovenous fistulas or vascular grafts.

What is the Modified Allen Test?

The Allen test for assessment of blood flow was originally developed by Edgar V. Allen in 1929 as a non-invasive method of assessing the patency of arteries in patients with Buerger disease, a recurring progressive inflammation, and clotting of arteries and veins of the hands and feet. Since then, it has been adopted as the Modified Allen test (MAT).

The difference between MAT and the original Allen test is that MAT efficiently evaluates the adequacy of blood circulation at one hand at a time. In contrast, the original Allen test compresses one artery of each hand at the same time.

MAT measures the competency and quality of the artery and should be performed prior to performing an ABG test.

The following are the proper steps in performing MAT:

  1. 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, close his or her hand tightly.
  2. Locate the radial and ulnar arteries – Face the patient and locate the radial and ulnar arteries. The radial artery is located close to the underside of the forearm while the ulnar artery is just below the radial artery. Make sure to locate 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. Tell the patient to relax his or her hand while doing this.
  6. Check whether the palm and fingers have blanched. Blanching means that you have completely occluded the radial and ulnar arteries with your fingers.
  7. Slowly release the pressure on the ulnar artery. If the patient hand flushes within 5 to 15 seconds, it means that the ulnar artery is patent or has a good blood flow. This is considered a positive MAT. In contrast, if flushing is not observed within 5 to 15 seconds, this result suggests that the ulnar artery has inadequate blood circulation and this is considered as a negative MAT. In this case, it is recommended not to puncture the radial artery of the same hand.
So now that you have a full understanding of arterial blood gases and ABG interpretation, let’s go through some practice questions so that we can really reinforce this information into your brain.

ABG Practice Questions for Respiratory Therapy Students:

Phew! You have finally made it all the way through our ultimate guide on Arterial Blood Gases and ABG Interpretation. By doing so, I know that you now have a great understanding of the ins and outs of ABGs.

With that being said, what better way to reinforce that knowledge into your brain than by going through practice questions? That is exactly why we listed out the absolute best ABG practice questions for you below. Are you ready to get started?

1. What are the causes of Respiratory Acidosis?
The build-up of CO2 in blood, hypoventilation, and increased dead space.

2. What can you correct Respiratory Acidosis?
Increase the number of BPM, increase the size of breaths (tidal volume), and decrease dead space.

3. What are the causes of Respiratory alkalosis?
Must have a low amount of CO2 in blood, hyperventilation, pain, and anxiety.

4. How can you correct respiratory alkalosis?
Decrease the number of BPM, medication for pain, and treat anxiety.

5. What are the causes of metabolic acidosis?
Low amount of HCO3 in blood, diarrhea, aspirin toxicity, diabetes, and renal failure.

6. How can you correct metabolic acidosis?
Stop or correct whatever is causing the issue, medication for diarrhea, and treat renal failure.

7. What are the causes of metabolic alkalosis?
Increase in HCO3 in the blood, vomiting, NG suctioning, and the ingestion of NaHCO3.

8. How can you correct metabolic alkalosis?
Stop or correct the vomiting, discontinue NG suctioning, and stop the NaHCO3 ingestion.

9. What is oxygenation?
It is represented by the PaO2; it is measured only of the oxygen dissolved in plasma.

10. What is the first step before doing an ABG?
Check the patient’s chart to confirm the doctor’s order and indication.

11. What is the preferred site for an ABG in adults?
Radial artery.

12. What is the longest time an ABG sample could go (without ice) without being analyzed?
15 minutes.

13. What test is performed to confirm collateral circulation before doing an ABG stick?
Modified Allen Test.

14. What is an adequate amount of blood for an ABG sample?
2 – 4 ml of blood.

15. What are some causes of metabolic acidosis?
Diarrhea, starvation, and diabetic ketoacidosis.

16. What are the 3 major hazards of an arterial puncture?
Bleeding, obstruction of the vessel, and infection.

17. What are the three major criteria for selection of the arterial puncture site?
Collateral blood flow, vessel accessibility, and peripheral structures.

18. What is collateral blood flow?
If it’s part of a collateral circulation system it can prevent loss of distal blood flow in the event of an arterial obstruction.

19. What is vessel accessibility?
The best vessel for puncture is one that is easy to palpate, relatively superficial and somewhat stable.

20. What are peripheral structures?
The best site for arterial punctures are those which do not have extremely sensitive adjacent structures such as nerves.

21. How to perform the Allen test?
Elevate the 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.

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?
Look at the PaO2.

24. An increase in CO2 causes the pH levels to become what?

25. A patient comes in with a pH of 7.52, a PaCO2 of 25, an HCO3 of 25, and a BE +1. What would be the interpretation of this blood gas?
Respiratory Alkalosis

26. The patient has a pH of 7.10, CO2 of 20, HCO3 of 10, and BE of -20, what is your interpretation of this blood gas?
Metabolic Acidosis

27. What do you look at in a blood gas to determine ventilation?

28. Can a blood gas be considered normal if the BE 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 CO2 and HCO3 are moving in the same direction then how would you first classify the gas?
Fully compensated.

30. If you get an ABG and it reads: pH is 7.56, CO2 is 42, HCO3 is 34, and BE is +5. How would you name this gas?
Acute/Uncompensated metabolic alkalosis

31. At what PH should we intubate the patient?
Generally at 7.2 and below.

32. Severe hypoxemia is classified as a PaO2 of less than?
40 mm Hg

33. What is a normal range for PaO2 on room air?
80-100 mm Hg

34. What is the range of moderate hypoxemia?
40-59 mm Hg

35. You get an ABG and it reads: pH is 7.42, CO2 is 43, HCO3 is 25, and BE is +2. How would you classify this gas?
Normal ABG

36. If the pH decreases below 7.35, then it is considered to be?

37. If the pH is above 7.45, it is considered to be?

38. If you are hypoventilating, your CO2 will do what?
It will increase.

39. If you were hyperventilating then your CO2 would do what?
It would decrease.

40. What are the drugs that cause a low pH?
Narcotics, barbiturates, acetazolamide (Diamox), 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?
A normal pH.

43. What are the potential causes of Respiratory Alkalosis?
Anxiety, Hypoxemia, and Pain.

44. Which organ system maintains the normal level of HCO3 at 24 mEq/L?
The renal system.

45. What is the limiting factor for H+ excretion in the renal tubules?
Insufficient buffers.

46. What acts as the “first-line” or immediate defense against the accumulation of H+ ions?
Blood buffer systems.

47. A primary respiratory problem is determined by?
If the PaCO2 is less than 35 mmHg or Greater than 45 mmHg.

48. A primary metabolic problem is when?
HCO3- is less than 33 mEq/L or greater than 26 mEq/L.

49. If a patient has a pH of 7.49, what would this define?

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?
Femoral, radial, and brachial.

52. Before a sample of capillary blood is taken, what should you do to the site?
Warm to 42 degrees Celsius and clean 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 needed for an arterial blood sample include?
ABG analyzer’s requirements, specific anticoagulant used, other tests that will be done.

55. After obtaining an arterial blood sample, what should you do?
Apply pressure to the site until bleeding stops, place sample in a transport container with ice slush, mix the sample by rolling and inverting the syringe.

56. Transcutaneous blood gas monitoring is indicated when what need exists?
To continuously analyze gas exchange in infants and children, to quantify the real-time responses to bedside interventions, to continuously monitor for hyperoxia in newborn infants.

57. What size needle would you recommend to obtain an ABG sample on an infant?
25 gauge.

58. The indications for arterial blood sampling by percutaneous needle puncture include?
Monitor the severity of a disease process, evaluate ventilation and acid-base status, evaluate a patient’s response to therapy.

59. After obtaining an arterial blood sample from an arterial Line, you would?
Flush the line and stopcock with heparinized intravenous solution, confirm that the stopcock port is open to the intravenous bag solution and catheter, confirm an undamped pulse pressure waveform on the monitor.

60. The patient parameters that should be assessed as part of arterial blood sampling include?
Temperature, position and activity level, and clinical appearance.

61. The clinical signs of acute respiratory alkalosis include?
Convulsions, dizziness, and paresthesia.

62. A low PaCO2 best describes what?
Respiratory Alkalosis

63. With partially compensated respiratory alkalosis, which of the following blood gas abnormalities would you expect to encounter?
Decreased HCO3, decreased PCO2, and increased pH.

64. The causes of Respiratory Acidosis in patients with normal lungs include?
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?
Aspirate at least 5mL of fluid or blood using a wasted syringe.

66. What is the equipment needed for capillary blood sampling?
Lancet, capillary tubes, and a warming pad.

67. When is capillary blood gas sampling indicated?
When an ABG analysis is needed but arterial access is not available.

68. The compensation for metabolic acidosis occurs through?
A decrease in blood CO2 levels.

69. What are the causes of Metabolic Alkalosis?
Diuretics, hypochloremia, and vomiting.

70. What clinical findings would you expect in a fully Compensated Respiratory Acidosis patient?
An elevated HCO3 and a pH between 7.35 and 7.39.

71. What is the normal pH range?

72. What is the normal range for PaCO2?

73. What is the normal range for HCO3?

74. What is the normal range for base excess?

75. What type of issues are we looking for when we look at the HCO3 and base excess values?
Metabolic issues

76. What type of issues are we looking for when we look at the PaCO2 values?
Ventilation status.

77. What does the PaO2 measure?
Oxygenation status.

78. What range of PaO2 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?
% MetHb

80. What ABG value would we look for in patients that have carbon monoxide poisoning or have been in a fire?
% COHb

81. In a given ABG, if the pH and CO2 values are going in DIFFERENT directions, what is this ABG considered to be?

82. In a given ABG, if the pH and HCO3 values are going in the SAME direction, what is this ABG considered to be?

83. When interpreting a given ABG, what values must be abnormal for it to be considered “partial”?
All values must be abnormal. (pH, CO2, and HCO3)

84. When interpreting a given ABG, what values must be normal 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”?
The pH must be normal.

86. How do you prevent pre-analytical errors in ABG samples?
Make sure the sample is: obtained anaerobically, properly anticoagulated, bubbles removed, and analyzed within 10 to 30 minutes.

87. How is CO2 transported?
45-55 mL of CO2 per 1 dl blood is transported by ionized bicarb, dissolved in plasma, and plasma protein transport.

88. How much blood is needed for an adequate ABG sample?
0.5 mL of blood is needed. This is usually enough to perform two tests.

89. For accurate ABG results, what are the components of quality control?
Recordkeeping, performance validation, preventative maintenance and function checks, automated calibration/verification, internal statistical quality control, and external quality control.

90. What are the reasons for drawing an ABG?
Sudden, unexplained dyspnea, acute shortness of breath/tachypnea, abnormal breath sounds, cyanosis, heavy use of accessory muscles, changes in ventilator settings, CPR, diffuse infiltrates in, chest x-ray, sudden cardiac arrhythmias, and acute hypotension.

91. What does the blood gas machine accuracy depend 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. 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?
Bicarbonate (HCO3) Base Excess (BE) or deficit Hemoglobin saturation (HbO2).

94. What are the benefits of indwelling catheters?
They can provide ready access for blood sampling. They allow for continuous monitoring of vascular pressures.

95. What are two site locations for indwelling catheters?
Normal routes are peripheral arteries (radial, brachial, pedal), femoral artery, central vein, and the pulmonary artery.

96. What can a good capillary blood gas sample provide and reflect?
Estimated arterial oxygenation and PCO2.

97. What can be used if frequent blood sampling is needed?
Arterial Cannulation.

98. What is hemoximetry?
Laboratory analytical procedure requiring invasive sampling of arterial blood

99. What determines the ventilation status of a patient?

100. What is the rule of thumb in regards to the PaCO2 and FiO2?
The PaCO2 should be about 5 times the FiO2.

101. Does oxygenation decrease with age?
Yes, it does.

102. What happens when an ABG is partially compensated?
The pH is out of range and the CO2 and HCO3 are going to the same direction.

104. What is the normal value for COHB?
Less than 3.0%.

105. What does the COHB indicate?
It indicates if the patient was exposed to carbon monoxide or a house fire.

106. What is the normal value for MetHb?
Less than 2.0%.

107. What does the MetHb indicate?
It indicates if the patient is a smoker.

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?
It represents a measurement of the overall acid-base balance and is used to assess the overall [H+] status of the blood.

110. What does the PaCO2 represent?
It represents the arterial CO2 level and is used to assess the ventilatory status.

111. What does the PaO2 represent?
It represents the oxygen tension level in the arterial blood and is used to evaluate the oxygenation status.

112. What does the HCO3 represent?
It 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?
It represents the base excess (or deficit) level of the blood and is used to indicate the metabolic aspect of acid-base balance.

114. What does the SaO2 represent?
It represents the level of saturation of hemoglobin (Hb) with oxygen and also provides a measure of arterial oxygenation.

115. What is compensation?
The altering of function of the respiratory or renal (metabolic) system in an attempt to correct for an acid-base disorder.

116. What is Hypoxemia?
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 (Alkalosis). As the minute ventilation decreases, the PaCO2 will increase and the pH will decrease (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?
It is a risky stick; huge veins and arteries, a fibrinolytic automatically rules out femoral sticks.

120. Describe the radial artery in regards to an ABG stick?
It has good collateral circulation, it is superficial and easy to palpate, it is the best artery for sticking, it’s not near any large veins, and the stick is relatively pain-free (hahaha, okay).

121. Describe the brachial artery in regards to an ABG stick?
It is a risky stick; it is near nerves and large veins. There is no collateral circulation. There is an increased risk for a venous sample.

122. What are the contraindications for an ABG stick?
A negative modified Allen’s test, avoid lesions or surgical shunts, avoid infection of PVD, avoid the femoral site on outpatients, and a high dose of an anticoagulant.

123. What are the indications for an arterial line?
Continuous ABP monitoring, and/or the need for repeated ABGs.

124. The CBG is an alternative to what?
The ABG procedure.

125. The CBG gives what?
It gives a rough estimate of the pH and PaCO2. The PO2 is of no value for estimating oxygenation because venous blood does not carry oxygen.

126. How is the ABG procedure done?
Blood is collected in a heparinized glass capillary tube and the site must be warmed before the procedure.

127. The site prep and handling of the CBG is?
The same as ABG sampling.

128. What are the sites for the CBG?
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 CO2.

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 overall tissue oxygenation.

133. Prior to an ABG draw, what should the Respiratory Therapist review in the patient’s chart?
Look for a low platelet count or increased bleeding time.

134. What must be evaluated prior to a radial stick?
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, the 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 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 Allen test is negative?
Try the other arm, then try the brachial artery.

138. What should the Respiratory Therapist do for a patient who needs frequent ABG’s?
Recommend the insertion of an indwelling arterial catheter.

139. What do bubbles in the ABG sample do?
They can cause the oxygen reading in the results to be falsely high.

140. How should the Respiratory Therapist handle that ABG sample after it has been drawn?
Remove any air bubbles, store in ice water to stop metabolism, analyze as soon as possible.

141. Room temperature samples must be analyzed how soon?
They must be analyzed within 10-15 minutes.

142. How long should pressure be applied to a stick wound?
It should be applied for 3-5 minutes or longer if there is a clotting problem.

143. What is the kidney’s role in acid-base balance?
To remove small quantities of acid and restore the buffer capacity of fluids by replenishing HCO3 (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 BE?
It allows for the analysis of pure metabolic components of acid-base balance, changes in met components alter acid-base, respiratory components do not.

146. Do changes in CO2 effect the Base Excess?
No, only metabolic changes alter the BE.

147. What are the common causes of respiratory acidosis?
Acute upper airway obstruction, severe diffuse airway obstruction (acute or chronic), massive pulmonary edema.

148. What are the common non-respiratory problems that can cause respiratory acidosis?
Drug overdose, spinal cord injury, neuromuscular diseases, head trauma, and trauma to thoracic cage.

149. How can you describe fully compensated respiratory acidosis?
The HCO3 is enough to bring the pH within the normal range.

150. If the expected level of HCO3 compensation is not occurring for acute or chronic acidosis, what should the Respiratory Therapist suspect?
You 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?
Around 70 mmHg.

152. Because CO2 causes systemic vasodilation, what cardiac manifestations should be expected?
Warm flush skin, bounding pulse, and arrhythmias.

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, i.e. hyperventilation.

154. What are the common causes of respiratory alkalosis?
Hyperventilation caused by pain, hypoxemia (PaO2 55-60), acidosis, and anxiety.

155. How do the kidneys compensate for respiratory alkalosis?
They excrete HCO3.

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?
During IPPB and mechanical ventilation.

158. How does the body compensate for metabolic acidosis?

159. What is the most common and obvious sign of metabolic acidosis?
Kussmaul’s breathing.

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?
Hypoventilation to retain PaCO2.

162. What do ABG results determine?
They are used to determine the oxygen level and management of mechanical ventilation.

163. What does the pH measure?
The pH measures the state of blood; acidic or basic.

164. What does the PaCO2 measure?
The PaCO2 measures the partial pressure of carbon dioxide.

165. What does the PaO2 measure?
The PaO2 measures the partial pressure of oxygen.

166. What does the HCO3 measure?
The HCO3 measures the concentration of bicarbonate (metabolic issues).

167. What three ways do we classify the primary problem of ABG’s as?
Normal, acidosis, or alkalosis.

168. What two types do we classify as the primary cause of ABG’s as?
Respiratory or metabolic.

169. Which of the parameters is the respiratory component?

170. Which of the parameters is the metabolic component?

171. What happens to the pH when there is an increase in H+?
The pH will decrease and become acidotic (less than 7.35).

172. What happens to the pH when there is a decrease in H+?
The pH will increase and become alkalotic (greater than 7.45).

173. Under what range of the pH will a patient have to be intubated?
Anything less than 7.2.

174. If the pH and PcO2 are going in opposite directions, what does this indicate?
A respiratory problem.

175. If the pH and HCO3 are going in the same direction, what does this indicate?
A metabolic problem.

176. What type of compensation is indicated when the pH, PCO2, and HCO3 are all out of range?
Partially compensated.

177. What type of compensation is indicated when either the pH or PCO2 is out of range?

178. What type of compensation is indicated when the pH is normal and the PCO2/HCO3 are out of range?
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 MetHb and the COHb?
An ABG analyzer and co-oximeter.

181. What is the most important value to examine when looking at ABG’s?

182. How is ventilation measured?
PaCO2 levels

183. How is oxygenation measured?
PaO2 levels

184. What two electrochemical oxygen analyzers are good for basic FiO2 monitoring?
Clark electrode and galvanic cell.

185. Where can blood gas samples be taken from?
Peripheral arteries, indwelling catheters, or via capillary sampling.

186. What is considered the gold standard of gas exchange analysis?

187. Why is the radial artery the preferred site for arterial blood sampling?
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 (lol, nope).

188. What are the indications for ABGs?
The need to evaluate ventilation, acid base, oxygenation, status and oxygen carrying capacity of blood; need to assess the patient’s response to therapy and/or diagnostic tests; need to monitor severity and progression of a documented disease process.

189. Blood errors in a sample can be caused by what?
Air in the sample, venous admixture, excess anticoagulant, and metabolic effects.

190. The sample should be analyzed within how many minutes?
15 minutes.

191. What are some hazards and complications of ABGs?
Bleeding, hematoma, infection, air/blood embolism, arterial spasm, occlusion, vessel damage. ischemia distal to the sample site, and necrosis distal to the sample site.

192. What do you want to obtain for a patient who just survived a house fire?
Get an ABG to check for carbon monoxide, and be sure to run the blood through co-oximeter.

193. A good rule of thumb when deciding if a person is well oxygenated or not based off of their PaO2 is?
5 x FiO2

194. What are the four main values you look at while trying to name a disorder based off on an ABG?
pH, PaCO2, HCO3 and Base Excess.

195. What is a simple description of an ABG?
It is a cornerstone in the diagnosis and management of oxygenation (PaO2) and acid-base (pH and PaCO2) disturbances.

196. What are the normal results for an Allen test?
The hand color flushes within 5-7 seconds.

197. What are some alternative methods for checking for collateral blood flow?
Doppler ultrasound, and pulse oximeter.

198. What are the ABG contraindications?
Negative Allen test, no ABG through a lesion or distal to a surgical shunt (dialysis), and be sure to show caution with patients that take anticoagulants.

199. Why do we choose the radial artery when sticking an ABG?
It is near the surface and relatively easy to palpate and stabilize, it has collateral circulation, the artery is not near any large veins, and the procedure is relatively pain-free (lol yeah, okay).

200. What should you label on the syringe of an ABG sample?
Date, time, patient name, O2 %, temperature (if abnormal), and your initials.

201. What are some hazards of an ABG?
Infection, bleeding, and obstruction of the vessel.

202. What should you document in the chart after obtaining an ABG?
Date, time, document puncture information, and verify that you sent the sample to the lab.

203. What does blood gas analyzer directly measure?
It measures: pH (Sanz electrode), PaCO2 (Serveringhaus electrode), and PaO2 (Clark electrode).

204. What does a blood gas analyzer indirectly measure?
HCO3, O2 saturation (this is done by calculating from the direct measurements).

205. What does co-oximeter measure?
It measures hemoglobin content and values related to hemoglobin binding: SaO2, %COHB , and %methemoglobin.

Final Thoughts

So there you have it! That wraps up our study guide on arterial blood gases and ABG Interpretation. I hope that this information was helpful for you. Understanding ABGs is definitely one of the most important things 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 wish you the best of luck on your journey towards becoming a Respiratory Therapist. Thank you so much for reading and as always, breathe easy my friend.