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By: Kevin Asp, CRT, RPSGT on July 9th, 2020

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Arterial Blood Gases (ABGs) in the Sleep Center

arterial blood gas | ABG

Interpreting an arterial blood gas (ABG) is a critical ability for respiratory therapists, doctors, nurses and other healthcare personnel —  including sleep technologists. It is particularly essential to monitor ABGs in patients who are critically ill. ABGs are a valuable tool for evaluating our patients in the sleep center presenting with complex health conditions like COPD, obesity hypoventilation, and neuromuscular conditions.

What Is an Arterial Blood Gas Test?

The ABG blood test measures pH or the acidity and the levels of carbon dioxide (CO2) and oxygen (O2) from blood taken from an artery. It is used for checking a patient's lung function, and evaluates how well they can move blood oxygen and remove carbon dioxide. 

For the body's cells to live, they require oxygen. When your patients inhale and exhale, their lungs are moving oxygen into their blood and pushing carbon dioxide out. This is referred to as gas exchange and provides the patients and their cells with the oxygen they require to survive.

Physicians order this test to obtain clues about the patient's heart, lung, and kidney function. Usually, they'll seek other testing too. And, while the test is usually performed in an ER and ICU setting, an ABG can also be drawn on patients in other healthcare settings, such as in a sleep center, depending on their diagnosis.

It may be helpful to have arterial blood gas testing performed on your sleep patients if they're having a hard time breathing to figure out what the issue is. Your patients might have have this test performed if they have a serious illness or injury.  The information can be helpful to you as the sleep technologist.


Advanced Sleep Titration Module

Anatomy and Physiology: Ventilatory Assessment and Control Assessment of Ventilation with ABG's

 

 


Why Is an ABG Test Done?

The physician might request an ABG test to:

  • Check for severe lung and breathing problems like cystic fibrosis, asthma or chronic obstructive pulmonary disease (COPD)
  • Determine how lung problem treatments are working
  • Determine if your patient requires oxygen or other breathing assistance 
  • Check your patient's acid-base balance. They could have extra acid in their body from a serious infection, kidney failure, diabetes complications, certain toxic ingestions or under-treated sleep apnea

 

What Do Arterial Blood Gases Measure?

An arterial blood gas test measures how much carbon dioxide and oxygen are in the blood. It might also be used to determine the blood's pH to determine how acidic it is.  The test is commonly known as an arterial blood gas (ABG) test or a blood gas analysis.

An ABG measures:

  • PaCO2 (partial pressure of carbon dioxide): Measures carbon dioxide dissolved in blood and carbon dioxide's ability to move out of the body.
  • PaO2 (partial pressure of oxygen): Measures oxygen dissolved in the blood and the oxygen's ability to move into the blood from the airspace of the lungs.
  • pH: Measures H+ (hydrogen ions) in blood. The blood's pH is typically between 7.35 and 7.45. A less than 7.35 pH is known as acid and a greater than 7.45 pH is known as basic (alkaline). 
  • O2CT (oxygen content) and O2Sat (oxygen saturation) values: The O2 content measures how much oxygen is in the blood, whereas oxygen saturation is the measurement of the amount of oxygen being carried by the hemoglobin in red blood cells.
  • HCO3 (bicarbonate):  HCO3 is a chemical (buffer) keeping the blood's pH from becoming too basic or too acidic.

Your patients' red blood cells carry carbon dioxide and oxygen throughout their bodies. These are referred to as blood gases.

As blood passes through the lungs, the O2 flows into their blood as the CO2 flows out of their blood and into their lungs. The ABG test will assist to determine how well their lungs can move the O2 into their blood and remove the CO2 from their blood.

Imbalances in your patients' O2, CO2, and pH levels can be an indication of the presence of a specific medical condition, including...

Sleep Center Related Conditions:

  • Chronic Obstructive Pulmonary Disease (COPD)
  • Obesity Hypoventilation 
  • Severe Obstructive Sleep Apnea
  • Neuromuscular conditions

Other Medical Conditions:

  • Heart failure
  • Kidney failure
  • A drug overdose
  • Uncontrolled diabetes
  • Shock
  • Chemical poisoning
  • Hemorrhage

Restrictive lung disease and obstructive lung disease and ABGs also go hand-in-hand.  The physician might order an ABG test when the patient is showing symptoms of any one of these medical conditions.

 

What Happens During an Arterial Blood Gas Test?

To perform this test, a small amount of blood is collected from an artery. It's a simple and safe procedure that takes minutes to complete.

Your patient can have an ABG test in the hospital, in their doctor's office, and even in your sleep center if you have a respiratory therapist or nurse on staff.  A small needle is used to collect a sample of arterial blood, typically from thee wrist. In some cases, blood may be taken from an artery on the inside of their arm above their elbow or in their groin.

Prior to the ABG test, a modified Allen test may be performed.  This is performed by applying pressure to the patient's wrist for a few seconds and then releasing pressure to determine if blood flow in the patient's hand is normal.

In preparation for the ABG test, the patient will need to inform you of all medications and supplements they may be taking, including:

  • Supplements
  • Medications
  • Vitamins

If your patient is on oxygen therapy, but can breathe without it, it is best to turn their supplemental oxygen off for 20 minutes so you can perform a "room air" test before drawing blood for the ABG.

You should inform your patient of any discomfort they may experience during or after the test. Let them know that when blood is collected from their artery, it could be more uncomfortable than if you were drawing blood from a vein. This is because their arteries are deeper than their veins and arteries also have sensitive nearby nerves.

Let the patient know they may experience a little dizziness or feel faint, lightheaded or nauseated while you're drawing their blood. Gently press for several minutes on the area after you remove the needle to lower the chance of bleeding or bruising.

 

How to Read Arterial Blood Gases

The first value you should be looking at is the pH to determine if your patient's level is above, below or within the normal range. If your patient's pH is > 7.45, they are in alkalosis. If the pH is < 7.35, they are in acidosis. Keep in mind the lower the pH number is, the greater the acid level in the patient's body. Even if your patient has a normal pH level, alkalosis or acidosis might still be present, since the body can compensate in order to balance the pH.

You will next evaluate the PaCO2 to see if the blood gas changes are metabolically driven or are due to the respiratory system.  Combined with the HCO3 value, you'll have a complete picture of the blood gas findings.

 

Who Draws Arterial Blood Gases?

Only a health worker for whom the testing is within the legal scope of practice for that health worker's position, typically a Respiratory Therapist,  can draw an arterial blood gas. 

A sample is obtained through a catheter placed in the patient's artery or using a needle and syringe  to puncture the artery. The syringes used for ABGs are pre-heparinized and are made for minimizing air exposure that will potentially change the blood gas values.

 

What Are the Normal Arterial Blood Gas Values?

Arterial blood gas interpretation is performed by a physician. You should, however, understand the basics of blood gas values.

Blood gas normal values, in general, include:

  • Bicarbonate (22 to 28 milliequivalents per liter)
  • Arterial blood pH (7.38 to 7.42)
  • Oxygen saturation (94 to 100%)
  • Partial pressure of O2 (75 to 100 mm Hg)
  • Partial pressure of CO2 (38 to 42 mm Hg)

Your patient's blood oxygen levels might be lower if they live above sea level.

Abnormal results could be a sign of a specific medical condition. For instance, a less than 7.35 blood pH could indicate metabolic acidosis often caused by shock, diabetic ketoacidosis or kidney failure.  A pH greater than 7.45 could indicate metabolic alkalosis often caused by low blood potassium or chronic vomiting.


Learn more about this and other advanced topics in the sleep titration  e-Learning modules

 

AAST Advanced Sleep Titration e-Learning Course 

 

About Kevin Asp, CRT, RPSGT

Because of the implementation of his best practices of Implementing Inbound Marketing in its Medical Practice, he turned the once stagnant online presence of Alaska Sleep Clinic to that of "The Most Trafficked Sleep Center Website in the World" in just 18 months time. He is the President and CEO of inboundMed and enjoys helping sleep centers across the globe grow their business through his unique vision and experience of over 27 years in sleep medicine.

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