Understanding Arterial Blood Gas Readings

How to Read ABG’s for Healthcare Students

Oct 23, 2009 Marian Henderson

Understanding arterial blood gas readings or ABG's is challenging for some healthcare students; this article aids in understanding ABG's.

An arterial blood gas reading or ABG is taken to measure the Ph of arterial blood. Ph of the arterial blood is normally between 7.35 and 7.45. A Ph above or below this range is dangerous to a patient.

Ph Level of the Blood

Acids have ph below 7 while alkalines (bases) have a Ph above 7. To actually relate the concept to reality instead of pure textbook, try the following experiment: retrieve a carbonated beverage and baking soda. Mix a tablespoon of baking soda with 1/4 cup of the beverage.

Visual Mnemonic for ABG

The result is similar to mixing vinegar and baking soda because an acid (carbonic acid or carbonated water) is mixed with a base/alkaline (sodium bicarbonate). The purpose of the experiment is to provide a visualization of Carbon Dioxide (CO2) and Bicarbonate (HCO3). Recalling this visualization aids in understanding blood gas values.

The student who memorizes the normal CO2 range but who additionally understands the principles, can recall that more CO2 means more carbonated beverage and therefore more acidic. Thus, the student has an understanding of the CO2 values.

Additionally, the student who understands that CO2 means carbon dioxide and that HCO3 is bicarbonate, will have a less complicated process of interpreting ABG values. The student will also understand that CO2 is acidic in water and HCO3 is alkaline/basic.

PH Balance Scale

The renal system and the respiratory system are responsible for maintaining the proper balance of Ph. The respiratory system does this by regulating CO2 and the renal system does this by regulating HCO3. The balance is like a balance scale with Carbon dioxide (CO2) on one side and bicarbonate (HCO3) on the other.

Referring back to the beverage and baking soda example – the carbonated beverage representing CO2 and carbonic acid is on one side and the baking soda representing HCO3 and (sodium) bicarbonate is on the other. These substances must be in the correct balance in the blood; otherwise the client’s life will be endangered as blood that is too acidic or too basic is not compatible with life.

The CO2 value must be between 35-45, and the HCO3 value must be between 22-26 to maintain the correct Ph balance of 7.35-7.45. Nurse educator, David Woodruff uses the analogy of a seesaw to explain the principle; the CO2 (carbon dioxide) and the HCO3 (bicarbonate) in the blood must balance one another to maintain the Ph of the blood.

Normal ABG Range

• Ph 7.35-7.45 Humans can’t live with blood ph that is too alkaline or too acidic. Ph less than 7.35 is acidic and ph more than 7.45 is alkaline.
• PaCO2 35-45 mm/hg The carbon dioxide level in the arterial blood. More than 45 is acidic and less than 35 is alkaline.
• PaO2 80-100 mm/hg The oxygen level in the arterial blood

"Pa" preceding CO2 and O2 means partial pressure measured in mm/hg

• O2 Sat. 95-100% Percent of oxygen saturation of the hemoglobin of the blood (measured by pulse oximeter).
• HCO3 22-26 Bicarbonate level of arterial blood (base/alkaline). More than 26 is alkaline, less than 22 is acidic.

The level of carbon dioxide (PaCO2) and the level of bicarbonate (HCO3) determine which system is causing a Ph imbalance. CO2 is related to respiratory function; if the blood ph is acidic and the CO2 level is above 45, the imbalance is respiratory acidosis, but if ph is alkaline and CO2 level is below 35 the imbalance is respiratory alkalosis.

Conversely, HCO3 is related to the metabolic function; if the blood ph is acidic and the HCO3 level is below 22, the imbalance is metabolic acidosis, but if ph is alkaline and HCO3 level is above 26, the imbalance is metabolic alkalosis.

Compensation is the body’s attempt to restore the correct ph balance by using the opposing system (renal system to respiratory system) to return the ph to normal. Compensation is indicated by values that are outside of the normal range but that help to bring the ph back to 7.35-7.45. David Woodruff’s ABG site explores compensation. Acidosis and alkalosis will either be compensated or uncompensated.

Respiratory System Controlling Carbon Dioxide

The level of carbon dioxide (acid) is controlled by the respiratory system and breathing pattern. Carbon dioxide is exhaled in the lungs and hyperventilating (rapid shallow breathing) gets rid of carbon dioxide.

Renal System Controlling Bicarbonate

The level of bicarbonate (alkaline/base) is controlled by the renal system and urination. Bicarbonate is excreted by the kidneys and urination gets rid of bicarbonate. The Workbook of Anatomy and Physiology by Janet and Pat Dupree states that the kidneys filter “1.2 liters of blood per minute” and remove “anything normally present in the blood that builds up to excessive levels.”

The kidneys respond to excessive levels of acid or alkaline in the blood by excreting the excess, but the process of lowering the acidic level via the bladder and urination is slower than the process of lowering the acidic level via hyperventilating the lungs i.e. the respiratory system will get rid of acid build-up faster by hyperventilation than the renal system will get rid of acid build-up by urination.

Basic Respiratory System Concepts

Hyperventilating (getting rid of carbon dioxide) can either be the respiratory system’s attempt to compensate for the renal system acid build-up, or hyperventilating may be causing alkaline build-up in the respiratory system. The answer requires analysis of lab values and assessment of the client.

If the hyperventilation is caused by anxiety, the healthcare provider will intervene by aiding the client in slowing his/her breathing rate and taking deeper breaths. But if hyperventilation is caused by the body’s attempt to compensate for renal system metabolic acidosis, the healthcare provider will intervene by taking measures to correct the malfunction in the renal system which will eliminate the cause of the hyperventilation.

In preparing for tests, students should strive for an understanding of materials instead of reliance upon rote memorization; the student benefits by increasing his/her comprehension.

References

Dupree, J. and Dupree, P. Anatomy and Physiology Workbook for Dummies. Hoboken, N.J.: Wiley. 2007.

The copyright of the article Understanding Arterial Blood Gas Readings in Career Advice is owned by Marian Henderson. Permission to republish Understanding Arterial Blood Gas Readings in print or online must be granted by the author in writing.

Baking Soda - Sodium Bicarbonate

Ph Balanced by CO2 and HCO3

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