Arterial Blood Gases - Abg's

Arterial Blood Gases-ABGs

What are ABGs?

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ABGs, or Arterial Blood Gases, are a measurement of the body's acid/base balance and oxygenation status.

pH -the concentration of hydrogen ions (H+)

normal pH=7.35-7.45

paCO2 -the partial pressure of carbon dioxide (acid) in the blood, some of which is on its way to the lungs to be "blown off" in respiration. normal paCO2= 35-45 mmHg

HCO3- -or bicarbonate (base, or alkali) is regulated mainly by the kidneys, and is part of the carbonic acid/bicarbonate buffer system.

normal HCO3- =22-26mEq/L

ABGs that inform about oxygenation status

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paO2 -the partial pressure exerted by oxygen in the blood. It affects how much O2 can dissolve in the blood that is destined for transport via the lungs to the tissues.

normal paO2 =75 to 100mmHg

O2 sat -(O2 saturation)-the ratio of the amount of oxygen in the arterial blood to the amount that the hemoglobin could carry

normal O2 sat is 94 - 100%

Why are ABGs important?

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Our survival depends upon the body's ability to regulate acid/base balance--to eliminate excess acids of normal metabolism and to compensate for disturbances due to illness or injury.

A myriad of enzymes, required for essential biochemical reactions in our cells, function best within a specific range of pH. Failure of the body to regulate acid/base balance can impair these critical enzyme functions.

By measuring arterial blood gases, and correlating the results with other diagnostic and clinical data, we can assess of how well these regulatory mechanisms are working. Proper intervention depends upon understanding how to interpret arterial blood gas results.

How does the body regulate acid/base balance?

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The lungs are involved in acid/base regulation via the carbonic/bicarbonate buffer system. Carbonic acid (H2CO3-) is converted to carbon dioxide (CO2) for excretion during respiration. High levels of carbonic acid may, for example, trigger faster and deeper respirations, as the body attempts to eliminate excess acid by breathing out more CO2.

The kidneys help to maintain the correct ratio of carbonic acid/bicarbonate by excreting or reabsorbing bicarbonate, and can generate bicarbonate if needed. The kidneys also help to excrete other metabolic acids that cannot be converted to gases for removal through respiration.

Assessing Acid/Base Balance

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It helps to have a system in mind for analyzing the results of an arterial blood gas measurement. Here is one simple method for thinking it through.

1. Look at the pH and determine if the patient is in acidosis or alkalosis:

2. If the pH is out of the normal range, determine if the primary problem is respiratory or metabolic.

if the paCO2 (acid) correlates with the abnormal pH, then the problem is respiratory examples--the patient is acidotic and the CO2 (acid) is high the patient is alkalotic and the CO2 (acid) is low

if the HCO3- (base or "alkali") correlates with the abnormal pH, then the problem is metabolic

examples--the patient is alkalotic and the HCO3- (base) is high the patient is acidotic and the HCO3- (base) is low.

3. Now look at the value (either paCO2 or HCO3-) that did NOT correlate with the pH.

If it is normal, no compensation is occurring If it is abnormal, but the pH remains abnormal, partial compensation is occurring If it is normal, and the pH is normal, complete compensation.

Compensation-Looking