The ABC’s of ABG’s - 2 Nursing CEs

Author: Kristi Hudson RN MSN CCRN

Written: 10/16/05

Updated: September 11, 2009

Course Objectives:

Upon completion of this course the student will be able to:

• Discuss the purpose of ABG testing
• Describe what an ABG is measuring
• List normal ABG values
• Explain the steps to completing a Modified Allen’s Test
• Differentiate between acidosis and alkalosis
• Have a better understanding of ABG interpretation

What is an ABG?

An arterial blood gas (ABG) is a blood test taken from an artery, that measures the amount of oxygen and carbon dioxide that is found in the blood. The purpose of this measurement is to determine the lungs effectiveness in moving oxygen and carbon dioxide into and out of the bloodstream.

What does an ABG measure?

• Partial pressure of oxygen (Pao2) – This indicates the ability of the lungs to move oxygen into the bloodstream.
• Partial pressure of carbon dioxide (PaCo2) – This indicates the ability of the lungs to retrieve carbon dioxide out of the bloodstream.
• pH – The pH is the measurement of hydrogen ions (H+) found in the bloodstream which indicates the acid or base balance of the blood.
• Bicarbonate (HCo3) – This is the most important buffer found in the bloodstream, it assists in returning the body from an acid state back to a normal range.
• Oxygen Content (o2CT) and Oxygen Saturation (o2 Sat) – Like the Pao2, these values provide information about the oxygen content found in the bloodstream. 

When Should an ABG be Ordered?

There are 4 major reasons to draw an ABG and they are:

• Assessment of oxygenation capacity – determine cause of pleuritic chest pain or rule out Pulmonary Embolism
• Assessment of oxygen pressure to guide therapy – prevention of vision problems in premature infants and monitoring risk of pleural disruption (Pneumothorax) in such disease processes as ARDS
• Assessment of respiratory adequacy – oxygen and carbon dioxide measurement to assist with assessment of ventilation rate, depth and pressure
• Assessment of acid-base balance – disease identification and determination of metabolic status

Performing a Modified Allen’s Test:

Choose the best site (radial preferred, but brachial and femoral can be used)

If the radial artery is chosen a Modified Allen’s Test must be performed before any attempt at artery puncture is attempted. The purpose of a Modified Allen’s Test is to determine sufficient collateral circulation from the ulnar nerve (which sits beside the radial artery), in the event that a thrombus in the radial artery should occur. To perform a Modified Allen’s Test; follow this procedure:

• Place the patients arm on a flat surface, palm of hand facing up and wrist supported on a rolled towel.
• Compress both the radial and ulnar arteries using the index and middle finger of one hand for several seconds (usually 10 to 15)
• Ask the patient to clench and unclench their fist until blanching of the skin occurs
• When blanching is noted, release the pressure of the ulnar artery only and assess the hands ability to return to normal color (this should occur within 5 to 15 secs.)
• If color returns a positive Modified Allen’s test has occurred which means that the ulnar artery can supply blood to the hand should any occlusion of the radial artery occur.

Note: It is at this point that only a trained individual may proceed with accessing the radial artery and attaining an Arterial Blood Gas.

Normal Values

Arterial	Mixed Venous
pH 7.35-7.45	pH 7.34-7.37
PaC02 35-45 mmHg	PvC02 44-46 mmHg
PaO2 80-100 mmHg	Pv02 38-42 mmHg
HC03 22-26 mEq/Liter	HC03- 24-30 mEq/Liter
SaO2 > 95%	Sv02  60-80%

• The level of the C02 is controlled by breathing so any derangements of carbon dioxide are considered to be a respiratory problem.
• The level of Bicarb in the blood is controlled by the renal system so any derangements of Bicarb are considered to be a metabolic problem.

A Step by Step Approach to Interpreting ABG’s:

Step 1: Determine primary abnormality

Determine Acidosis versus alkalosis:

• pH <7.35: Acidosis
• pH >7.45: Alkalosis

Step 2: Determine is it is Respiratory or Metabolic:

• For Metabolic disorders remember the Ph changes in the same direction as the Bicarb or Co2.
• For Respiratory disorders remember the Ph changes in the opposite direction as the Bicarb or Co2.

For Example:

• Metabolic Acidosis: Serum pH/Serum Bicarb/Co2 all decreased
• Metabolic Alkalosis: Serum pH/Serum Bicarb/Co2 all increased
• Respiratory Acidosis: Serum pH decreased - Serum Bicarb/Co2 increased
• Respiratory Alkalosis: Serum pH increased - Serum Bicarb/Co2 decreased

Step 3: Determine if the Acidosis/Alkalosis is Partially or Completely Compensated:

Complete Compensation is simple if you remember one key concept. The body only “JUST” compensates back to normal range and then it stops; so if the pH is in normal range, determine which end of normal and that will tell you what the original problem was.

For Example:

Compensated ABG with pH 7.36 (the patient had an acidosis)

Compensated ABG with pH 7.44 (the patient had an alkalosis)

Partial Compensation works the same way only the pH is not yet within normal limits.

Examples for Practice: (answers below)

1. pH 7.51, pCO2 40, HCO3- 31

a. Normal
b. Uncompensated metabolic alkalosis
c. Partially compensated respiratory acidosis
d. Uncompensated respiratory alkalosis
2. pH 7.33, pCO2 29, HCO3- 16 (everything is decreased)
a. Uncompensated respiratory alkalosis
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
 3. pH 7.40, pCO2 40, HCO3- 24
a. Normal
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis

4. pH 7.12, pCO2 60, HCO3- 29 (decreased pH/increased bicarb/Co2)
a. Uncompensated metabolic acidosis
b. Uncompensated respiratory acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
 5. pH 7.48, pCO2 30, HCO3- 23
a. Uncompensated metabolic alkalosis
b. Uncompensated respiratory alkalosis
c. Partially compensated respiratory alkalosis
d. Partially compensated metabolic alkalosis
6. pH 7.62, pCO2 47, HCO3- 30
a. Uncompensated metabolic alkalosis
b. Uncompensated respiratory alkalosis
c. Partially compensated respiratory alkalosis
d. Partially compensated metabolic alkalosis
7. pH 7.30, pCO2 59. HCO3- 28
a. Uncompensated metabolic acidosis
b. Uncompensated respiratory acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis

Answers and Rationale:

1. B – pH is high, Bicarb is high, Co2 is normal and not attempting to correct the problem so this metabolic alkalosis is uncompensated.

2. D – pH is low, Bicarb is low, Co2 is low and attempting to correct the problem (but has not completely helped) so this is partially compensated metabolic acidosis.

3. A – pH, Bicarb and Co2 are within normal ranges so this is a normal ABG

4. C – pH is low, Co2 is high, Bicarb is high and attempting to correct the problem (but has not completely helped) so this is partially compensated respiratory acidosis

5. B – pH is high, Co2 is low, Bicarb is normal and not attempting to correct the problem so this is an uncompensated respiratory alkalosis

6. D – pH is high, Bicarb is high, Co2 is high and attempting to correct the problem (but has not completely helped) so this is a partially compensated metabolic alkalosis

7. C – pH is low, Co2 is high, Bicarb is high and attempting to correct the problem (but has not completely helped) so this is a partially compensated respiratory acidosis.

References

Marini, J., J. MD. Wheeler, A., P. (2009). Critical care medicine: the essentials. (4th ed.). Lippincott, Williams & Wilkins. Philadelphia

Hargrove-Huttel, R., A. (2008). Lippincott’s review series: medical-surgical nursing. (2nd ed.). Lippincott, Williams & Wilkens. Philadelphia