Arterial Blood Gas

Interpretation

Sherry L. Knowles, RN, CCRN, CRNIOrlando Regional Medical Center 2009

Objectives

Recognize Signs & Symptoms of Respiratory Failure

Understand Ventilation and Perfusion Mechanics

Analyze and Interpret Acid Base Disturbances

Identify Appropriate Treatments for Abnormal ABG’s

Objectives

Recognize normal and abnormal values for pH, PaO2, PaCO2, SaO2 and HCO3.

Relate the pH scale to acidosis and alkalosis. Discuss the respiratory and metabolic mechanisms

involved in controlling the body's acid-base balance. Interpret basic arterial blood gas values and relate

these values to patient conditions. Anticipate appropriate therapies for acid-base

correction.

Why Do ABG’s?

1) Check oxygenation

2) Check the pH (acid base balance)

3) Define the problem

4) Determine the treatment

Fundamentals

All human cells require oxygen.

Breathing (ventilation) brings oxygen in and CO2 out of the lungs.

Oxygen is absorbed into the bloodstream through the alveoli.

Hemoglobin molecules carry oxygen to the tissues.

Hemoglobin

Carries Oxygen

Has 4 Binding Sites

Hemoglobin + 4 Oxygen = Oxyhemoglobin

Hemoglobin Binding Sites

When all of 4 sites are occupied, the hemoglobin molecule cannot hold any more.

Molecules, other than oxygen, can attach to the oxygen binding sites.

If enough hemoglobin binding sites are occupied with molecules other than oxygen, severe tissue hypoxia can result.

Hypoxia can occur even in the presence of 100% oxygen. This can be a life-threatening condition.

Carboxyhemoglobin (HbCO)

Carboxyhemoglobin (HbCO) is a hemoglobin molecule that has carbon monoxide attached where the oxygen should be.

Exposure to car exhaust, or other chemicals can cause carbon monoxide (CO) to attach to hemoglobin binding sites, instead of oxygen and thus compete with the oxygen for the limited number of binding sites.

The blood will exhibit a cherry red color.

Methemoglobin (MetHb)

Methemoglobin (MetHb) is produced when exposed to certain poisons or due to a genetic condition that affects the hemoglobin molecule.

The hemoglobin molecule is saturated with methemoglobin (MetHb) and competes with oxygen for the hemoglobin binding sites.

Methemoglobin (MetHb) changes blood to a brownish color.

Oxygen and Hemoglobin

CO2's affinity for hemoglobin is much less than O2's affinity for hemoglobin.

When CO2 and O2 are both available, hemoglobin will accept oxygen rather than CO2.

In the oxygen rich environment of the alveoli, hemoglobin carries oxygen.

Oxygenated blood then travels through the body.

Acid Base Balance

Understanding the cause of an acid-base imbalance is the key to treating it.

The Respiratory component of acid base balance affects the pH within minutes.

The Metabolic component of acid base balance can take days to affect pH.

Buffer System

Carbonic Acid - Bicarbonate Buffer System

CO2 + H2O <--> H2CO3 <--> (HCO3-) + (H+) carbon dioxide + water <--> carbonic acid <--> bicarbonate + hydrogen ion

Note: The two headed arrows indicate that the process is reversible

CO2

When combined with water, carbon dioxide becomes carbonic acid (H2CO3)

Carbon dioxide is an acid when dissolved in water.

Carbon dioxide is a product of metabolism.

As long as cells are functioning, CO2 is produced.

HCO3

Bicarbonate = HCO3.

HCO3 increases in response to high CO2.

Metabolic changes take days to affect pH.

Basic ABG Components

pH

PaCO2

HCO3

PaO2

pH

1. The pH scale ranges from 1 to 14.

2. pH 7 is Neutral

3. Low pH is Acid.

4. High pH is Alkaline.

pH

Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb)

Normal blood pH = 7.35-7.45

pH < 7.35 = acidosis

Ph > 7.45 = alkalosis

PaCO2

CO2 has several forms in the blood.

Like oxygen, some is dissolved directly into the plasma. The PaCO2 is the measurement of the partial pressure of carbon dioxide dissolved in the plasma. It is measured in mm Hg (millimeters of mercury).

The rest is found in the red blood cells on a hemoglobin molecule.

PaO2

About 3% of the body's oxygen is dissolved in the plasma.

PaO2 is a measurement of the partial pressure of oxygen dissolved in the plasma only. It is measured in mm Hg.

The PaO2 does not tell us about the body's total oxygen content, but it does indicate how much oxygen was available in the alveoli to dissolve in the blood.

SaO2

The remainder of the body's oxygen is carried attached to hemoglobin molecules.

SaO2, or oxygen saturation, measures the degree to which oxygen is bound to hemoglobin.

Sa02 is expressed as a percentage.

Ventilation

Controls CO2 levels

CO2 = Ventilation

High CO2 = Hypoventilation

Low CO2 = Hyperventilation

Respiratory Mechanism

1) Respiratory Mechanism (depth and rate of breathing) controls CO2.

2) CO2 in solution is an acid.

3) Higher PaCO2 causes acidosis (lower pH), or neutralizes alkalosis.

4) Lower PaCO2 causes alkalosis (raises pH.), or neutralizes acidosis.

Metabolic Mechanism

Bicarbonate = Alkaline = HCO3.

HCO3 increases in response to high CO2.

Metabolic changes take days to affect pH.

Compensation

“Compensation" is the body's normal response to normalize pH

– By neutralizing the opposite acid base mechanism. Example: If the pH is high because of respiratory

alkalosis (low CO2): – Alkaline HCO3- will decrease to neutralize the pH. – In this case, the abnormal bicarb is not a metabolic problem; it

is a metabolic solution to a respiratory problem. It is important to determine which is the cause and

which is the effect. If you treat the compensatory abnormality, you make the

pH even more abnormal.

Normal ABG Values 

pH = 7.35-7.45 7.4 (+/- 0.5) 

PaCO2 = 35-45 40 (+/- 5)

HCO3 = 22-26 24 (+/- 2)

PaO2 = 80-100 90 (+/- 10)

SaO2 = 94-100 97 (+/- 3) 

Norms Quick Reference

Steps to ABG Interpretation

1) Determine Acidosis or Alkalosis.

2) Evaluate the Respiratory Mechanism

3) Evaluate the Metabolic Mechanism

STEP 1

Step 1.

Use pH to determine Acidosis or Alkalosis.

  Normal or Compensated =   Acidosis = Alkalosis =

 

7.35-7.45

< 7.35 > 7.45

 

STEP 1

Step 1. Use pH to determine Acidosis or Alkalosis.

   ph

 < 7.35  7.35-7.45  > 7.45

 Acidosis Normal or

Compensated Alkalosis

STEP # 2

Step 2. Use PaCO2 to

look at the Respiratory Mechanism

   PaCO2

 < 35  35 -45 > 45

• Tends toward alkalosis • Causes high pH • Neutralizes low pH

 Normal or

Compensated

• Tends toward acidosis • Causes low pH • Neutralizes high pH

STEP 3

Step 3.

Use HCO3 to look at the Metabolic Mechanism

   HCO3

 < 22  22-26 > 26

• Tends toward acidosis • Causes low pH • Neutralizes high pH

 Normal or

Compensated

• Tends toward alkalosis Causes high pH • Neutralizes low pH

Interpretation

 High pH Low pH

 Alkalosis Acidosis 

High HCO3Low

PaCO2High

PaCO2Low HCO3

 MetabolicRespiratory

Respiratory Metabolic

Compensation

“Compensation" is the body's normal response to normalize pH

– By neutralizing the opposite acid base mechanism. Example: If the pH is high because of respiratory

alkalosis (low CO2): – Alkaline HCO3- will decrease to neutralize the pH. – In this case, the abnormal bicarb is not a metabolic problem; it

is a metabolic solution to a respiratory problem. It is important to determine which is the cause and

which is the effect. If you treat the compensatory abnormality, you make the

pH even more abnormal.

Normal ABG Values 

Norms Quick Reference

pH = 7.35-7.45 7.4 (+/- 0.5) 

PaCO2 = 35-45 40 (+/- 5)

HCO3 = 22-26 24 (+/- 2)

PaO2 = 80-100 90 (+/- 10)

SaO2 = 94-100 97 (+/- 3) 

Combined Disturbances

A “Combined Disturbance” occurs when the PaCO2 and

HCO3- both alter the pH in the same direction.

A high PaCO2 and low HCO3- (acidosis).

Low PaCO2 and high HCO3- (alkalosis).

VERY RARE

OxyHemoglobin Dissociation Curve 

This curve describes the relationship between available oxygen and amount of oxygen carried by hemoglobin.

Oxygen-Hemoglobin affinity changes with:• variation in pH *CO2 *temperature *2,3,-DPG

Once the PaO2 reaches 60 mm Hg the curve indicates that there is little change in saturation above this point.

– So, PaO2 of 60 or more is usually considered adequate. – At PaO2 of less than 60 even small changes will greatly

reduce the SaO2.

Question 1

If the pH is 7.23, the PaCO2 is 50, and the HCO3 is 24 what is the likely diagnosis?

 

RESPIRATORY ACIDOSIS

Question #2

If the pH is 7.49, the PaCO2 is 25, and the HCO3 is 22 what is the likely diagnosis?

RESPIRATORY ALKALOSIS

Question # 3

If the the pH is 7.56, the PaCO2 is 39, and the HCO3 is 38, what is the likely diagnosis?

METABOLIC ALKALOSIS

Question # 4

If the pH is 7.35, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis?

   COMPENSTATED METABOLIC ACIDOSIS  

Question # 5

If the pH is 7.30, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis?

   PARTIALLY COMPENSTATED

METABOLIC ACIDOSIS   

The End

OxyHemoglobin Dissociation Curve 

Shift Left InterpretationIncreased affinity for O2Hemoglobin does not unload so easilySaO2 may be higher at given PaO2, but O2 delivery is less

CausespHPCO2Temperature2, 3-DPG (use of bank blood can cause this )

Shift Right InterpretationDecreased affinity for O2Hemoglobin unloads O2 more easily than usualSaO2 may be less, but O2 delivery is higherCausespHPCO2Temperature2, 3-DPG