Factors Affecting Validity of Arterial Blood Gases Results ...
Arterial Blood Gases
-
Upload
sherry-knowles -
Category
Health & Medicine
-
view
22 -
download
1
description
Transcript of Arterial Blood Gases
Arterial Blood Gas
Interpretation
Sherry L. Knowles, RN, CCRN, CRNIOrlando Regional Medical Center 2004
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).
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