ABG Interpretation

Steven Podnos MD

Why Do ABG’s?

1) Check oxygenation

2) Check the pH (acid base balance)

3) Ventilatory Status

4) Determine the treatment

Basic ABG Components

• pH

• PaCO2

• HCO3

• PaO2

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.

Buffer System

Carbonic Acid - Bicarbonate Buffer System

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

NOTE THAT “CO2” on Lyte panel is HCO3• Note: The two headed arrows indicate that the process is reversible

Acid/Base-General Considerations

PH = Increased - Alkalosis Decreased - Acidosis

PCO2 = Increased – Acidosis Decreased – Alkalosis

HCO3 = Increased – Alkalosis Decrease - Acidosis

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.

A. Metabolic Acidosis

• Results from renal failure, Lactic acidosis, DKA

B. Respiratory Acidosis Hypoventilation• Results from respiratory failure, instrinsic or

extrinsic.

C. Metabolic Alkalosis

• Results from Gastric Suction, vomiting,

D. Respiratory Alkalosis Hyperventilation• Results from Head Trauma, Fever, Emotions, Salicylate Ingestion, Shock at an early stage

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.

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.

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.

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.

Steps to ABG Interpretation

1) Determine Acidosis or Alkalosis.

2) Evaluate the Respiratory Mechanism

3) Evaluate the Metabolic Mechanism

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

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 HCO3 Low PaCO2 High PaCO2 Low HCO3

Metabolic Respiratory 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.

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

Question

• If the pH is 7.30, the PaCO2 is 50, and the HCO3 is 24 what is the likely diagnosis?

Question 1

• If the pH is 7.30, the PaCO2 is 50, and the HCO3 is 24 what is the likely diagnosis?

Acute uncompensated RESPIRATORY

ACIDOSIS

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

Question

Question #2

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

ACUTE RESPIRATORY ALKALOSIS,

Question

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

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

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

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

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

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