ABG interpretation

ABG

INTE

RPRETATI

ON

FAY

ALI

AL- B

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INTRODUCTION:

Interpreting an arterial blood gas (ABG) is a crucial skill for physicians, nurses, respiratory therapists, and other health care personnel. ABG interpretation is especially important in critically ill patients.

Arterial blood gasses provide information about :

• Oxygenation• ventilation• acid-base balance.

6 STEPS TO INTERPRET ABG RESULT:Step 1#: analyze the PaO2 and the SaO2

Step 2#: analyze the pH

Step 3#: analyze the CO2

Step 4#: analyze the HCO3

Step 5#: match the CO2 or the HCO3 with the pH.

Step 6#: does the CO2 or the HCO3 go opposite direction of the pH?

The PaO2 only tells you if you need to give a patient more

oxygen or if you need to cut back on supplemental

oxygen.

These are normal values for arterial blood gases.

opH= 7.35 – 7.45oPaCO2= 35 – 45 mmHgoPaO2= 80 – 100 mmHgoHCO3= 23 – 26 MEq/L

When interpreting ABGs, the

most important

values to look at are pH,

PaCO2, and HCO3.

OXYGENATI

ON

STEP 1#: ANALYZE THE PAO2 AND THE SAO2

PaO2 is depend upon : Age FiO2Patm

paO2 : Partial pressure of oxygen molecules dissolved in plasma.

PAO2:

If the patient is hypoxemic, the low oxygen content in his blood will be reflected in low PaO2 and SaO2 values.

Mild hypoxemia ,PaO2 of (60-79) mmHg.moderate hypoxemia, (40-59) mm Hg.severe hypoxemia, less than 40 mm Hg.

SAO2:

Normal range (96% - 100%)

P/F RATIO: P:F ratio is the ratio of arterial oxygen

concentration to the fraction of inspired oxygen. It reflects how well the lungs absorb oxygen from expired air.

Based on Berlin definition:

• mild (from 200 to 300),

• moderate (from 100 to 200)

• and severe (≤100 mmHg).

O2 CONTENT :

Arterial oxygen content CaO2 is the amount of oxygen bound to hemoglobin plus the amount of oxygen dissolved in arterial blood.

CaO2 = (Hgb x 1.36 x SaO2) + (0.0031 x PaO2)

Normal CaO2 ranges from (16 – 22) ml O2/dl.

SCENARIO:A patient who presented to the ER with headache and dyspnea & h/o exposure to smoke in a closed room. His first blood gases showed Pao2 80 mmHg, PaCO2 38 mmHg, pH 7.43. and oxygenation was judged normal.He was sent out from the ER and returned a few hours later with mental confusion.This time both SaO2 and COHb were measured; paO2 79 mmHg,PaCO2 31 mmHg, pH7.36,SaO2 53%, carboxyhemoglobin 46%.

CO poisoning

CO POISONING: • SaO2 = NORMAL “false value on pulse

oximetry”

• PaO2 = NORMAL

Symptoms: Cherry red color instead of cyanosis (even though there is Hypoxia), its because of the red pigment of CO, headache and dizzness .

CARBOXYHEMOGLOBIN TEST

CO-OXIMETRY

CARBOXYHEMOGLOBIN TEST: Headache and nausea can begin when levels are

10 to 20%. Levels > 20% commonly cause vague dizziness, generalized weakness, difficulty concentrating, and impaired judgment. Levels > 30% commonly cause dyspnea during exertion, chest pain (in patients with coronary artery disease), and confusion. Higher levels can cause syncope, seizures, and obtundation. Hypotension, coma, respiratory failure, and death may occur, usually when levels are > 60%.

ACID-B

ASE BALA

NCE

6 STEPS TO INTERPRET ABG RESULT:Step 1#: analyze the PaO2 and the SaO2

Step 2#: analyze the pH

Step 3#: analyze the CO2

Step 4#: analyze the HCO3

Step 5#: match the CO2 or the HCO3 with the pH.

Step 6#: does the CO2 or the HCO3 go opposite direction of the pH?

STEP 2#: ANALYZE THE pH:

pH

< 7.35 7.35-7.45 > 7.45

Acidosis Normal or Compensated Alkalosis

•This is usually the primary disorder•Remember: an acidosis or alkalosis may be present even if the pH is in the normal range (7.35 – 7.45) HOW?? When it is fully compensated and that will be described later.

STEP 3#: ANALYZE THE CO2

PaCO2

< 35 35 -45 > 45

• Tends toward alkalosis

• Causes high pH

Normal orCompensated

• Tends toward acidosis

• Causes low pH

STEP 3#: ANALYZE THE HCO3:

HCO3

< 22 22-26 > 26

• acidosis• Causes low pH

Normal orCompensated

• Alkalosis• Causes high pH

STEP 5+6#: MATCH THE CO2 OR THE HCO3 WITH THE pH.Acidosis Respiratory pH ↓ PaCO2 ↑ HCO3 N

Acidosis Metabolic pH ↓ PaCO2 N HCO3 ↓

Alkalosis Respiratory pH ↑ PaCO2 ↓ HCO3 N

Alkalosis Metabolic pH ↑ PaCO2 N HCO3 ↑

Acidosis mixed pH ↓ PaCO2 ↑ HCO3 ↓

What is the relationship between the direction of change in the pH and the direction of change in the PaCO2?In primary respiratory disorders, the pH and PaCO2 change inapposite directions; in metabolic disorders the pH and PaCO2 change in the same direction.

Metabolic

Respiratory

Acute chronicAnion gap

Alkalosis Acidosis

pH

PaCO2 Inversely

Proportionally

IF IT’S RESPIRATORY: ACUTE OR CHRONIC?

Expected changes in pH for a 10-mmhg change PaCO2 resulting from either primary respiratory acidosis or respiratory alkalosis:

• Respiratory acidosispH↓ by 0.08 pH↓ BY

0.03• Respiratory alkalosis

pH↑by 0.08 pH↑by 0.03

Acute Chronic

COMPENSATED OR NOT:The body compensates for acid-base imbalance

through either the lungs or the kidneys.

If PaCO2 is abnormal and pH is normal, it indicates compensation.

pH > 7.4 would be a compensated alkalosis. pH < 7.4 would be a compensated acidosis.

Respiratory compensation Complete within 24hrs

Metabolic compensation Complete within several days

Both the respiratory or renal compensation almost never over compensates.

PARTIALLY COMPENSATED:

With partially compensated ABGs, the body has begun to correct the acid-base problem via either the lungs or the kidneys.

The pH has not returned to the normal range.

CASE #1 : A patient’s ABGs pH= 7.30; PaCO2=31

mmHg; PaO2=76 mmHG; HCO3=17 mEq/L.

First, look at the pH. What does this pH value indicate?

alkalosis

acidosis



The above ABG represents what kind of acidosis ?Respiratory

acidosis

Metabolicalkalosis

Metabolic acidosis

Respiratory alkalosis



Fully compensated

patially Compensated

Uncompensated

CASE #2 : A patient’s blood gases are pH= 7.35;

PaCO2= 33; PaO2= 88; HCO3= 18.

What does the pH indicate?

alkalosis

acidosis


PaCO2= 33; PaO2= 88; HCO3= 18.

Based on this information, the above ABG represents what kind of acidosis?

Respiratory acidosis

Metabolicalkalosis

Metabolic acidosis

Respiratory alkalosis


PaCO2= 33; PaO2= 88; HCO3= 18.

patially Compensated

Uncompensated

Fully compensated

IF IT’S METABOLIC: ANION GAPAnion gap measures the difference between

anions (-) and cations (+) present in blood.

Normal value = 10 – 12 mmol/L

AG = Na – (HCO3 + Cl)

CAUSES OF HIGH-ANION-GAP METABOLIC ACIDOSIS Elevated anion

gap represents metabolic acidosis.

REFERENCES:• Interpreting Arterial Blood Gases :WILLIAM C.

Pruitt CPFT, MBA, AND MICHAEL JACOBS, RN, CCRN, CEN,MSN.

• Interpretation of Arterial Blood Gases (ABGs)David A. Kaufman, MD Chief, Section of Pulmonary, Critical Care & Sleep Medicine.

• Clinical Blood Gases,2nd Edition, Assessment & Intervention: W.Malley,Imprint:Saunders. ISBN :9780721684222

• 6 Easy steps to ABG analysis,E-booklet, David W. Woodruff, MSN, RN-BC, CNS, CMSRN, CEN.

FOR YOUR ATTENTION

Presented by: Fay Ali Al-Buainain

Healthcare

ABG interpretation