- ABG INTERPRETATION -

Question 1

What do ABGs measure?

Answer 1

• Partial pressure of O2
• pH
• Partial pressure of CO2
(PaCO2)
• Bicarbonate (HCO3) concentration
• Base excess
• Anion gap

Question 2

Discuss the role of PaO2 in oxygenation

Answer 2

• partial pressure of oxygen in arterial blood
• guide to efficiency of gas exchange from the lungs into the blood
• normal 80-100mmHg on 21% of FiO2
• should be assessed in conjunction with FiO2

Question 3

What is the estimated PaO2, relative to FiO2

Answer 3

FiO2 40% - PaO2 200mmHg
FiO2 60% - PaO2 300mmHg
FiO2 80% - PaO2 400mmHg

Question 4

Discuss the role of SaO2 in oxygenation

Answer 4

• the percentage of Hb molecules saturated with O2
• measure of their ability to carry O2
• important for oxygen content and delivery to the tissues
• normal 95-100% on 21% of FiO2
• should be assessed in conjunction with FiO2

Question 5

Discuss full, partial and uncompensated metabolic and respiratory abnormalities

Answer 5

• Full compensation – pH within normal limits with values of respiratory and
metabolic components outside normal range and in opposite directions
• Partial compensation – compensation goes in the same direction as the
cause but unable to normalise pH
• Uncompensated – one system has made no correction

Question 6

List the causes of respiratory acidosis

Answer 6

• hypoventilation
• CNS/respiratory centre depression
• airway obstruction
• lung disease
• neuromuscular interference

Question 7

List the causes of metabolic acidosis

Answer 7

increased production H+
(ketoacidosis
• altered cellular metabolism (lactic acidosis)
• ingestion toxins increase lactate production
• renal failure (decreased excretion H+)
• excessive loss HCO3
(diarrhoea, renal tubular acidosis)

Question 8

List the causes of respiratory alkalosis

Answer 8

• hyperventilation/CNS irritation
• hypoxia/hypoxaemia
• anxiety
• mechanical ventilation
• fever/sepsis
• hyperthyroidism
• lung disorders
• liver failure
• drugs - salicylates

Question 9

List the causes of metabolic alkalosis

Answer 9

• excess loss H+ (vomiting, NG suctioning, Cl- wasting diarrhoea)
• excess HCO3 - (diuretic therapy, overuse antacids, citrate in blood transfusion)

Question 10

Discuss the role of the Anion Gap

Answer 10

The anion gap is the difference between the plasma concentration of the major measured cation (Na+ & K+ ) and the sum of the measured anions (Cl- & HCO3 - )
It is useful in identifying the cause of metabolic acidosis

Normal anion gap < 18 mEq/L

AG = (Na+ + K+ ) – (HCO3 - + Cl)

A high AG means there is an accumulation of acid in body fluids that is not being buffered (loss of HCO3 - without a subsequent increase in Cl)

Question 11

Discuss the normal ranges on an ABG

Answer 11

pH 7.35 – 7.45
PaO2 80 – 100 mmHg
SaO2 95 - 100%
PaCO2 35 – 45 mmHg
HCO3
- 22 – 26 mEq/L
Base Excess -2 - +2
Anion Gap <18

Question 12

Discuss a clinical approach to ABG analysis

Answer 12

1. Look at oxygenation (PaO2)
   Hypoxia?
2. Look at pH
   Is the pH normal?
   Is your patient acidotic (pH < 7.35)
   Is your patient alkolitic (pH > 7.45)
3. Look at PaCO2
   to determine if respiratory or metabolic
   Is pCO2 normal?
   Is pCO2 increased? (acidosis or compensation)
   Is pCO2 decreased? (alkolosis or compensation)
4. Look at HCO3
   - Is the HCO3 normal?
   Is the HCO3 increased (alkolosis or compensated)
   Is the HCO3 decreased? (acidosis or compensated)
5. Look at anion gap
   Na+ + K+ – Cl- + HCO3 <18 accumulation of acid in body fluid (lactic, ketones, uric, phosphoric etc.)