Arterial Blood Gas Analysis

Question 1

What is arterial blood gas analysis?

Answer 1

An invasive method for measuring oxygenation, carbon dioxide clearance, and acid-base balance (H+/pH) in the arterial blood, oxygen saturation, blood pH, and bicarbonate values. This makes it a more comprehensive measure than pulse oximetry

Question 2

ABG values: Normal

Answer 2

pH: 7.35 - 7.45
PaCO2: 35-45mmHg
HCO3-: 22-26mmol/L
BE: -2 to +2 mEq/L

Question 3

ABG values: Acidaemia

Answer 3

pH: <7.35
PaCO2: >45mmHg
HCO3-: <22
BE: <-2

Question 4

ABG values: Alkalaemia

Answer 4

pH: >7.45
PaCO2: <35mmHg
HCO3-: >26
BE: >+2

Question 5

What are the four types of acid-base disturbance?

Answer 5

1. Respiratory acidosis
2. Respiratory alkalosis
3. Metabolic acidosis
4. Metabolic alkalosis

Question 6

What respiratory components affect pH?

Answer 6

The PaCO2 is regulated by the lungs and can be used to determine if an acid-base disturbance is caused by the respiratory system.
- Hypoventilation caused by high PaCO2 is therefore acidosis (increased CO2 in blood via respiratory acidosis)
- Hyperventilation caused by low PaCO2 is therefore alkalosis (decreased CO2 in blood via respiratory alkalosis)

Question 7

What metabolic components affect pH?

Answer 7

The bicarbonate ion (HCO3-) is the acid-base component regulated by the kidneys. Acting as the body’s buffer, the kidneys retain or excrete alkalotic HCO3- as needed
- If the HCO3- decreases, the reaction shifts to the right (process of metabolic acidosis), H+ is increased to reduce pH (acidaemia)
- If the HCO3- increases, the reaction shifts to the left (process of metabolic alkalosis), H+ is decreased to increase pH (alkalaemia)

Question 8

Define acidaemia

Answer 8

A low pH level in the blood

Question 9

Define alkalaemia

Answer 9

A high pH level in the blood

Question 10

Define acidosis

Answer 10

A physiological process that lowers the blood pH (causing acidaemia)

Question 11

Define alkalosis

Answer 11

A physiological process that raises the blood pH (causing alkalaemia)

Question 12

What is hypoxaemia and its consequences?

Answer 12

Abnormally low oxygenation of arterial blood (low PaO2/SaO2). Consequences of acute hypoxaemia include:
o Tachycardia, increased cardiac output, arrhythmias, hypotension (peripehral vasodilation)
o PaO2 <55mmHg leads to short term memory alteration, confusion, euphoria
o PaO2 <30mmHg leads to loss of consciousness

Question 13

What is hypoxia?

Answer 13

When O2 delivery to tissues is inadequate to maintain normal tissue oxygenation/meet metabolic needs (hypoxaemia is one cause of tissue hypoxia)

Question 14

What two methods are used to interpret partial pressure of oxygen (PaO2)?

Answer 14

METHOD 1: PaO2 should be approx FiO2 (% O2) x 5 (e.g. if breathing in RA, should be 21x5 = 105mmHg)
METHOD 2: P/F ratio (normal is 350-450) where P is the PaO2 in mmHg and F is the FiO2 as a decimal. The smaller the value, the worse the patients gas exchange and oxygenation (e.g. 80/0.21 = 381)

Question 15

What are the normal values for FiO2, PaO2 and SaO2?

Answer 15

FiO2 (RA) = 21% (0.21)
PaO2 = 80-100mmHg
SaO2 = 95-100%

Question 16

What are hypoxaemia values for FiO2, PaO2 and SaO2?

Answer 16

FiO2 = N/A
PaO2 = <80mmHg
SaO2 = <95%

Question 17

What are the severe hypoxaemia (respiratory failure) values for FiO2, PaO2 and SaO2?

Answer 17

FiO2 = N/A
PaO2 = <60mmHg
SaO2 = <90%

Question 18

What is the effective of CO2 on ventilation?

Answer 18

The level of CO2 in the blood is the most important influence on the regulation of ventilation. The blood level of CO2 acts directly on the respiratory centres in the medulla, affecting CSF pH

Question 19

How do we interpret ABG results?

Answer 19

1. Determine the nature of the primary disturbance (by looking at the pH to see if it is normal, acidic or alkaline)
2. Determine the cause of the primary disturbance (respiratory shows in PaCO2, metabolic shows in HCO3- and BE)
3. Determine whether there is compensation occurring via the other system (are the opposite ranges out of balance also?)

Question 20

Physiotherapy implications of ABG result interpretation

Answer 20

If you have ABG results, you can definitively determine whether a patient has a GAS MOVEMENT impairment or not. You can also now define the gas movement impairment further as either:
- Reduced O2 movement (low PaO2 – or lower PaO2 than should be for given FiO2 or low P/F ratio)
- Reduced CO2 movement (high PaCO2/respiratory acidosis)