Arterial Blood Gas (ABG)

Question 1

What is the purpose of an Arterial Blood Gas

Answer 1

It is used to determine if the patient’s metabolic state (respiratory/Metabolic Acidosis/ Alkalosis)

Question 2

Where is arterial blood gas taken from?

Answer 2

The Radial Artery
and less commonly the Femoral Artery

Question 3

What are the five (5) components of arterial blood gas?

Answer 3

pH
HCO-3
PaCO2
PaO2
O2 sat

Question 4

What is the normal range for pH

Answer 4

pH : 7.35-7.45

Question 5

What is the normal range for PaCO2

Answer 5

35-45 mmHg

Question 6

What is the normal range for HCO-3

Answer 6

22-28 mEq/L

Question 7

Which test is done before commencing an ABG in a patient?

Answer 7

Allen’s test

Question 8

Why is Allen’s Test done before commencing an ABG

Answer 8

To assess collateral flow to the hands

Question 9

List the steps in Allen’s Test

Answer 9

1) Ask the pt to raise their hand above their head for 30s (to exsanguinate the hand)

2) Occlude the ulnar artery and ask the pt to make a tight fist

3) Ask the pt to open their hand rapidly

4) If the hand return to its original colour it means that the hand is adequately perfused by the ulnar artery

+ Allen’s Test

Question 10

What does a negative Allen’s test indicate

Answer 10

It indicates that the patient’s hand is not adequately perfused by the ulnar artery and the ABG should not be done at the radial artery

Question 11

List 4 contraindications for ABG

Answer 11

• negative Allen’s Test
• Burns over the site of the radial artery
• Signs of vascular disease
• Infections/Wounds overlying the radial artery

Question 12

What are the ABG changes seen in a Respiratory Alkalosis

Answer 12

inc. pH
dec. CO2
dec. HCO-3

Question 13

Define Respiratory alkalosis

Answer 13

This is a condition marked by inc. pH and dec. CO2 & HCO-3 in the blood due to sustained hyperventilation

Question 14

List some causes of Respiratory alkalosis

Answer 14

Causes can be divided into two groups:

Hypoxia and Respi Centre Stimulation

Hypoxia: Pulmonary Embolism, Pulmonary fibrosis, Pulmonary oedema, pneumonia, severe anemia, high altitude, heart failure

Respi Centre: Anxiety, Pain, Gram - Sepsis, Pregnancy, CNS disorders
Drugs(Asa, progesterone)

Question 15

List some drugs that can cause Respiratory Alkalosis

Answer 15

-Aspirin (direct stimulation of respi centre in medulla)
-Progesterone (stim ventilation by activate progesterone receptors in the CNS)

Question 16

What are two steps in management of Respi Alk

Answer 16

-Manage the underlying cause

-Rebreathe into a closed bag to allow CO2 levels to rise

Question 17

What is Respiratory Acidosis

Answer 17

This is a condition marked by a decrease in blood pH and an increased in CO2 and HCO3- due to hypoventilation

Question 18

Respiratory Acidosis is usually due to what type of respiratory failure ?

Answer 18

Type II

Question 19

What is the difference between type I and type II respiratory failure?

Answer 19

Type I respiratory failure occurs when the respiratory system cannot adequately provide oxygen to the body, while Type II respiratory failure occurs when the respiratory system cannot adequately remove CO2

Question 20

List some causes of respiratory acidosis

Answer 20

Causes of respiratory acidosis can be classified in three groups

Airway obstruction: Asthma, foreign body, COPD, pulmonary edema, pneumothorax

Neuromuscular Dx: Myasthenia Gravis, Gullain Barre syn, poliomyelitis

Respi centre depression:
Drugs ( narcotics, anaesthetics, sedative), Trauma, inc. ICP

Question 21

What is the management of respiratory acidosis

Answer 21

Ventilatory support
correct the cause

Question 22

Define Metabolic Acidosis

Answer 22

Metabolic Acidosis is defined as when an acid accumulates in the body resulting in a decreased plasma bicarbonate (decrease in pH and HCO3- )

Question 23

What are the two types of metabolic acidosis?

Answer 23

Wide Anion Gap
Normal/Non Anion Gap

Question 24

How is anion gap calculated ?

Answer 24

(NA+ + K+) - (HCO-3 + Cl-)

Question 25

What is the range for a normal anion gap

Answer 25

5-11mmol/L

Question 26

What are the three top differentials for Wide Anion Gap Metobolic Acidosis

Answer 26

Renal Failure
Lactic Acidosis
Diabetic Ketoacidosis

Question 27

List the differentials for wide anion gap metabolic acidosis

Answer 27

M - methanol
U - uraemia
D - DKA (acc of ketones with >Gluc)
P - paraldehyde
I -isoniazide
L- lactic acidosis
E - ethlene glycol
S - salycilate

Question 28

What is the usual cause of non-anion gap metabolic acidosis

Answer 28

This usually occurs when there is loss of bicarb from the ECF

Question 29

List differentials for non-anion gap metabolic acidosis

Answer 29

H - hyperalimentation
A - Acetazolamide
R - Renal Tubular Acidosis
D - Diarrhea
U - uteroentericfistula
P - pancreatoduodenal fistula

Question 30

How does renal tubular acidosis cause non- anion gap metabolic acidosis?

Answer 30

This occurs through:
decreased HCO-3 reabsorption
Decreased Na reabsorption
Decreased Acid secretion ( potassium and H+ not secreted since Na not reabsorbed rem. the Na/K pump)

Question 31

Define metabolic alkalosis

Answer 31

This is defined as an increase in pH and HCO-3 due to loss of fluids

Question 32

List some differentials for metabolic alkalosis

Answer 32

Initiating factors:
-GI (vomiting, NG loss)
-Diuretics (Thiazide, Loop) renal loss H+

Maintenance factors
- Volume depletion
- hyperaldosteronism (inc. water /Na retention therefore secreting K+/H+)
- hypokalemia