Arterial Blood Gas (ABG) Flashcards
What is the purpose of an Arterial Blood Gas
It is used to determine if the patient’s metabolic state (respiratory/Metabolic Acidosis/ Alkalosis)
Where is arterial blood gas taken from?
The Radial Artery
and less commonly the Femoral Artery
What are the five (5) components of arterial blood gas?
pH
HCO-3
PaCO2
PaO2
O2 sat
What is the normal range for pH
pH : 7.35-7.45
What is the normal range for PaCO2
35-45 mmHg
What is the normal range for HCO-3
22-28 mEq/L
Which test is done before commencing an ABG in a patient?
Allen’s test
Why is Allen’s Test done before commencing an ABG
To assess collateral flow to the hands
List the steps in Allen’s Test
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
What does a negative Allen’s test indicate
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
List 4 contraindications for ABG
- negative Allen’s Test
- Burns over the site of the radial artery
- Signs of vascular disease
- Infections/Wounds overlying the radial artery
What are the ABG changes seen in a Respiratory Alkalosis
inc. pH
dec. CO2
dec. HCO-3
Define Respiratory alkalosis
This is a condition marked by inc. pH and dec. CO2 & HCO-3 in the blood due to sustained hyperventilation
List some causes of Respiratory alkalosis
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)
List some drugs that can cause Respiratory Alkalosis
-Aspirin (direct stimulation of respi centre in medulla)
-Progesterone (stim ventilation by activate progesterone receptors in the CNS)
What are two steps in management of Respi Alk
-Manage the underlying cause
-Rebreathe into a closed bag to allow CO2 levels to rise
What is Respiratory Acidosis
This is a condition marked by a decrease in blood pH and an increased in CO2 and HCO3- due to hypoventilation
Respiratory Acidosis is usually due to what type of respiratory failure ?
Type II
What is the difference between type I and type II respiratory failure?
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
List some causes of respiratory acidosis
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
What is the management of respiratory acidosis
Ventilatory support
correct the cause
Define Metabolic Acidosis
Metabolic Acidosis is defined as when an acid accumulates in the body resulting in a decreased plasma bicarbonate (decrease in pH and HCO3- )
What are the two types of metabolic acidosis?
Wide Anion Gap
Normal/Non Anion Gap
How is anion gap calculated ?
(NA+ + K+) - (HCO-3 + Cl-)
What is the range for a normal anion gap
5-11mmol/L
What are the three top differentials for Wide Anion Gap Metobolic Acidosis
Renal Failure
Lactic Acidosis
Diabetic Ketoacidosis
List the differentials for wide anion gap metabolic acidosis
M - methanol
U - uraemia
D - DKA (acc of ketones with >Gluc)
P - paraldehyde
I -isoniazide
L- lactic acidosis
E - ethlene glycol
S - salycilate
What is the usual cause of non-anion gap metabolic acidosis
This usually occurs when there is loss of bicarb from the ECF
List differentials for non-anion gap metabolic acidosis
H - hyperalimentation
A - Acetazolamide
R - Renal Tubular Acidosis
D - Diarrhea
U - uteroentericfistula
P - pancreatoduodenal fistula
How does renal tubular acidosis cause non- anion gap metabolic acidosis?
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)
Define metabolic alkalosis
This is defined as an increase in pH and HCO-3 due to loss of fluids
List some differentials for metabolic alkalosis
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
