ABGs

Question 1

What are the indications for an ABG?

Answer 1

• To assess oxygenation
• Monitoring oxygen therapy administration/ patients on ventilators
• To assess acid-base status
• Monitoring patients with acid-base abnormalities
• To assess suspected dyshaemoglobinemias

Question 2

What are the contraindications for ABGs?

Answer 2

• Poor collateral flow/ non palpable pulse
• Arm with IV infusion in progress
• Abnormal or infectious skin processes at the puncture site
• Arterial graft, surgical shunt, or AV fistula in the arm
• History of arterial spasm after ABG
• History of clotting disorders or anticoagulant use
• Known, or suspected, aneurysm at the puncture site

Question 3

What are the complications of ABGs?

Answer 3

• Vascular thrombosis or spasm
• Distal and proximal embolus
• Bleeding or haematoma formation
• Pain, infection, and local damage
• Venous sample
• Vaso-vagal reaction

Question 4

What is the rationale for site selection?

Answer 4

Radial = preferred
Superficial so easy to access and easy to apply pressure after

Femoral if:

• Absent ulnar circulation (modified Allen’s test) or other CI for radial e.g underlying skeletal trauma
• Patient is in shock/ peripherally shut down

However, never do femoral on children due to risk of septic arthritis!! or of extensive vascular disease (risk of emboli)

Brachial is not used due to arterial depth (risk of impaired circulation distally) and close proximity to median N

Question 5

How do you assess ulnar circulation?

Answer 5

Modified Allen’s test:

1. Raise the patient’s hand and occlude the radial and ulnar circulations
2. Ask the patient to make a fist for 20 seconds
3. Ask the patient to open their hand, observe for blanching
4. Release pressure on the ulnar artery, and ensure that colour returns to the hand within 7 seconds

Question 6

Which needles do you use for radial vs femoral artery?

Answer 6

Radial = 23g (blue)
Femoral = 21g (green)

Question 7

For how long do you put pressure on artery after ABG?

Answer 7

5 mins

Question 8

How long after taking sample should it be analysed?

Answer 8

15 mins

Question 9

What should you do with the sample after it has been taken before it is analysed?

Answer 9

Invert syrringe

push out a few drops of blood before putting it into the analyser (clots can break the machine)

Question 10

Normal range for pH?

Answer 10

7.35-7.45

Question 11

Normal base excess?

Answer 11

+/- 2

Question 12

Normal range for bicarb?

Answer 12

22-26

Question 13

Normal range for PaO2?

Answer 13

11-13

Question 14

Normal range for PaCO2?

Answer 14

4.7-6

Question 15

Normal lactate?

Answer 15

less than 2

Question 16

What other things can you measure with an ABG?

Answer 16

• SaO2
• Hb
• Glucose
• Electrolytes

Question 17

Causes of Respiratory Acidosis?

Answer 17

Hypoventilation (T2RF)

• lung disease
• NMJ dysfunction
• Mechanical lung dysfunction

Question 18

Causes of Respiratory Alkalosis?

Answer 18

Hyperventilation

• Anxiety
• Hypoxia
• Acute pulmonary insult (T1 RF)

Question 19

Causes of Metabolic Acidosis?

Answer 19

High anion gap (new acid added to body or retaining H+)

• Lactic acidosis (lactate can be raised in sepsis, poor tissue perfusion, seizure, LF or as a SE of salbutamol or metformin)
• DKA
• Drugs/ toxins (e.g. salicylate, ethylene glucol, methanol)
• Renal Failure

Low/ normal anion gap (losing HCO3-)

• RTA
• Hypoaldosteronism e.g. medications, Addison’s
• GI tract losses (diarrhoea, fistula)

Question 20

Causes of Metabolic Alkalosis?

Answer 20

Loss of H+

• Vomiting
• Hyperaldosteronism