Arterial Blood Gases: Skill and Interpretation

Question 1

Why would you do an ABG

Answer 1

Assessment of respiratory patients, acutely unwell
Metabolic, respiratory state
Sodium, potassium, lactate, glucose, haemoglobin

Question 2

Contraindications with ABG sampling

• absolute
• relative

Answer 2

Absolute

• poor collateral circulation
• cellulitis at side
• AV fistula
• peripheral vascular disease

Relative
-impaired coagulation

Question 3

Describe what can be found in the ABG syringe

-how should you transport it

Answer 3

Can contain heparin = lowers pH, form a thin film on the surface, discard excess liquid before use

Blue 22G needle

Remove any air bubbles

Transported quickly or put on ice

Question 4

What would you before you start?

Answer 4

Introductions
Explain procedure, gain consent

Allen’s test

• occlude radial, ulnar artery
• raise arm in air, clenched fist => pale hand
• release ulnar artery => if sufficient, hand reperfused

Question 5

How would you perform ABG sampling

-preparation of equipment

Answer 5

ABG syringe, needle
Alcohol wipe, gauze, tape
Gloves
Sharps bin

Discard any extra heparin

Question 6

How would you perform an ABG sampling

-procedure

Answer 6

Gently extend wrist to 30 degrees
Palpate, identify center of arterial pulsation
Clean with alcohol wipe, allow to dry

Sample blood at 90 degrees
-aim for 3ml blood, the high velocity blood flow should push plunger up
Remove needle, press firmly down on area with gauze, cotton wool for minimum 3-5mins
Engage safety guard, discard needle
Cap ABG syringe, label sample with 3 patient identifiers and amount of O2 they’re on.

Question 7

What would you do post sampling?

Answer 7

Tape cotton wool on hand
Wash hands
Take syringe promptly to ABG machine on ice

Document

• clinical indication for measurement
• O2 therapy and interpretation of results
• subsequent plan

Question 8

Risks associated with ABG sampling

Answer 8

Bleeds

• hematoma
• hemorrhage esp in ACd

Infection
Nerve damage
Arterial blockage
False aneurysm formation

Needlestick injury
Local anaesthetic toxicity if used in sampling

Question 9

ABG interpretation nornal ranges of

• pH
• pCO2
• pO2
• HCO3
• base excess
• lactate
• anion gap

Answer 9

pH 7.35-7.45
pCO2 4.5-6.0kPa
pO2 - 11-13
-FiO2% - 10 if on O2
HCO3 22-26
Base excess -2 - 2
Lactate U2
Anion gap 6-12

Question 10

Common causes of

• resp acidosis
• resp alkalosis

Answer 10

Resp acidosis
Hypoventilation
-lung disease (COPD, asthma attack, pulmonary edema)
-CNS depression
-Mechanical lung dysfunction (obese, GBS, MG, kyphoscoliosis)

Resp alkalosis
Hyperventilation
-anxiety
-hypoxia
-acute pulmonary insult (PE, pneumonia, asthma attack, pulmonary edema)

Question 11

Common causes of

• met acidosis
• met alkalosis

Answer 11

Calculate anion gap!

Met acidosis
Normal anion gap - H retention OR HCO3 loss
-Addisons
-Bicarb loss - diarrhea, renal tubular necrosis
-Chloride
-Drugs (acetazolamide, CAinh)

Increased anion gap - acid gain

• Methanol
• Uremia
• DKA
• Propylene glycol
• Iron
• Lactate
• Ethylene glycol
• Salicylates

Met alkalosis
Acid loss
-vomiting, diuretics, CS
-hyperaldosteronism

Question 12

Types of respiratory failure

• T1
• T2

Answer 12

T1 - hypoxia
Poor ventilation - airway obstruction, asthma, COPD,
Poor perfusion - PE

T2 - hypoxia, hypercarbia
Alveolar hypoventilation - obstructive, restrictive lung disease, reduced RR, NMD, thoracic wall disease