Arterial blood gas

Question 1

How is a Modified Allen’s test performed?

Answer 1

1. Patient clenches fist
2. Apply pressure over the radial and ulnar artery to occlude both vessels
3. Ask patient to open hand which should now appear blanched.
4. Remove pressure from the ulnar artery whilst maintaining pressure over radial artery
5. If there is adequate perfusion from ulnar artery, normal colour should return within 5-15 seconds.

Question 2

What is the importance of Modified Allen’s test in an ABG?

Answer 2

Assess the collateral arterial supply of the hand from the ulnar artery i.e., to make sure the hand isn’t completely reliant on the radial artery for its blood supply, in which case sampling should be avoided.

Question 3

What is the technique for doing an ABG?

Answer 3

1. Remove equipment from packaging, attach needle to pre-heparinised ABG syringe, position hand with wrist extended by 20-30 degrees
2. Palpate radial artery and identify a distal site where artery is most pulsatile
3. Clean area for 30 seconds and allow to dry
4. Wash hands again and don an apron and gloves
5. Administer subcutaneous lidocaine and aspirate to ensure not in a blood vessel before injecting the LA. Allow at least 60 seconds for the LA to work
6. Flush through the heparin from the syringe of the ABG
7. Palpate radial artery around 1cm proximal to the planned puncture site
8. Hold ABG syringe like a dart and insert at 30-45 degrees angle
9. Advance needle towards pulsation until you feel sudden reduction in resistance and see flashback. The syringe should now fill in a pulsatile manner
10. Once required amount of blood has been collected, remove the needle and apply firm pressure.
11. Gauze, discard needle, place cap on ABG syringe and expel any air from the sample if present

Question 4

What are the indications for an ABG? (x7)

Answer 4

To assess oxygenation levels, respiratory derangements, metabolic derangements, acid-base status, carboxyhaemoglobin in CO poisoning, assess lactate (for sepsis), and gain preliminary results for electrolytes and Hb.

Question 5

What are the contraindications of ABG? (x4)

Answer 5

Peripheral vascular disease, cellulitis surrounding site, and arteriovenous fistula in situ are absolute contraindications. Impaired coagulation is a relative contraindication.

Question 6

What are sources of sampling error in ABGs? (x3)

Answer 6

Presence of air in the sample distorts blood gases, an improper quantity of heparin in syringe, delay in specimen delivery (increases paCO2)

Question 7

What are the complications of an ABG? (x5)

Answer 7

Haematoma, nerve damage, arteriospasm, arterial aneurysm, vaso-vagal response of fainting

Question 8

What is the normal range for pH in an ABG?

Answer 8

7.35-7.45

Question 9

What is the normal range for pCO2 in an ABG?

Answer 9

4.5-6.0 kPa

Question 10

What physiological factors result in low pCO2? (x2)

Answer 10

Large tidal volume, increased RR.

Question 11

What is the normal range for pO2 in an ABG?

Answer 11

10-14 kPa

Question 12

What is the normal range for HCO3 in an ABG?

Answer 12

22-26 mmol/l

Question 13

What is the buffer equation that controls pH in the blood?

Answer 13

CO2 + H2O <=> H2CO3 <=> HCO3- + H+ catalysed by carbonic anhydrase

Question 14

What is the normal range for BE in an ABG?

Answer 14

-2 to +2

Question 15

What is the normal range for lactate in an ABG?

Answer 15

0.5-1 mmol/l but less than 2 when acutely ill.

Question 16

How is lactate produced?

Answer 16

By-product of anaerobic respiration. Therefore, it is raised in exercise, hypoxia and ischaemia.

Question 17

What drugs induce hyperlactatemia?

Answer 17

Metformin.

Question 18

What is the normal range for Hb in an ABG?

Answer 18

115-160 g/l

Question 19

What is the normal range for Na+ in an ABG?

Answer 19

135-145 mmol/l

Question 20

What is the normal range for K+ in an ABG?

Answer 20

3.5 – 5.5 mmol/l

Question 21

What is the normal range for Ca2+ in an ABG?

Answer 21

1.10 – 1.35 mmol/l

Question 22

How must serum calcium be corrected? Note about ABG?

Answer 22

Corrected for amount of albumin present in the blood as calcium is protein bound in circulation. ABG samples will not correct this since it does not measure serum albumin.

Question 23

!!! What happens to serum calcium in acid-base disturbances? How does this manifest physically?

Answer 23

Acidosis increases ionised calcium levels due to DECREASED protein binding. Opposite trend in alkalosis. In alkalosis, patients may experience peri-oral paraesthesia which is caused by a reduction in calcium dissociation (hypocalcaemia).

Question 24

What is the normal range for Cl in an ABG?

Answer 24

95 – 105 mmol/l

Question 25

What is FO2Hb?

Answer 25

Fractional haemoglobin – measure of how much of the Hb is oxyHb.

Question 26

What is FCOHb?

Answer 26

Fractional carboxyhaemoglobin – measure of how much of the Hb has CO bound reversibly to it

Question 27

How much added affinity does CO have to Hb compared with oxygen?

Answer 27

x200

Question 28

What is FMetHb?

Answer 28

Fractional methaemoglobin – formed when ferrous iron ions (Fe2+) in the heme groups are oxidised to ferric iron (Fe3+). Methaemoglobin is unable to combine with oxygen.

Question 29

How do you interpret an ABG? (x7)

Answer 29

1. Introduce
2. pH
3. Metabolic vs respiratory
4. Compensation?
5. If respiratory, acute or chronic?
6. Respiratory failure?
7. Anion gap?

Question 30

How do you introduce an ABG? (x4 parts)

Answer 30

State what the investigation is (ABG), who from, date and time taken, and whether they were on breathing air or oxygen delivery.

Question 31

What characterises acute and chronic respiratory acidosis/alkalosis?

Answer 31

Acute sees no alteration in bicarbonate levels. In chronic cases, the kidney will attempt to buffer the pH abnormalities by increasing/decreasing bicarbonate. If there is significant increase/decrease, then the event is chronic.

Question 32

What are the signs of Type 1 respiratory failure on an ABG?

Answer 32

Reduction in oxygen content of the blood less than 8 kPa.

Question 33

What are the signs of Type 2 respiratory failure on an ABG?

Answer 33

Reduction in oxygen content of the blood less than 8 kPa and rise in CO2 content above 6 kPa.

Question 34

!!! How does the mechanism and aetiology of Type 1 and Type 2 respiratory failure differ?

Answer 34

Type 1 is a failure of oxygen transfer from the air to blood; Type 2 is a failure of ventilation due to inability of lungs to move air in and out effectively. Type 1 is caused by pneumonia (failure of oxygen to get to and through alveoli), PE (failure of blood to pass through some lung tissue), and pulmonary fibrosis (failure of oxygen to diffuse through fibrosed lung). Type 2 is caused by opiate overdose (reduced RR), COPD (excess mucus and airway resistance), and neuromuscular disease (MND or Guillain-Barre syndrome).

Question 35

What is the anion gap? Significance?

Answer 35

Difference between the measured cations (Na+, K+) and anions (Cl-, HCO3-). The total number of cations should be equal to the total number of anions, but the ABG doesn’t measure all types of ions. As such, the anion gap represents these missing or unmeasured anions. The anion gap is calculated with a metabolic acidosis to help work out underlying aetiology.

Question 36

What is the normal range for anion gap?

Answer 36

6 – 12 mEq/l

Question 37

What are the causes of a raised anion gap acidosis?

Answer 37

MUD PILES: methanol, uraemia, diabetic ketoacidosis, paracetamol, iron/isoniazid (antibiotic), lactate, ethanol/ethylene glycol, salicylates (aspirin)

Question 38

What is normal anion gap acidosis? Doesn’t make sense to me!

Answer 38

Acidosis but normal anion gap due to loss of bicarbonate levels. As there is only one other buffering ion (chloride), the acidosis must be compensated for by increasing chloride. This is known as hyperchloremic acidosis. HCO3 can be lost through diarrhoea, or due to drugs which increases excretion.