Chapter 15: Blood Gas Analysis & Pulse Oximetry

Question 1

What is the 6 step approach of interpreting ABG?

Answer 1

1. How is the patient?
2. Is the patient hypoxaemic?
3. What is the pH?
4. What has happened to the PaCO2?
5. What has happened to the Bicarb?
6. Are there any other important values to consider?

Question 2

What is the normal PaO2?

Answer 2

1. 10-13 kPa
2. On RA

Question 3

What is “rule of thumb” relationship between PaO2 & FiO2 in a patient with normal oxygenation?

Answer 3

1. PaO2 should be about 10 less than FiO2

Example:
FiO2 of 40% O2
PaO2 = +/- 30 kPa

Question 4

Normal pH range:

Answer 4

7.35 - 7.45

Question 5

Normal PaCO2 range:

Answer 5

4.6 - 6.0 kPa

Question 6

What does it mean if pH < 7.35 & PaCO2 > 6.0 kPa?

Answer 6

Respiratory acidosis

Question 7

What does it mean if pH > 7.45 & PaCO2 < 4.6 kPa?

Answer 7

Respiratory alkalosis

Question 8

What does it mean if pH < 7.35 & HCO3 < 22?

Answer 8

Metabolic acidosis

Question 9

What does it mean if pH > 7.45 & HCO3 > 26?

Answer 9

Metabolic alkalosis

Question 10

What is normal HCO3 level?

Answer 10

22 - 26

Question 11

What is base excess?

Answer 11

1. Measure of the amount
2. Of excess acid or base
3. In the blood
4. As a result of metabolic derangement

Question 12

What is the normal base excess values?

Answer 12

-2 to +2

Question 13

What does a base excess more NEGATIVE than -2 suggest?

Answer 13

Metabolic acidosis

Question 14

What does a base excess GREATER than +2 suggest?

Answer 14

Metabolic alkalosis

Question 15

What is a valuable indicator of improving tissue perfusion after ROSC?

Answer 15

Lactate

Question 16

How can VBGs be useful when assessing acutely unwell patients?

Answer 16

1. Useful for monitoring metabolic issues
2. When no clinical indication of any resp. compromise

E.g. DKA

Question 17

Potential sources of error from pulse oximetry:

Answer 17

1. Other haemoglobins - carboxyHb (CO poisoning), metHb (congenital), foetal Hb (sickle cell disease)
2. Dyes - methylene blue
3. Nail varnish
4. High ambient light
5. Motion artefact
6. Reduced pulse volume - hypotension, vasoconstriction, low CO, hypothermia

Question 18

Pulse oximeters are not affected by:

Answer 18

1. Anaemia
2. Jaundice

Question 19

What type of O2 to give to critically ill patients? (peri-arrest situation)

Answer 19

1. NRB mask with reservoir
2. @ 15 L/min

Question 20

Features of type 1 resp. failure:

Answer 20

1. Low PaO2 < 8 kPa
2. Normal PaCO2 < 7 kPa

Safe to give high concentration O2
Keep sats 94-98%

Question 21

Features of type 2 resp. failure:

Answer 21

1. Low PaO2 < 8 kPa
2. Raised PaCO2 > 7 kPa

Keep sats 88-92%

Question 22

What is a by-product of anaerobic respiration (due to cardiac arrest) and how does it change the metabolic state of the body?

Answer 22

1. Lactate
2. Causes metabolic acidosis
3. Causing decrease in HCO3 (used up to buffer excess acid)