Clinical Blood Gases

Question 1

why do we measure blood gases

Answer 1

to assess very sick patients
to diagnose resp failure
to diagnose metabolic problems

Question 2

oxygen haemoglobin dissociation curve

Answer 2

Hb takes up o2 in lung
Hb liberates o2 in tissue

Question 3

what initiates a right shift in the o2/Hb dissociation curve

Answer 3

increase co2
increase H+
increase temp
increase 2,3-DPG

Question 4

how saturated does haemoglobin tend to be

Answer 4

97%

Question 5

how is oxygen carriage quantified

Answer 5

Hb saturation
- very easy to do
- assuming Hb is normal, is an accurate reflection of oxygen content
arterial blood gases
- more complicated and invasive
- PaO2 reflects haemoglobin saturation but is a measure of partial pressure of O2 in blood

Question 6

how to measure Hb saturation

Answer 6

oxygenated blood red
deoxygenated blood blue
using absorption spectroscopy, is possible to estimate degree of saturation of haemoglobin
SpO2, pulse oximetry

Question 7

how to measure ABG

Answer 7

single arterial puncture technique
- radial a.
- femoral a.
- brachial a.
measurement from in-dwelling arterial catheter or a-line
- only really an option in HDU/ITU

Question 8

what does blood gas measure

Answer 8

pao2
paco2
H+/pH
bicarbonate
some may measure electrolytes and Hb
other forms of Hb - carboxyhaemoglobin

Question 9

normal blood gas values

Answer 9

H+ 36-44 nmol/L
PO2 12-15 kPa
PCO2 4.4-6.1
HCO3- 21-27.5
BE +2 to -2 mmol/L

Question 10

oxygenation kPa

Answer 10

partial pressure of o2 in air is 21 kPa
total pressure in atmosphere is 100 kPa
21% of air is oxygens, therefore 21% of total pressure is partial pressure of oxygen
depends on environment

Question 11

oxygenation

Answer 11

normal PaO2 = 12-15 kPa
breathing air
at normal atmospheric pressure
because amount of o2 available, alveolus (PAo2) around 14-15 kPa

Question 12

carbon dioxide

Answer 12

normal PaCO2 = 4.4-6.1 kPa
pco2 and co2 content vary with ventilation
more v = low pco2 and content
less v = high pco2 and content
hypoventilation causes build up of alveolar co2 and therefore less likely to be removed from blood
increase in blood co2 leads to acidosis

Question 13

carbon monoxide poisoning

Answer 13

CO produced from incomplete combustion of hydrocarbons
CO bind to Hb in place of O2 to form carboxyhaemoglobin
also interferes with mitochondrial respiration
death by asphyxia
treatment is high concentration o2 - displaces CO from Hb

Question 14

resp failure

Answer 14

low o2 level in blood - hypoxaemia
resp failure = pao2 < 8 kPa
caused by vq mismatch or hypoventilation (or both)

Question 15

asthma and resp failure

Answer 15

hypoxaemia suggest significant asthma attack
bronchospasm and mucous plugging causes ventilation defects and vq mismatch
T2 resp failure develops when severe bronchospasm causes hypoventilation of alveoli or exhaustion
patient need o2 to survive
invasive ventilation may be required

Question 16

copd and resp failure

Answer 16

copd - mixture of chronic airway inflammation and narrowing and emphysema
problems with vq mismatch and hypoventilation
may present acutely with resp failure T1 or T2
may have chronic T2 resp failure in advanced disease
treat resp failure with o2 but with caution in chronic T2

Question 17

what is h+ increased by

Answer 17

increased pco2 - respiratory acidosis
increase in acid production or decrease in excretion - metabolic acidosis

Question 18

acute vs chronic type 2 resp failure

Answer 18

acute hypoventilation - e.g. due to opiate toxicity leads to hypoxia, hypercapnia and acidocis
chronic hypoventilation e.g. neuromuscular disease or severe copd leads to hypoxia and hypercapnia but may not have acidosis due to compensation

Question 19

what does increased bicarbonate retention by the kidney compensate for

Answer 19

acidosis

Question 20

what does co2 normally stimulate

Answer 20

ventilation

Question 21

what are some patients with chronic type 2 resp failure dependent on to stimulate breathing

Answer 21

hypoxia

Question 22

what can a sudden increase in po2 with oxygen therapy worsen

Answer 22

hypoventilation

Question 23

what is respiratory alkalosis not usually associated with

Answer 23

respiratory failure

Question 24

what is resp alkalosis caused by

Answer 24

hyperventilation

Question 25

what are there low levels of with resp alkalosis

Answer 25

low pco2 and low h+

Question 26

metabolic problems involving blood gases

Answer 26

excess acid production by body - e.g. lactic acidosis or diabetic ketoacidosis
kussmal breathing - clinical sign of acidosis as a compensatory mechanism to increase co2 removal from blood
full compensation difficult - need to treat underlying cause of increased acid load

Question 27

how is bicarbonate interpreted

Answer 27

bicarbonate calculated by blood gas machine
hco3- increased by increase in pco2
hco3- decreased by increase in acid production or decrease in excretion

Question 28

actual bicarbonate

Answer 28

calculated with actual h+ and pco2 values

Question 29

standard bicarbonate

Answer 29

calculated with actual h+ and pco2 of 5.3kPa
only influenced by metabolic effects

Question 30

base excess

Answer 30

amount of base needed to be removed from a litre of blood at normal pco2 in order to bring h+ back to normal
- calculated with normal co2, so only looks at metabolic component
- normal value is zero (-2 to 2 mmol/L)
- big negative value indicated metabolic acidosis
- positive value seen in compensated resp acidosis

Question 31

when is the patient in respiratory failure

Answer 31

if the po2 is low given the inspired o2

Question 32

when is it type 1 resp failure

Answer 32

when co2 is low or normal

Question 33

when is it type 2 resp failure

Answer 33

when co2 is high

Question 34

what is high h+

Answer 34

acidosis

Question 35

what is low h+

Answer 35

alkalosis

Question 36

cause of acidosis - high pco2

Answer 36

respiratory

Question 37

cause of acidosis - standard bicarbonate low or base excess has large negative value

Answer 37

metabolic