15. Blood gas analysis and pulse oximetry

Question 1

6 questions to ask when interpreting a blood gas

Answer 1

1) how is the pt
2) hypoxaemic?
3) pH?
4) PaCO2?
5) bicarb?
6) any other imp values to consider?

Question 2

what is the partial pressure of a gas?

Answer 2

the contribution it makes to the total pressure of the mixture

Question 3

what is atmospheric pressure?
what about partial pressure of oxygen in the atmosphere?
this gives an arterial PaO2 of ___kPa

Answer 3

close to 100kPa
21kPa
10-13 (in normally functioning lungs)

Question 4

when you give a pt 40% oxygen, what is the partial pressure of oxygen you are giving them?

Answer 4

40kPa (almost twice that which they were breathing in air)

Question 5

normal arterial PaO2 on room air?

Answer 5

10-13kPa

Question 6

why is the PaO2 always lower than the atmospheric partial pressure?

Answer 6

because the air is humidified as it’s breathed in, then it mixes with expired CO2 in the alveoli, both of which dilute the oxygen a little

Question 7

rough rule of thumb, PaO2 should be about ___ less than the inspired concentration in a healthy pt

Answer 7

10

i.e. in a pt breathing 40% oxygen, would expect the PaO2 to rise to 30kPa

Question 8

the acidity/ alkalinity of the blood is determined by the concentration of _____ ions

normal levels in the body?

Answer 8

hydrogen (the greater the conc, the more acid)

normally very low, around 40nmol/L

Question 9

normal pH of arterial blood?

Answer 9

7.35-7.45
(36-44 H+)

Question 10

T/F: small changes in pH represent big changes in H+ concentration

Answer 10

true

Question 11

why does too much CO2 make pts acidaemic?

Answer 11

because CO2 combines with water to generate hydrogen ions and bicarbonate

Question 12

normal PaCO2?

Answer 12

4.7-6.0 kPa

Question 13

T/F: the respiratory centre in the brain stem is usually very sensitive to blood H+ concentraiton

Answer 13

true - within a few minutes of sending a rise in hydrogen concentration it stimulates the person to breathe more increasing CO2 excretion

Question 14

T/F: The lungs are the primary mechanism by which [H+] is adjusted by regulating PaCO2

Answer 14

true

Question 15

normal base excess and bicarbonate?

Answer 15

BE -2 to +2
Bicarbonate 22-26

Question 16

_______ is the most important buffer for hydrogen ions in the body and is generated by the ______

Answer 16

bicarbonate
kidneys

Question 17

what is an excess of bicarbonate normally caused by?

Answer 17

loss of acid, e.g. from persistent vomiting

will cause excessive buffering of hydrogen ions and produce a metabolic alkalosis

Question 18

what is base excess a measure of?

Answer 18

the amount of acid or base that would need added to neutralise the pH of their blood

BE of +8 means 8mmol strong acid would be needed to return pH to normal > indicates metabolic alkalosis

BE of -8 means 8mmol strong base would be needed to return pH to normal > indicates metabolic acidosis

Question 19

T/F: metabolic compensation takes several days

Answer 19

true

Question 20

T/F: during cardiac arrest, ABG values correlate closely with the severity of hypoxaemia, hypercapnia and acidosis in the tissues

Answer 20

false - poor correlation so interpret with caution

VBG values may reflect more accurately the acid-base state of the tissues

Question 21

inspired oxygen concentration on 6L face mask?

Answer 21

about 40%

Question 22

where is the best site for arterial cannulation?

Answer 22

the radial artery

Question 23

why is ABG more accurate than pulse oximetry

Answer 23

the relationship between oxygen saturation and PaO2 is a sinusoid shape on the oxyhaemoglobin dissociation curve

an initial decrease from a normal PaO2 is not accompanied by a drop of similar magnitude in the oxygen saturation of the blood (>early hypoxaemia may be masked)

Question 24

T/F: pulse oximeters may cause a falsely low reading in anaemia or jaundice

Answer 24

false - not affected by these factors

may, however, get false readings in presence of other Hbs (CO poisoning, sickling red cells), imaging dyes (methylene blue), nail varnish, high ambient light levels, motion artefact and reduced pulse volume (hypotension, low CO, vasoconstriction, hypothermia)

Question 25

T/F: pulse oximetry does not provide a reliable signal during CPR

Answer 25

true - relies on the presence of a pulse for reliable measurement