Blood Gas Analysis

Question 1

what does blood gas analysis do?

Answer 1

measures the partial pressures of gases in the blood

Question 2

what is an acid?

Answer 2

a proton donor

Question 3

what is a base?

Answer 3

a proton acceptor

Question 4

what is normal body pH?

Answer 4

7.35-7.45

Question 5

what happens to [H+] when pH falls?

Answer 5

[H+] increases in bigger and bigger steps as pH falls

Question 6

why is knowledge of a patients blood pH important?

Answer 6

effects rate of enzymatic reactions - has big impact on physiology

pH is often deranged in disease

Question 7

what are the limits of pH change the body can cope with?

Answer 7

death usually occurs if pH is less than 6.8 (acidaemia) or greater than 7.6 (alkalaemia)

Question 8

what does electroneutral mean?

Answer 8

all negative and positive charges in the body add up

Question 9

how does the body try and mitigate any charge differences?

Answer 9

H+ is generated from water

Question 10

what is classed as acidaemia?

Answer 10

blood pH less than 7.35

Question 11

what is classed as alkalaemia?

Answer 11

blood pH more than 7.45

Question 12

which systems does the body use to maintain a normal pH?

Answer 12

chemical buffers, respiratory and renal systems

Question 13

what are the most key parts in the maintenance of normal pH?

Answer 13

phosphates, proteins and the bicarbonate/carbonic acid system

Question 14

what is the bicarbonate/carbonic acid system?

Answer 14

H2O + CO2 <–> H2CO3 <–> H+ + HCO3-

Question 15

what is a buffer?

Answer 15

any particle capable of accepting or donating H+

Question 16

how do buffers work?

Answer 16

act to soak up any excess H+ and release it when necessary

Question 17

how quickly do buffers work?

Answer 17

fast - seconds

Question 18

give some examples of buffers in the body

Answer 18

bicarbonate (HCO3-)
haemoglobin
blood proteins
phosphate
lactate

Question 19

what can cause pH derangements?

Answer 19

respiratory acidosis or alkalosis

metabolic acidosis or alkalosis

Question 20

what is respiratory acidosis/alkalosis?

Answer 20

a change in PCO2 leading to a change in pH

Question 21

what is metabolic acidosis/alkalosis?

Answer 21

a change in anything else that can affect pH (not pCO2)

Question 22

which ion is usually involved in metabolic acidosis/alkalosis?

Answer 22

usually see a change in HCO3-, either due to loss/gain or its role as a buffer to other acids

Question 23

what is the relationship between arterial CO2 (PaCO2) and ventilation?

Answer 23

inversely proportional

Question 24

what type of breathing will cause PaCO2 to fall?

Answer 24

hyperventilation

Question 25

what type of breathing will cause PaCO2 to rise?

Answer 25

hypoventilation

Question 26

how quickly can the respiratory system change PaCO2?

Answer 26

rapidly - within seconds/minutes of sensing a pH change

Question 27

which parts of the brain/body sense changes in PaCO2?

Answer 27

medulla and carotid body

Question 28

what is the advantage of the quick response of the respiratory system to changes in pH?

Answer 28

allows the body to respond to sudden changes in pH e.g. exercise

Question 29

what is the main way of getting rid of excess acid from the body?

Answer 29

renal system

Question 30

how quickly does the renal system act to regulate ion levels in the body?

Answer 30

slowly - hours to days

Question 31

what is acute respiratory acidosis?

Answer 31

increased PCO2 due to hypoventilation

Question 32

what is the body's response to acute respiratory acidosis?

Answer 32

compensatory response is to increase HCO3- (buffer to mop up excess H+)

Question 33

how do the kidneys react to chronic respiratory acidosis?

Answer 33

more acid is excreted and HCO3- retained

Question 34

what causes respiratory alkalosis?

Answer 34

a fall in PCO2 caused by increased ventilation (pain, stress, hypoxaemia)

Question 35

what signifies metabolic acidosis? why does this occur?

Answer 35

signified by decreased HCO3- due to direct loss of bicarbonate or its consumption due to excessive levels of acids

Question 36

how does the body respond to metabolic acidosis?

Answer 36

respiratory compensation - a fall in PCO2 due to increased ventilation

Question 37

what signifies metabolic alkalosis? why does this occur?

Answer 37

signified by increased HCO3- compensatory increase, usually due to loss of chloride ions or albumin

Question 38

how does the body respond to metabolic alkalosis?

Answer 38

respiratory compensation - an increase in PCO2 due to decreased ventilation

Question 39

what is base excess?

Answer 39

amount of acid required to titrate 1L of blood to a pH of 7.4 at 37C and a PaCO2 of 40mmHg

Question 40

is base excess positive or negative?

Answer 40

can be either

Question 41

what is the relevance of base excess in blood gas interpretation?

Answer 41

fixes the PaCO2 at 40mmHg - means any remaining pH change is due to metabolic processes

Question 42

what is normal PCO2 (mmHg)?

Answer 42

arterial 36-46 venous 38-48

Question 43

what is normal HCO3- (mmol/l)?

Answer 43

22-29 (arterial and venous)

Question 44

what is normal base excess (mmol/l)

Answer 44

2-5 (arterial and venous)

Question 45

what are normal arterial values for blood when breathing room air?

Answer 45

90-100mmHg

Question 46

what is the normal venous value for blood?

Answer 46

40-50mmHg

Question 47

what is hypoxaemia defined as?

Answer 47

less than 80mmHg oxygen in the blood

Question 48

what is the normal PaO2 under anaesthesia?

Answer 48

400-500mmHg (due to 100% oxygen being given)

Question 49

which samples can be used for acid-base balance?

Answer 49

arterial or venous

Question 50

which samples can be used for gas exchange values in the body?

Answer 50

ideally arterial sample, blood taken over a few breaths samples should be collected anaerobically

Question 51

can glass syringes be used for sample collection?

Answer 51

yes - impermeable to gas must keep sample on ice

Question 52

which type of syringe is best to use if blood sample analysis will be delayed?

Answer 52

glass - impermeable to gas

Question 53

how long can purpose-designed plastic syringes hold a sample?

Answer 53

5-10 mins at room temperature

Question 54

what do purpose-designed plastic collection syringes for blood gas analysis contain?

Answer 54

heparin

Question 55

why do we discard the first drop of blood from the sample?

Answer 55

often clotted

Question 56

what common artifacts are seen with blood gas analysis?

Answer 56

air contamination - CO2 unexpectedly low saline contamination - high chloride clotting - low PCV/HCT

Question 57

when is saline contamination of blood gas samples often seen?

Answer 57

when blood is sampled from an IV catheter