Blood Gas Analysis Flashcards
1
Q
what does blood gas analysis measure?
A
partial pressure of gases in the blood
2
Q
what other measurements are incorporated into blood gas analysis?
A
pH
electrolytes which affect pH
3
Q
what is the relationship between acids and protons?
A
acids are proton donors
4
Q
what is the relationship between bases and protons?
A
bases are proton acceptors
5
Q
what ion is linked to pH?
A
Hydrogen
6
Q
what is the relationship between hydrogen ions on pH?
A
pH is linked to H+ ion concentration. More H+ means lower pH
7
Q
what is the normal blood pH?
A
7.4
(7.35-7.45)
8
Q
what effect can small changes in pH have on H+?
A
can lead to big changes in H+ ion concentration
9
Q
what happens to H+ concentration as pH falls?
A
H+ increases in larger and larger steps
10
Q
why is it essential that pH is maintained in a narrow range?
A
enzymes are designed to work at a specific pH
11
Q
what is the effect on enzymes of a change in pH?
A
rate of enzymatic reactions can be altered
enzymes can lose function and denature which has massive impact on physiology
12
Q
what impact does disease have on pH?
A
pH often deranged in disease
13
Q
other than disease how else may pH be altered?
A
iatrogenic (IVFT or drugs)
14
Q
at what pH is death a risk?
A
less than 6.8
or above 7.6
15
Q
what is a pH of above 7.45 known as?
A
alkalaemia
16
Q
what is a pH of below 7.35 known as?
A
acidaemia
17
Q
what must add up within the body to ensure it is electroneutral?
A
all negative and positive charges
18
Q
what are common ions found within the body?
A
Na+
K+
Mg2+
Cl-
HCO3-
Ca2+
19
Q
what other substances in the body aid determination of pH?
A
albumin
ketones
20
Q
what is generated from water to balance charge differences?
A
H+
21
Q
what range is normal pH?
A
7.35-7.45
22
Q
does pH within the body vary between species?
A
yes between mammals
23
Q
what does the suffix osis mean?
A
process of pH becoming more acidic or alkaline
24
Q
what systems in the body are involved in maintenance of normal pH?
A
chemical buffers
respiratory system
renal system
25
what are the most important chemical systems involved in maintenance of normal pH?
phosphates
proteins
bicarbonate/carbonic acid system
26
what is described by the Henderson Hasselbalch equation?
relationship between pH, bicarb and CO2
27
describe the relationship between water, CO2, H+ and bicarb
excess CO2 is combined with water to give H+ ions and bicarb
excess H+ ions are combined with bicarb to form water and CO2
28
how is carbon dioxide excreted?
through the lungs
29
how are H+ ions excreted?
through the kidneys
30
what are buffers?
any particle capable of accepting or donating H+
31
what is the role of buffers?
act to soak up excess hydrogen and release it when necessary
32
how quickly do buffers work within the body?
fast - within seconds
33
what are the main buffers?
bicarbonate
heamoglobin
blood proteins (albumin)
phosphate
lactate
34
why is CO2 an acid?
within solution it is able to combine with water and form carbonic acid
35
what is the enzyme which forms carbonic acid from CO2 and water?
carbonic anhydrase
36
what are the main types of pH derangement?
respiratory acidosis
respiratory alkalosis
metabolic acidosis
metabolic alkalosis
37
what is respiratory acidosis and alkalosis caused by?
change in PaCO2 leading to a change in pH
38
what is metabolic acidosis and alkalosis caused by?
change in any other factor which may affect pH
39
what is the most common cause of metabolic acidosis and alkalosis?
change in HCO3- (bicarb)
40
what changes in bicarb can lead to metabolic acidosis and alkalosis?
loss or gain
to its role as a buffer to other acids
41
what does respiratory acidosis and alkalosis involve?
abnormalities in the concentration (partial pressure) of CO2
42
what is the arterial concentration of CO2 inversely proportional to?
ventilation
43
what effect does hyperventilation have on PaCO2?
level falls
44
what effect does hypoventilation have on PaCO2?
level increases
45
what effect does falling PaCO2 have on pH?
rises pH
46
what effect does rising PaCO2 have on pH?
lowers pH
47
how rapidly can the respiratory system respond to changes in pH?
very quickly
acts in response to pH changes brought on by exercise
48
what systems in the body measure pH changes?
medulla
carotid body
49
what is one of the major routes of excretion of acid from the body?
kidneys
50
what substance amount is regulated by the kidney?
HCO3- (and other ions)
51
what organ regulates the amount of bicarbonate in the body?
kidneys
52
how rapidly does the renal system work to clear excess acid?
slowly - hours to days
53
what causes respiratory acidosis?
increased PaCO2
54
what is the compensatory response to increased PaCO2?
increase HCO3-
55
when may acute respiratory acidosis occur?
anaesthesia
respiratory obstruction
56
what is the difference in response between acute and chronic respiratory acidosis?
acute sees a smaller increase in HCO3-, chronic has more support from the kidneys so more acid is excreted and more HCO3- retained
57
why is there increased levels of HCO3- in chronic respiratory acidosis?
kidneys have longer to react
58
what is respiratory alkalosis caused by?
fall in PaCO2
59
what is a fall in PaCO2 caused by?
increased ventilation (pain, stress, hypoxaemia)
60
what is the difference in response between acute and chronic respiratory alkalosis?
acute shows a small decrease in HCO3-
chronic shows a much larger decrease as the kidneys have longer to excrete HCO3-
61
why are there decreased levels of HCO3- in chronic respiratory alkalosis?
the kidneys have longer to excrete HCO3-
62
what signifies metabolic acidosis?
decreased HCO3-
63
what is decreased HCO3- due to?
direct loss of bicarbonate
consumption of bicarbonate due to excessive levels of acids
64
what is suggested by decreased HCO3-?
metabolic acidosis
65
what is the respiratory compensation for metabolic acidosis?
fall in PaCO2 (hyperventilation)
66
what signifies metabolic alkalosis?
increased HCO3-
67
what is indicated by raised HCO3-?
metabolic alkalosis
68
what causes increased HCO3-?
compensatory increase usually due to loss of chloride ions or albumin
69
what is the respiratory compensation for metabolic alkalosis?
increase in PaCO2 caused by decreased ventilation
70
is it possible to have acidosis and alkalosis at the same time?
yes - mixed disorder
71
describe an example of mixed respiratory and metabolic derangements
colicing horse -
respiratory acidosis due to distention of the abdomen preventing effective respiration
metabolic acidosis due to GI obstruction leading to build up of lactic acid
72
what does base excess describe?
amount of acid required to titrate 1L of blood to a pH of 7.4 at 37 degrees and PaCO2 of 40mmHg
73
what is the purpose of using base excess calculations?
removes respiratory component so metabolic can be assessed
74
how does bases excess make interpretation of blood gas easier?
fixes PaCO2 at 40mmHg so that any remaining pH change is due to metabolic process
75
what is PaCO2 fixed at during base excess analysis?
40 mmHg
76
what is indicated by a positive base excess?
metabolic alkalosis
77
what is indicated by negative base excess?
metabolic acidosis
78
what is the normal pH range for arterial blood?
7.34-7.48 (7.4)
79
what is the normal pH range for venous blood?
7.36-7.43 (7.4)
80
what is the normal PaCO2 range for arterial blood?
35-45 mmHg (40)
81
what is the normal PaCO2 range for venous blood?
38-48mmHg (40)
82
what is the normal HCO3 range for arterial blood?
22-29 mmol/l (24)
83
what is the normal HCO3 range for venous blood?
22-29 mmol/l (24)
84
what is the normal base excess range for arterial blood?
2-5 mmol/L (4)
85
what is the normal base excess range for venous blood?
2-5 mmol/L (4)
86
what is the other gas measured alongside CO2 during blood gas analysis?
oxygen
87
what is the normal arterial pressure of oxygen on room air?
90-100 mmHg
88
what is the normal venous value of oxygen at room air?
40-50 mmHg
89
how does arterial partial pressure usually compare to fresh inspired?
arterial partial pressure ~5x FiO2
90
why is venous oxygen partial pressure lower then arterial?
due to oxygen extraction for metabolism
91
what is hypoxaemia defined as?
less than 80mmHg PaO2
92
what PaO2 is expected under anaesthesia when the patient is maintained on 100% O2?
400-500mmHg
93
at 95% oxygen saturation what is the PaO2 of oxygen?
80mmHg
94
what other blood gas values are measured by blood gas machines?
electrolytes
lactate
haematocrit
glucose
some will also provide calculated values (e.g. base excess)
95
what is used to provide the sample for blood gas analysis?
venous or arterial blood sample
96
what is the best sample for learning about gas exchange within the body?
arterial
97
when should an arterial blood gas sample be taken?
ideally over a few breaths
98
under what conditions should a blood gas sample be taken?
anaerobically
99
why do blood gas samples need to be taken anaerobically?
to prevent air from altering the reading
100
what syringe is best used for blood gas if sample analysis is going to be delayed?
glass
101
where should samples be kept if blood gas analysis is to be delayed?
on ice in a glass syringe
102
why should blood gas samples be kept on ice if analysis is delayed?
to prevent blood metabolism from altering the sample
103
what can plastic blood gas sampling syringes be used for?
short term (5-10 min) storage at room temperature
104
what is usually found within blood gas syringes?
anticoagulant such as heparin
105
what is the role of heparin in blood sampling?
prevention of clot formation
106
how should blood be handled before blood gas analysis is run?
roll syringe of blood in hand to mix anticoagulant
discard the first drop of blood
107
why does the first drop of a blood gas sample need to be disguarded?
often clotted
108
what are the common artifacts found in blood gas analysis?
air contamination
saline
clotting
109
how is air contamination of a blood gas sample shown?
CO2 unexpectedly low
O2 closer to 150mmHg
110
how is saline contamination of a blood gas sample shown?
high chloride
111
how is saline contamination of a blood gas sample caused?
when sampling from catheters
112
how is clotting of a blood gas sample shown?
low PVC / HCT
113
what is the anion gap?
difference between cations and anions
114
what is the anion gap value usually?
15-25mmHg
115
what is demonstrated by an increased anion gap?
lactic acidosis
ketoacidosis
116
what is indicated by a normal anion gap?
GI bicarbonate loss
117
what is indicated by a decreased anion gap?
hypoproteinaemia
118
what is measured by the anion gap?
the difference between the negatively charged and positively charged electrolytes in blood
119
what does an increased anion gap suggest about blood acidity?
blood is more acidic (acidaemia) (more + ions)
120
what does a decreased anion gap suggest about blood acidity?
blood is more alkaline (alkalaemia) (more - ions)
121
what acid base derangement is the anion gap used to measure?
metabolic acidosis or alkalosis
122
