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Unit 13 > Arterial blood gases lecture > Flashcards

Arterial blood gases lecture Flashcards

1
Q

What is the normal range of PaO2 in blood gas results?

A

Arterial: 10-13 kPa, 75-100mmHg, 95%
Veins: 4.0-5.5 kPa, 30-40mmHg, 7-75%
Alveolar gas: 14.2 kPa, 106mmHg.

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2
Q

What is the normal pH range in blood gas results?

A

Arterial 7.35 to 7.45
Venous 7.33 to 7.44

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3
Q

What is the normal PaCO2 in blood gas results?

A

Arterial 4.7-6.0 kPa, 35-45mmHg
Venous: 5.3 to 6.7 kPa, 40-50 mmHg
Alveolar gas: 4.8kPa, 36mmHg

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4
Q

What are is normal range of actual HCo3- in mM?

A

Arterial 22-28mM
Venous 24-30mM

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5
Q

What is the normal range of standard HCO3- in mM?

A

Arterial 22-28 mM
Venous 24-30 mM

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6
Q

What is the normal range of base excess?

A

-2 to +2 in arterial and venous
Units = mEq/L.

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7
Q

How do you convert between pH and nM?

A

Increase/decrease in pH by 0.3 is equaivalent to halfing/doubling nM

Good base values to know are:
pH = -log[H+]
[H+] = 10 ^-pH

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8
Q

What control the homeostasis of pH?

A

Respiratory and metabolic systems work together to maintain a stable pH.
Even for the respiratory system, pH maintenance is the priority not CO2 or O2 conc

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9
Q

What is the definition of an acid?
What is the equation to represent this?

A

A proton donor
HA (reversible arrow) H+ + A-
Where HA is the acid

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10
Q

What is the definition of a base?
What is the equation to represent this?

A

A proton acceptor
H+ + B (reversible arrow) HB+
Where B is the base.

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11
Q

What is meant by a conjugate base?
Give an example.

A

A substance formed by the loss of a H+ in the forward reaction, will now react in the backwards reaction to gain a H+ (hence is an base.

A- tends to be the conjugate base

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12
Q

What is a conjugate acid?
Give an example.

A

A substance formed by the gain of a H+ in the forward reaction, hence will now in the backwards reaction act as a base to loose a H+
HB+ tends to be the conjugate acid.
What remains of base after reaction

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13
Q

What is the naming relationship between an acid and its conjugate base?

A

xxx acid - acid
xxxate - conjugate base

Aspartic acid is the acid and aspartate is the conjugate base.
Carbonic acid is the acid and bicarbonate is the conjugate base.

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14
Q

How does the pharmacology of lidocaine relate to acid base conjugate pairs?

A

The base lidocaine is inactive
The conjugate acid lidocaine is the active form
Is a pro-drug.

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15
Q

What is the purpose of a Ka value?

A

Indicates how strongly an acid dissociates
A higher Ka value indicates a stronger acid.

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16
Q

How do you calculate Ka?

A

Ka = [H+][A-]/ [HA]

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17
Q

What is pKa?
How do you calculate it?

A

Is the minus log of Ka
Indicates the strength of an acid
THe smaller the pKa value the stronger the acid
pKa = -log[ka]
Hence Ka = 10^-pka

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18
Q

What is the Henderson-Hasselbalch equation?

A
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19
Q

What fact can be derived from the Henderson-Hasselbach equation?

A

pH = pKa
Only when the acid is 50% dissociated.

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20
Q

How does pKa / pKb relate to the acid/base property?

A

Each acid /base always has a constant Ka/ Kb
When pH = pka this acid/base is divided equally between protonared and unprotonated forms.

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21
Q

How do you calculate kb?

A

Kb = [BH+][OH-]/ [B]

or Kw/ Ka.

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22
Q

What is the partial pressure of oxygen in inspired air?

A

PiO2 = 21 kPa.

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23
Q

What is partial pressure?

A

The total pressure an individual gas would exert on its own in the same total volume and at the same temperature.

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24
Q

What is an ideal gas?

A

Where the total pressure is the sum of all constituents partial pressures.

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25
Q

How do you convert between Pa and mmHg?

A

1kPa = 1000Pa
1 mmHg or 1 torr = 133 Pa.

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26
Q

How do you convert between different units of pressure?

A

1kPa = 1000Pa
1 bar = 100kPa
1 atm = 100 kPa (101.325)
1 torr = 1mmHg = 133 Pa = 0.133kPa
1PSI = 6.89 kPa.

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27
Q

What are the normal gas pressures in dry air?

A
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28
Q

What is Henry’s law at the gas liquid interface?

A

The concentration in liquid phase is proportional to the partial pressure in a gaseous phase.

The concentration of a gas dissolved in a liquid is directly proportional to the pressure of the gas as it makes contact with the gas surface.

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29
Q

How is the amount of gas in blood commonly expressed?

A

In kPa or mmHg
1 kPa = 7.5mmHg.

30
Q

What is the normal distribution of Oxygen in the blood?

A

98% bound to Hb
2% dissolved in plasma - only this exerts pressure.

31
Q

What is the concentration of oxygen in the blood?

A

Arterial : 10.0 mmol/L
Venous: 7.5mmol/L

32
Q

What is the normal distribution of Carbon dioxide in blood?

A

95% as bicarbonate
5% dissolved in plasma (exerts pressure)
1% are carbimino-Hb (and other proteins, very small amount as carbonic acid.

33
Q

What is the normal concentration of carbon dioxide in the blood?

A

Arterial 21.5 mmol/L
Venous 23.5mmol/L

34
Q

Why is their more carbon dioxide than oxygen in the blood in terms of mmol/L but less in terms of kPa?

A

Carbon dioxide is 25 times more soluble than Oxygen.
This means for any given partial pressure there will be more carbon dioxide dissolved in blood than oxygen, therefore increases the mmol/L of CO2 more.
CO2 has a smaller KH than O2.

35
Q

What is the role of the carbonic anhydrase enzyme?

A

Catalyses the conversion of CO2 and H20 to carbonic acid (H2CO3) in both reactions.
Very rapid
Is a perfect enzyme aka only limited by the rate of diffusion of the molecules.

36
Q

How does respiratory effort effect pH homeostasis?

A

Resp rate can change Pa CO2 rapidly
Lung pathology can gradually change PaCO2

37
Q

How does metabolic function effect pH homeostasis?

A

Kidenys regulate HCO3- more slowly
Many pathologies can change HCO3- and other acids/bases.

38
Q

What is the normal pH of Blood?
Normal [HCO3-]
Normal [CO2]

A

pH = 7.4
HCO3- = 22-28 mmol.l-1
CO2 = 1.1 to 1.4 mmol.l-1

39
Q

On an ABG what does low CO2 and high ph indicate?

A

Respiratory alkalosis

40
Q

On an ABG what does high CO2 and low pH indicate?

A

respiratory acidosis

41
Q

On an ABG what does low HCO3- and low pH indicate?

A

Metabolic acidosis

42
Q

On an ABG what does a high pH an a high HCO3- indicate?

A

Metabolic alkalosis.

43
Q

What pathologies can cause the different acid/base imbalances?

A

Hyperventilation or high altitide = respiratory alkalosis
Trauma or hypoventilation or obstruction in the lungs = respiratory acidosis
Diabetic ketoacidosis - metabolic acidosis
Chronic vomiting - metabolic alkalosis.

44
Q

What are the five steps in interpreting arterial blood gas results?

A

0 = oxygenation
A = acidosis/alkalosis
B - Buffers/bases
C - compensation/chronic
D - differential diagnosis

45
Q

How do you interpret the oxygenation status of a patients from an ABG?

A

Conisder kPa of oxygen
Normal is 10-13kPa
Is below 8 kPa indicates respiratory failure.
(note this pressure is where the drop in the oxygen dissociation curve becomes more steep)

46
Q

What are the two different types of respiratory failure?

A

Type 1 - hypoxaemic (normal or low PaCO2)
Type 2 - hyercapnic

47
Q

What are the features of type 1 respiratory failure?

A

Hypoxaemic (low pO2)
Normal or low PaCO2 (normocapnia)
Impaired gas exchange at alevolar-capillary interface.
V/Q mismatch
Normally caused by parenchymal lung damage

48
Q

What are the key features of type 2 respiratory failure?

A

High PaCO2 (hypercapnic)
Also hypoxemic
Due to global underventilation seen in COPD or respiratory muscle weakness.
Problem tends to be in the mechanism of breathing.

49
Q

Why is oxygen exchange more vulnerable than Co2 exhcnage to compromise in the lungs?

A

O2 exchange in more vulnerable than CO2 exchange:
as Co2 more soluble so more rapid diffusion
only 10% of total carbon dioxide is lost in the lungs, compared to 25% of O2 gained principles of Ficks Law means oxygen has a lower diffusion capacity.

50
Q

How do you determine if an ABG is acidosis or alkalosis?

A

Check pH valuve
Normal - 7.35 to 7.45
If below this then acidic if above this then alkalosis.

51
Q

How do you check for buffering in arterial blood gas results?

A

Buffers can be HCO3- or CO2
Start by checking pH for acidosis or alkalosis
Then look at CO2 - if aligns with pH then is respiratory xyz, if is opposite then expected then compensating, if in norm range then norm

Then look at HCO3-, if not metabolic in cause, if HCO3- opposite to expected then compensating.

52
Q

What are the two different values of HCO3-?

A

Actual - derived from pH and PaCO2 using the HH equation
Estimates the [HCO3-] actually in the plasma
Can assess both metabolic and respiratory causes of acid-base disturbances

Standard - calculated or derived from PaCO2 and PaO2 and temp to give a value if blood was perfectly equilibrated, aka what the value of bicarbonate would be if ventilation hence CO2 was normal. Is a measure of metabolic imbalance. In healthy lungs in norm circumstances this value will be the same as the actual

53
Q

What is base excess?
Why is it relevant to interpreting an ABG?

A

Base excess - measures all bases not just HCO3-.
HCO3- accounts for 75% of total buffering, but remaining 25% can affect pH of blood under some conditions
In analysis, normally provides similar information to sHCO3-
Value represents the amount of acid/base causing a deviation from pH7.40 at normal paCO2 and temp
Units : mEq/L

54
Q

What do the values of base excess indicate?

A

BE >+2 mEq/L = high HCO3- = metabolic alkalosis
BE <-2 mEq/L = low HCO3- = metabolic acidosis.

55
Q

What is the importance of BE and HCO3- in a respiratory cause acidosis/alkalosis?

A

Indicates if acute or chronic.
Kidney excretion is slow to adjust - so if compensation in BE/HCO3- shown then indicates chronic cause.

BE would be low ins alkalosis
BE would be high in acidosis.

56
Q

Can you have a mixed respiratory and metabolic cause for a alkalosis/acidosis on a ABG?

A

Yes
High Co2 and low HCO3- = acidosis
LOw CO2 and high HCO3- = alkalosis

57
Q

What are the different pathological conditions to consider when interpreting an ABG result to indicate the differential diagnosis?

A

Lungs
V/Q mismatch, increased or decreased respiratory rate
Metabolic and kidney
Gain/loss of acid/base, increased or decreased HCO3- elimination.

58
Q

What two factors are required for affective gas exchange?

A

Adequate flow of gas to alveoli
Flow of blood through pulmonary capillaries

This requires functioning of the ventilatory air pump and the gas exchange surface.

59
Q

What is the response to a low V/Q ratio in the lungs?

A

Poor alveolar ventilation compared to blood flow in one area of the lungs
Results in poorly ocygenated blood in affected area
This poorly oxygenated blood mixed with blood from normally ventilated side leading to a reduction in PaO2.
This is compensated for by hypoxic vasoconstriction - helps divert blood to well-ventilated alveoli, increase PaO2.

60
Q

What pathology tends to have a low V/Q ratio?

A

Obstruction of the airway
Severe asthma attack.

61
Q

What is the physiological process and response of a high V/Q ratio in the lungs?

A

Inadequate blood flow compared to ventilation in one region of the lungs
Reduction in effective exchange across the alveolar membrane - reduced PaO2
Results in a physiological dead space (air in and waiting but not possible for exchange hence dead)
Response is bronchoconstriction and reduced surfactant help divert air to well perfused alveoli.

62
Q

How does the kidney respond to respiratory acidosis?

A

Liver reduces urea production and produces more glutamine
Kidney produces more glutamate dehydrogenase and PEPCK to catalyse the breakdown of glutamine into NH4+ and HCO3- in the PCT.
HCO3- is retained by the kidney and NH4+ is lost in urine
Results in increased plasma HCO3- and increased plasma pH.

63
Q

Describe the process in the kidney by which new HCO3- is produced during respiratory acidosis.

A

Na and glutamine co-transporter increase uptake into cytoplasm from the luminal and basolateral membrane.
In mitochondria glutamine is converted to phosphophoenolypyruvate, then to glucose in the cytoplasm. THe conversion from PEP to glucose produces HCO3-.
HCO3- is reabsorbed through a Na+ HCO3- cotransporter in the basolateral membrane
Glucose is also reabsorbed
NH4+ is also produced as a byproduct of the mitochondrial reactions, Dissocatiates into NH3 and H+. Secreted through luminal membrane (NH3 simple diffusion, H+ by antiporter with Na+ or through ATPase), reform NH4 which is excreted.

64
Q

How does the kidney respond to respiratory alkalosis?

A

Can decrease plasma [HCO3-]
Increased number and activity of Type B intercalated cells
This increases the excretion of HCO3- into the tubule lumen - increasing the concentration in the urine.
THis decreases the plasma HCo3- and decreases plasma pH.

65
Q

What metabolic factors can affect acid/base balance?

A

Kideny failure (acidosis - decrease HCO3-)
Alkaline tide (alkalosis - inc HCO3-)
Keton bodies (inc H+)
Lactic acid (inc H+)
Vomiting (decrease H+)

66
Q

What is Kussmaul breathing?

A

Respiratory response to metabolic acidosis - aims to increase the rate (hyperpnoea) and tidal volumes of ventilation
Increase CO2 removal from blood stream, dec plasma Co2 and increase pH.

67
Q

What different pathologies can cause Kussmaul breathing>

A

K = ketones (diabetic ketoacidosis)
U = uremia
S = sepsis
M = methanol
A = aldehydes
U = (un used)
L = lactic acid

68
Q

How do the lungs compensate for increased blood pH?
Metbaolic acidosis

A

Lungs excrete excess non-volatile acids int he form of CO2
Excess H+ in blood, is combined with HCO3- catalysed by carbonic anyhdrase to form CO2 + H2O.

69
Q

What are the different methods of physiological buffering with increased H+/decreased pH of the blood?

A

Excess H+
1) incorporated into proteins (mainly in liver)
2) used to convert HPO42- to H2PO4-
3) Combined with HCO3- (and carbonic anhydrase) to produced Co2 and H2O - CO2 is expired via the lungs
In the kidney - HCO3- production increases

70
Q

What is the reflexive method of increases the ventilation rate?

A

Increased firing of peripheral chemoreceptors - travel up glassopharangeal and vagus nerve to the NTS
Increased firing of central chemoreceptors.
Signals integrate into the central pattern generator in the brain stem
Results in more frequent phrenic and intercostal nerve activity
Causes more forceul and frequent contraction fo the diaphragm and intercostal muscles.
This increases the rate and depth of breathing (inc ventilation rate)

Unit 13 (22 decks)

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