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Respiratory/Musculoskeletal > CM- Interpretation of blood gases > Flashcards

CM- Interpretation of blood gases Flashcards

1
Q

What are the 3 physiological processes that can be assessed by an arterial blood gas analysis?

A
  1. alveolar ventilation
  2. acid-base status
  3. oxygenation
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2
Q

What values are measurable when taking an arterial blood gas?
What values need to be derived?

A

Measured:

  1. pH (7.35-7.45)
  2. pCO2 (35-45)
  3. p02 (>70mmHg)
  4. SaO2% (>95%)

Derived:

  1. HCO3 (22-26mmol/L)
  2. base excess/deficit (-2.4 to 2.3)
  3. blood O2 content (18-22mL/dL)
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3
Q

What is ventilation? In a healthy person what percent of air reaches functional alveoli?
What is the rest of the air considered?

A

It is the amount of air taken into the lungs/time.
70% of air reaches functional alveoli supplied by functional capillaries and do gas exchange (alveolar ventilation)
30% does not take part in gas exchange (never reaches the alveoli OR there isn’t good blood supply) and the portion of ventilation is dead-space ventilation

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4
Q

Total ventilation = _______________+__________________.

Which type of ventilation do we care most about?

A

Total ventilation = alveolar ventilation + dead space ventilation.

We care most about alveolar ventilation because that is the oxygen that is supplied to functional capillaries

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5
Q

Is it possible to assess alveolar ventilation at the bedside?

A

NO! you can use the respiratory rate and estimated tidal volume to make a guess about TOTAL ventilation, but you do not know how many of the capillaries are being supplied with oxygen.

Someone breathing heavy and fast might STILL have alveolar hypoventilation where someone breathing slowly and normally may have alveolar hyperventilation

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6
Q

In steady state, the amount of CO2 secreted into the blood by tissue is equal to the amount _______________. This means that the blood level of C02 is directly proportional to ____________ and inversely proportional to _____________.

A

C02 secreted into blood = C02 exhaled by the lung

This means that blood C02 is directly proportional to C02 production in the tissue and inversely proportional to alveolar ventilation.

pC02 = VC02x 0.863/ VA

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7
Q

What does an elevated pCO2 tell us?

A

That alveolar ventilation is reduced compared to the amount of C02 produced in the tissue.

Hypercapnia and hypoventilation*

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8
Q

You cannot describe a patient as hyper or hypoventilating unless you know what value?

A

pC02

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9
Q

What are the 2 ways that CO2 can be eliminated from the body?

A
  1. alveolar respiration - fastest and quantitatively most important
  2. Kidney- slower and smaller quantity
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10
Q

Why is CO2 accumulation dangerous?

A
  1. it can cause a fall in extracellular pH

2. elevated alveolar CO2 reduces alveolar O2 concentation

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11
Q

What are the 4 main physiological processes that cause alveolar hypoventilation?

A

Reduced total ventilation:

  1. inadequate airway
  2. failure of respiratory pump
  3. failure of the brain to initiate a breath

Normal total ventilation:
4. increased dead-space ventilation

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12
Q

What are 2 clinical examples of inadequate airway that could cause alveolar hypoventilation?

A
  1. status asthmaticus

2. acute epiglottitis

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13
Q

What are 2 clinical examples of situations where the respiratory pump would fail causing alveolar hypoventilation?

A
  1. myasthenia gravis

2. high cervical cord lesions

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14
Q

What are examples of the brain failing to initiate a breath that could cause alveolar hypoventilation?

A
  1. Opiate overdose

2. respiratory compensation for metabolic alkalosis

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15
Q

What are 2 situations where dead space ventilation would increase cause alveolar hypoventilation?

A
  1. severe restrictive lung disease/advanced obstructive lung disease
  2. PE
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16
Q

What is the major cause of alveolar hyperventilation?

A

increased central respiratory drive - the brain initiates breaths that are too deep, too frequent or both

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17
Q

What equation is used to assess acid-base balance in the body?
Which variables in the equation are measured and which are derived?

A

Henderson- Hasselbalch

pH = 6.1 + log [HCO3]/(0.03paCO2)

paCO2, pH can be measured by ABG, the HCO3 is derived

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18
Q

You get a blood workup and the pCO2 is increased, the pH is low and the HCO3 is normal.
What is the problem?

A

This is an acute respiratory acidosis

Within days, the kidney will compensate by reducing urinary loss of HCO3 and the pH will increase.

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19
Q 
If a patient has:
1. normal pH
2. elevated HCO3
3. elevated CO2 
What is the problem?
A

This is chronic respiratory acidosis with renal compensation

20
Q

You get a patient workup and the pCO2 is low, pH is high and HCO3 is normal.
What is the problem?
In a few days, if the problem continues, what do the lab values look like?/

A

acute respiratory alkalosis

In a few days, the kidneys will compensate by increasing urinary output of HCO3.
pCO2 will still be low, pH will be normal and HCO3 will be low

21
Q

What is a common cause of acute metabolic acidosis?

How does the brain respond to this?

A

If the tissue is not getting enough oxygen, the tissue switches to anaerobic metabolism and lactic acid is produced. It diffuses into the serum and the pH lowers.
pH is low so the brain responds by initiating respiratory hyperventilation, reducing pCO2 to bring it back to normal

22
Q

What is oxygenation and what are the 3 main questions we need to ask about it?

A

Oxygenation- blood getting from the environment to the lungs to the blood to mitochondria of tissue.

  1. are the lungs getting oxygen from the environment to the blood?
    2, is the blood getting oxygen to tissues?
  2. is the tissue doing a good job of taking up oxygen?
23
Q

What calculation tells you if the lungs are doing a good job of getting oxygen to the blood?
To determine this, what measured values do you need? What must you calculate?

A

A-a gradient which is the difference between oxygen in the alveoli and oxygen in the arterial blood

pAO2 is calculated using the equation FiO2 (Pb-47)-1.25(paCO2) or

pAO2 = 150 - 1.25(paCO2)

You can measure paCO2 and paO2
Then you do pA02-paO2

24
Q

Why is PAO2 not the same as inspired air?>

A
  1. inspired air gets saturated with water vapors on the way down to the alveoli
  2. CO2 from the blood to the alveoli is rapid and alveolar pCO2= blood pCO2
25
Q

PiO2 = ___________ (___-____)

A

Inspired air = %oxygen in the air (barometric pressure- water vapor)

PiO2 = FiO2 (Pb- 47)

26
Q

What is FiO2 at sea level? Pb?

A 
FiO2 = 0.21
Pb = 760
27
Q

What is the equation to calculate PAO2?

A

PAO2 = FiO2 (Pb-47) - 1.25(paCO2)

28
Q

What is a normal A-a gradient? Why isn’t it zero?

A

It is usually 10-15mmHg.

It is not zero because:

  1. diffusion of O2 from alveolus to capillary is too slow
  2. inadequate ventilation of some alveoli that are perfused normally
29
Q

What is a diffusion block?

When would this become most evident?

A

It is when the alveolar-capillary barrier is thickened and the diffusion of oxygen is slowed. Ex. scar tissue
This is NOT enough to reduce arterial pO2 at rest, but it becomes evident when the transit time of blood through the capillaries is shortened (exercise)

30
Q

What are causes of decreased arterial oxygen that would still give normal A-a gradient?

A
  1. decreased FiO2
  2. decreased Pb
  3. alveolar hypoventilation (increased PaCO2)
31
Q

What are causes of decreased paO2 with an increased A-a gradient?

A
  1. diffusion block
  2. VQ mismatch
  3. shunt
32
Q

How can you differentiate between a VQ mismatch and a shunt?

A

VQ mismatch can be repaired if the patient breaths 100% oxygen.
Shunts do not increase pO2 even if the patient breaths 100% oxygen

33
Q

Clinically, a RtoL shunt is suspected if paO2 is below what value? or if SaO2 is below what value when the patient is breathing 100% oxygen?

A

PaO2 < 95%

34
Q

What does it mean if there is an increased VQ ratio?

A

Ventilation is normal or increased and there is inadequate perfusion of the alveoli. There will be increased dead space ventilation and decreased alveolar ventilation while total remains the same

35
Q

What happens to “normal” PaO2 with age?

A

It decreases, so the A-a gradient gets larger with age

36
Q

What value tells us the amount of oxygen in the arterial blood?

How is it calculated?
What is the normal value?

A

CaO2 is the blood oxygen content. It is made up of the oxygen being carried by Hb and the blood that is freely dissolved in blood.

CaO2 = (HbxSaO2X1.34) + (paO2 x0.003)

Hb =14 SaO2 = 98%, paO2 = 100 so.

CaO2 should be around 18.7ml/dl

37
Q

What is the volume of oxygen that 1 gram of Hb can carry?

What is the volume of oxygen that dissolves in a dL of blood?

A
  1. 34 mL

0. 003

38
Q

Why does anemia reduce blood content?

A

Most of the blood content comes from O2 bound to Hb and only a very small portion from dissolved O2 in the blood.
If the person is anemic, there will be less Hb to bind to and the content will decrease.

39
Q

What are situations that would make the Hb dissociation curve shift to the right?

A

anything that makes it easier to give up O2:

  1. hyperthermic
  2. acidotic
40
Q

What would make the Hb dissociation curve shift to the left?

A

Anything that makes the Hb hold O2 tighter:

  1. hypothermia
  2. alkalosis
41
Q

At paO2 > ______, the hemoglobin is almost fully saturated and any change in paO2 will not have significant change in SaO2.
At paO2

A

paO2>90%

PaO2

42
Q

What oxygen factors change with CO poisoning?

A

paO2 is NOT affected but SaO2 and Ca02 are both vastly decreased

43
Q

What paO2 and Sa02 values are the “hump” of the Hb dissociation curve?

A 
PaO2 = 40-50mmHg
SaO2 = 75-85%
44
Q

How does the SpO2 (pulse oximetry measurement of oxygen saturation) differ from the true SaO2?

A
  1. SpO2 cannot represent SaO2 when it is below 85%
  2. SpO2 does not distinguish between carboxyhemoglobin and oxyhemoglobin
  3. SpO2 gives NO infomration about alveolar ventilation or acid-base
  4. SpO2 is inaccurate in poor extremity circulation (Reynauds)
45
Q

What is an example of when the lung gets enough oxygen, the oxygen content is fine, but the tissue still does not get enough oxygen delivered?

A

When there is hypoperfusion due to hypotension.

Even though the lungs are working and the content is fine, not enough volume of blood reaches the tissue

46
Q

What is an example of when adequate oxygen is delivered to the tissue, but the tissue is not able to utilize it as the final e- receptor in the mitochondria?

A

CN poisoning which irreversibly inactivates mitochondrial enzymes

Respiratory/Musculoskeletal (36 decks)

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