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MS2 Respiratory > 9/9- Arterial Blood Gases in Lung Disease. > Flashcards

9/9- Arterial Blood Gases in Lung Disease. Flashcards

1
Q

Arterial blood gases are indicators of the ____of the lungs?

What do they provide information about?

A

Arterial blood gases are indicators of the adequacy of the functions of the lungs

Provide information about:

  • Alveolar ventilation
  • Oxygenation
  • Acid-base balance
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2
Q

What are some normal arterial blood gas values?

A

pH: 7.35 - 7.45

PaCO2: 35-45 mmHg

PaO2: > 70mmHg (age dependent!)

HCO3: 22-26 mEq/L

Note: small ‘a’ = arterial; large ‘A’ = alveolar!

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3
Q

What is alveolar ventilation (VA)? Typical volume?

A

The amt of air that reaches the alveoli and takes part in gas exchange

  • Alveolar ventilation: ~4.2 L/min
  • Alveolar volume: ~ 350 mL (per breath)
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4
Q

What is dead space ventilation (VD)?

A

The amt of air that does not take part in gas exchange

  • Dead space ventilation: 1.8 L/min
  • Dead space volume: 150 mL (per breath)
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5
Q

What is the minute ventilation equation and typical value?

A

VE = minute ventilation = TV x RR

  • Typ ~ 6L/min
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6
Q

A low PaCO2 is what? What does it signify?

A
  • PaCO2 under 35
  • Signifies hyperventilation
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7
Q

A high PaCO2 is what? What does it signify?

A
  • PaCO2 > 45
  • Signifies hypoventilation
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8
Q

In a steady state, the amount of CO2 added to the blood (VCO2) = ______?

A

In a steady state, the amount of CO2 added to the blood (VCO2) = amount excreted by the lungs (200 mL/min)

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9
Q

What two things does PaCO2 depend on?

A
  • Alveolar ventilation
  • CO2 production

((PCO2 = VCO2/VA x 0.863))

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10
Q

What are the two broad causes of increased PaCO2?

A
  • Alveolar hypoventilation
  • Increased CO2 production (rare)
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11
Q

What may cause hypoventilation (and thus increased PaCO2)?

A

Decreased minute volume (VE)

  • CNS depression
  • Respiratory muscle paralysis

Increase dead space (VD)

  • Severe obstructive lung disease
  • Severe restrictive lung disease

Combination of both

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12
Q

What may cause increased CO2 production (and thus increased PaCO2)?

A
  • Overfeeding
  • Hyperthyroidism
  • Fever
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13
Q

What may cause decreased PaCO?

A

Alveolar hyperventilation

  • Pain
  • Anxiety
  • Liver failure
  • Early sepsis
  • Pulmonary embolism
  • Acute asthma
  • Metabolic acidosis
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14
Q

Oxygen delivery system (in terms of oxygen pressure)?

A

PiO2 -> PAO2 -> PaO2 -> PvO2

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15
Q

What do the following oxygen parameters measure:

  • Partial pressure of oxygen (PaO2)
  • Arterial oxygen saturation (SaO2)
  • Arterial oxygen content (CaO2)
A

Partial pressure of oxygen (PaO2)

  • Oxygen molecules dissolved in plasma (not bound to hemoglobin)

Arterial oxygen saturation (SaO2)

  • % heme binding sites saturated with oxygen

Arterial oxygen content (CaO2)

  • Amount of oxygen bound to hemoglobin and dissolved in plasma
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16
Q

What is the major determinant of pulmonary and arterial PaO2?

A

PAO2

  • PAO2 defines the upper limit of PaO2
  • PaO2 can never be higher or equal to PAO2
  • The difference is the alveolar-arterial oxygen difference (PAO2 - PaO2)
17
Q

How is PAO2 calculated?

A

Alveolar gas equation

18
Q

What is PAO2 - PaO2?

What does it reflect?

A
  • The difference between alveolar and arterial oxygen
  • Reflects V/Q mismatch
19
Q

What is normal V/Q ratio?

  • When is it higher? lower?
A

Normal: V/Q = 1

  • V/Q > 1: increased dead space
  • V/Q < 1: venous admixuture/shunting
20
Q

T/F: A-a difference (PAO2 - PaO2) is constant with age

A

False!

  • PAO2 decreases with age
21
Q

Which parts of the lung get the highest ventilation? Perfusion?

A

The base of the lung gets the most ventilation and perfusion! (although doesn’t have the highest V/Q ratio)

22
Q

What part of the lung has the highest V/Q ratio?

A

Apex (upper portion) of the lungs

23
Q

What shifts the oxygen dissociation curve left (higher affinity)?

A
  • Lower pH
  • Decreased pCO2
  • Lower temperature
24
Q

What is hypoxemia? Hypoxia?

A

Hypoxemia = low oxygen in blood and plasma (low CaO2, PaO2, SaO2)

Hypoxia = decreased oxygen delivered to the tissues; takes into account Hb, cardiac output, and oxygen uptake (VO2)

25
Q

What are some nonrespiratory causes of hypoxemia (low PaO2)?

How does each effect P(A-a)?

A

Nonrespiratory:

  • R to L intracardiac shunt (increased A-a)
  • Decreased PIO2 from low barometric pressure or low FiO2 (normal A-a)
26
Q

What are some respiratory causes of hypoxemia (low PaO2)?

How does each effect P(A-a)?

A

Respiratory:

  • Diffusion barrier (increased A-a)
  • Hypoventilation/increased PCO2 (normal A-a)
  • R to L intrapulmonary shunt (increased A-a)
  • V/Q mismatch (increased A-a)
27
Q

What are some artifactual causes of hypoxemia (low PaO2)?

How does each effect P(A-a)?

A

Artifact:

  • Very high WBC count (increased A-a)
  • Patient hyperthermia (increased A-a)
28
Q

What are the main categories of causes of hypoxia (3)?

A
  1. Hypoxemia
  2. Reduced oxygen delivery to tissues
  3. Reduced tissue oxygen uptake
29
Q

Breakdown of causes of hypoxia:

  1. Hypoxemia

-

  1. Reduced oxygen delivery to tissues

-

  1. Reduced tissue oxygen uptake

-

A

1. Hypoxemia

  • Reduced PaO2
  • Reduced oxygen content (including severe anemia)
  • Reduced SaO2 (CO intoxication, methemoglobinemia…)

2. Reduced oxygen delivery to tissues

  • Reduced CO
  • L to R systemic shunt

3. Reduced tissue oxygen uptake

  • Mitochondrial poisoning
  • Left shift of oxygen dissociation curve
30
Q

What are the primary acid-base disorders (primary event and compensatory event)?

A
31
Q

What may cause respiratory acidosis?

A
  • CNS depression
  • Chest wall dysfunction
  • Obstructive lung diseases
  • Hypercapnic respiratory failure
32
Q

What may cause respiratory alkalosis?

A
  • Anxiety
  • Sepsis
  • Liver failure
  • Acute pulmonary insult; (eg; pulmonary embolism, asthma)
33
Q

What may cause metabolic acidosis?

A

Increased anion gap (normochloremic):

- Lactic acidosis

- Ketoacidosis

(also diabetes, alcoholism, starvation, uremia, methanol, ethylene glycol, salicylates, paraldehyde)

Normal anion gap (hyperrchloremic):

- Diarrhea

- Renal tubular acidosis

(also early renal failure, small bowel losses, ureteral diversions, anion exchange resins, ingestion of CaCl2, acid infusion)

34
Q

What may cause metabolic alkalosis?

A
  • Diuretics
  • Corticosteroids
  • Nasogastric suctioning
35
Q

What is anion gap? Normal value?

A

Anion gap (AG) = Na - (Cl + HCO3)

  • Normal value = 10 (10-12ish)
36
Q

What information do you need to interpret ABGs?

A
  • Fraction of inspired oxygen (FiO2)
  • Barometric pressure
  • Clinical data and age of patient
  • Serum electrolytes and bicarbonate level
37
Q

Steps in interpreting ABGs?

A
38
Q

Case

  • 65 yo man, hx of COPD
  • SOB following an URTI

ABGs on room air:

  • pH = 7.2
  • PaCO2 = 56 mmHg
  • PaO2 = 50 mmHg

Analyze

A
  1. Pt is acidemic
  2. Disturbance is respiratory (high PaCO2)
  3. Respiratory acidemia is acute (increase PaCO2 by 16, decrease pH by 0.12)
  4. Pt is hypoxemic and hypercapneic on room air

PAO2 = 0.21 (760-47) - 56/0.8 = 80

PAO2 - PaO2 = 80 - 50 = 30 (elevated)

Most common cause of elevation is VQ mismatch, cause of increased PaCO2 is hypoventilation

39
Q

Case

  • 20 yo man, Hx of IV drug abuse
  • Altered mental status

ABGs on room air:

  • pH = 7.08
  • PaCO2 = 80 mmHg
  • PaO2 = 45 mmHg
A
  1. Pt is acidemic
  2. Disturbance is respiratory (high PaCO2)
  3. Respiratory acidemia is acute (increase PaCO2 by 40, decrease pH by 0.32)
  4. Pt is hypoxemic and hypercapneic on room air

PAO2 = 0.21 (760-47) - 80/0.8 = 50

PAO2 - PaO2 = 50-45 = 5 (normal)

Most common cause of this disturbance is hypoventilation secondary to CNS depression (overdose)

MS2 Respiratory (26 decks)

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