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

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Flashcards in 9/9- Arterial Blood Gases in Lung Disease. Deck (39):
1

Arterial blood gases are indicators of the ____of the lungs?

What do they provide information about?

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

Provide information about:

- Alveolar ventilation

- Oxygenation

- Acid-base balance

2

What are some normal arterial blood gas values?

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!

3

What is alveolar ventilation (VA)? Typical volume?

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)

4

What is dead space ventilation (VD)?

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)

5

What is the minute ventilation equation and typical value?

VE = minute ventilation = TV x RR

- Typ ~ 6L/min

6

A low PaCO2 is what? What does it signify?

- PaCO2 under 35

- Signifies hyperventilation

7

A high PaCO2 is what? What does it signify?

- PaCO2 > 45

- Signifies hypoventilation

8

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

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

9

What two things does PaCO2 depend on?

- Alveolar ventilation

- CO2 production

((PCO2 = VCO2/VA x 0.863))

10

What are the two broad causes of increased PaCO2?

- Alveolar hypoventilation

- Increased CO2 production (rare)

11

What may cause hypoventilation (and thus increased PaCO2)?

Decreased minute volume (VE)

- CNS depression

- Respiratory muscle paralysis

Increase dead space (VD)

- Severe obstructive lung disease

- Severe restrictive lung disease

Combination of both

12

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

- Overfeeding

- Hyperthyroidism

- Fever

13

What may cause decreased PaCO?

Alveolar hyperventilation

- Pain

- Anxiety

- Liver failure

- Early sepsis

- Pulmonary embolism

- Acute asthma

- Metabolic acidosis

14

Oxygen delivery system (in terms of oxygen pressure)?

PiO2 -> PAO2 -> PaO2 -> PvO2

15

What do the following oxygen parameters measure:

- Partial pressure of oxygen (PaO2)

- Arterial oxygen saturation (SaO2)

- Arterial oxygen content (CaO2)

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

16

What is the major determinant of pulmonary and arterial PaO2?

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

How is PAO2 calculated?

Alveolar gas equation 

18

What is PAO2 - PaO2?

What does it reflect?

- The difference between alveolar and arterial oxygen

- Reflects V/Q mismatch

19

What is normal V/Q ratio?

- When is it higher? lower?

Normal: V/Q = 1

- V/Q > 1: increased dead space

- V/Q < 1: venous admixuture/shunting

20

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

False!

- PAO2 decreases with age

21

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

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

22

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

Apex (upper portion) of the lungs 

23

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

- Lower pH

- Decreased pCO2

- Lower temperature

24

What is hypoxemia? Hypoxia?

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

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

How does each effect P(A-a)?

Nonrespiratory:

- R to L intracardiac shunt (increased A-a)

- Decreased PIO2 from low barometric pressure or low FiO2 (normal A-a)

26

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

How does each effect P(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

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

How does each effect P(A-a)?

Artifact:

- Very high WBC count (increased A-a)

- Patient hyperthermia (increased A-a)

28

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

1. Hypoxemia

2. Reduced oxygen delivery to tissues

3. Reduced tissue oxygen uptake

29

Breakdown of causes of hypoxia:

1. Hypoxemia

-

2. Reduced oxygen delivery to tissues

-

3. Reduced tissue oxygen uptake

-

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

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

31

What may cause respiratory acidosis?

- CNS depression

- Chest wall dysfunction

- Obstructive lung diseases

- Hypercapnic respiratory failure

32

What may cause respiratory alkalosis?

- Anxiety

- Sepsis

- Liver failure

- Acute pulmonary insult; (eg; pulmonary embolism, asthma)

33

What may cause metabolic acidosis?

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

What may cause metabolic alkalosis?

- Diuretics

- Corticosteroids

- Nasogastric suctioning

35

What is anion gap? Normal value?

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

- Normal value = 10 (10-12ish) 

36

What information do you need to interpret ABGs?

- Fraction of inspired oxygen (FiO2)

- Barometric pressure

- Clinical data and age of patient

- Serum electrolytes and bicarbonate level

37

Steps in interpreting ABGs?

38

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

1. Pt is acidemic

2. Disturbance is respiratory (high PaCO2)

3. Respiratory acidemia is acute (increase PaCO2 by 16, decrease pH by 0.12)

6. 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

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

1. Pt is acidemic

2. Disturbance is respiratory (high PaCO2)

3. Respiratory acidemia is acute (increase PaCO2 by 40, decrease pH by 0.32)

6. 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)