9/9- Arterial Blood Gases in Lung Disease. Flashcards by Kathryn Kudlaty

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

How well did you know this?

Not at all

Perfectly

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!

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

What is the minute ventilation equation and typical value?

VE = minute ventilation = TV x RR

Typ ~ 6L/min

How well did you know this?

Not at all

Perfectly

A low PaCO2 is what? What does it signify?

PaCO2 under 35
Signifies hyperventilation

How well did you know this?

Not at all

Perfectly

A high PaCO2 is what? What does it signify?

PaCO2 > 45
Signifies hypoventilation

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

What two things does PaCO2 depend on?

Alveolar ventilation
CO2 production

((PCO2 = VCO2/VA x 0.863))

How well did you know this?

Not at all

Perfectly

What are the two broad causes of increased PaCO2?

Alveolar hypoventilation
Increased CO2 production (rare)

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

Overfeeding
Hyperthyroidism
Fever

How well did you know this?

Not at all

Perfectly

What may cause decreased PaCO?

Alveolar hyperventilation

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

How well did you know this?

Not at all

Perfectly

Oxygen delivery system (in terms of oxygen pressure)?

PiO2 -> PAO2 -> PaO2 -> PvO2

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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)

How is PAO2 calculated?

Alveolar gas equation

What is PAO2 - PaO2?

What does it reflect?

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

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

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

False!

PAO2 decreases with age

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)

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

Apex (upper portion) of the lungs

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

Lower pH
Decreased pCO2
Lower temperature

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)

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)

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)

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)

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

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

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

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

What may cause respiratory acidosis?

- CNS depression - Chest wall dysfunction - Obstructive lung diseases - Hypercapnic respiratory failure

What may cause respiratory alkalosis?

- Anxiety - Sepsis - Liver failure - Acute pulmonary insult; (eg; pulmonary embolism, asthma)

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)

What may cause metabolic alkalosis?

- Diuretics - Corticosteroids - Nasogastric suctioning

What is anion gap? Normal value?

Anion gap (AG) = Na - (Cl + HCO3) - Normal value = 10 (10-12ish)

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

Steps in interpreting ABGs?

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

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)