9/9- Arterial Blood Gases in Lung Disease. Flashcards
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
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!
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)
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)
What is the minute ventilation equation and typical value?
VE = minute ventilation = TV x RR
- Typ ~ 6L/min
A low PaCO2 is what? What does it signify?
- PaCO2 under 35
- Signifies hyperventilation
A high PaCO2 is what? What does it signify?
- PaCO2 > 45
- Signifies hypoventilation
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)
What two things does PaCO2 depend on?
- Alveolar ventilation
- CO2 production
((PCO2 = VCO2/VA x 0.863))
What are the two broad causes of increased PaCO2?
- Alveolar hypoventilation
- Increased CO2 production (rare)
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
What may cause increased CO2 production (and thus increased PaCO2)?
- Overfeeding
- Hyperthyroidism
- Fever
What may cause decreased PaCO?
Alveolar hyperventilation
- Pain
- Anxiety
- Liver failure
- Early sepsis
- Pulmonary embolism
- Acute asthma
- Metabolic acidosis
Oxygen delivery system (in terms of oxygen pressure)?
PiO2 -> PAO2 -> PaO2 -> PvO2
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
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)?
- Hypoxemia
- Reduced oxygen delivery to tissues
- Reduced tissue oxygen uptake
Breakdown of causes of hypoxia:
- Hypoxemia
-
- Reduced oxygen delivery to tissues
-
- 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
- Pt is acidemic
- Disturbance is respiratory (high PaCO2)
- Respiratory acidemia is acute (increase PaCO2 by 16, decrease pH by 0.12)
- 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
- Pt is acidemic
- Disturbance is respiratory (high PaCO2)
- Respiratory acidemia is acute (increase PaCO2 by 40, decrease pH by 0.32)
- 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)
