Flashcards in Assessment of V/Q Deck (7)
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1
Dead space vs. V/Q mismatch vs. shunt
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V/Q: Ratio of ventilation to perfusion. Quantitative measure of gas exchange. Best value would be 1. Normal is .8.
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Local regulation of V/Q occurs to limit either ventilation or perfusion to normalize the V/Q
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Dead space is the ventilation of un-perfused airway or alveoli
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Anatomic - trachea, bronchi, bronchioles
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Alveolar - ventilation of unperfused alveoli
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Vphys = Vanat + Valv
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Causes work without benefit, doesn’t usually cause hypoxemia unless disease is severe, decreases with exercise
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↑ PaCO2
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Shunt: Blood passing through capillaries that does not get oxygenated.
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Bronchial circulation is 1-2% of CO
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Shunt is the extreme of low V/Q
V/Q: Ratio of ventilation to perfusion. Quantitative measure of gas exchange. Best value would be 1. Normal is .8.
-
Local regulation of V/Q occurs to limit either ventilation or perfusion to normalize the V/Q
Dead space is the ventilation of un-perfused airway or alveoli
-
Anatomic - trachea, bronchi, bronchioles -
Alveolar - ventilation of unperfused alveoli -
Vphys = Vanat + Valv -
Causes work without benefit, doesn’t usually cause hypoxemia unless disease is severe, decreases with exercise -
↑ PaCO2
Shunt: Blood passing through capillaries that does not get oxygenated.
-
Bronchial circulation is 1-2% of CO -
Shunt is the extreme of low V/Q
2
Patterns of ventilation impact on amount of dead space (+causes)
- Rapid shallow breathing will only ventilate trachea, bronchi (minimal alveoli)
- RR @ 40x/minw/250 ml per breath ==> maximize dead space breathing
- Breathe deeply 10x/min at 1000 ml per breath ==> perfuse the alveoli maximally thus minimizing your dead space ventilation.
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Causes:
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Rapid, shallow breathing
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PE
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Decreased CO
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Mechanical Ventilation- snorkeling, being on a ventilator
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Emphysema
- RR @ 40x/minw/250 ml per breath ==> maximize dead space breathing
- Rapid, shallow breathing
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PE -
Decreased CO -
Mechanical Ventilation- snorkeling, being on a ventilator -
Emphysema
3
Major causes of low V/Q and shunts
- Low V/Q:
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Regional resistance (bronchitis, asthma)
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Hypoventilation
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Diffusion Defec
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t
Emphysema
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Interstitial lung disease
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Congenital heart disease
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Pulmonary fistula
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Vascular lung tumor
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Shunt: Filled alveolar space
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Heart failure (transudate)
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Pneumonia / ARDS (adult respiratory distress syndrome) (exudate)
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Regional resistance (bronchitis, asthma) -
Hypoventilation -
Diffusion Defec -
t
Emphysema -
Interstitial lung disease -
Congenital heart disease -
Pulmonary fistula -
Vascular lung tumor
Shunt: Filled alveolar space
- Heart failure (transudate)
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Pneumonia / ARDS (adult respiratory distress syndrome) (exudate)
4
very low V/Q vs. shunt
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V/Q will respond well to increased FiO2 (fraction of inspired oxygen).
V/Q will respond well to increased FiO2 (fraction of inspired oxygen).
5
Pulse oximetry measurement characteristics and common problems
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= ratio of Deoxygenated Hb : Oxygenated Hb
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Ratio = SpO2
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Problems:
Hb may be bound to something other than O2
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Carboxy-Hb absorbs in the red band (where oxygenated Hb absorbs)
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Sensitive to movement, temperature, lighting, nailpolish
- Met-Hb (3+) can’t bind O2 and increases O2 affinity at other sites. Also absorbs @ red.
= ratio of Deoxygenated Hb : Oxygenated Hb
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Ratio = SpO2
Problems:
Hb may be bound to something other than O2
-
Carboxy-Hb absorbs in the red band (where oxygenated Hb absorbs) -
Sensitive to movement, temperature, lighting, nailpolish - Met-Hb (3+) can’t bind O2 and increases O2 affinity at other sites. Also absorbs @ red.
6
Causes of hypoxemia (5)
- Normal A-a Gradient
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Altitude
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Hypoventilation (obesity, central apnea, neuromuscular disease, drugs)
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Increased A-a Gradient
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Diffusion limitation (extreme exercise, interstitial lung disease during exertion)
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Low V/Q
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Shunt
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Altitude -
Hypoventilation (obesity, central apnea, neuromuscular disease, drugs)
Increased A-a Gradient
- Diffusion limitation (extreme exercise, interstitial lung disease during exertion)
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Low V/Q -
Shunt
7
Calculation of A-a gradient
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10 or below is normal
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= PAO2 - PaO2
10 or below is normal
= PAO2 - PaO2
