Assessment of V/Q Flashcards
If you have:
normal arterial PO2= 40mmHg and PCO2= 45mmHg normal alveolar PO2= 104 mmHg and PCO2= 40mmHg
What will be the values of PO2/PCO2 after passing by the alveolus?
PO2= 104 mmHg (same as alveolus)
PCO2= 40 mmHg (same as alveolus)
Gases equilibrate in normal lungs
V/Q = 1
- What does V/Q of 0 mean?
- What about V/Q of infinity?
- Normal perfusion, no ventilation
- Normal ventilation, no perfusion
*There is no oxygenation of blood with these extremes
What would V/Q be in the upper lobes?
The lower lobes?
What’s the average V/Q
Upper = 2.5
Lower = .6
Average = .8
How does your body regulate each of the following?
- High V/Q
- Low V/Q
- Increase local airway resistance to try to decrease ventilation and decrease V/Q
* In this situation alveolar PCO2 drops - Hypoxic vasoconstriction to try and decrease perfusion and increase V/Q
* In this situation alveolar O2 drops
What are the following values at sea level and Denver?
- PaO2
- PaCO2
- Sat
- CaO2
- CaCO2
Sea level
- 104
- 40
- 97
- 20.7
- 44
Denver
- 80
- 35
- 95
- 19
- 42
What effect does dead space have on V/Q?
- High VQ
- work without benefit
- Generally doesn’t cause hypoxemia (decreased PaO2)
- Can cause ^ PaCO2
- Decreases with exercise
Elevated PaCO2 is abnormal. But what are some things that can increase PaCO2 and how can your body compensate?
- Increased CO2 production (exercise)
- Dead space increases PaCO2
Body compensates by increasing minute ventilation
Which of the following minute ventilations would give you the highest alveolar ventilation?
- 250 ml x 40 /min
- 500 ml x 20 /min
- 1000 ml x 10 /min
- Alveolar vent = 4
- alveolar vent = 7
- alveolar vent = 8.5
**bigger breaths are better because you are getting more into your alveoli. Short quick breaths don’t give you as much alveolar ventilation (deadspace ventilation)
What are some causes of dead space?
- Rapid shallow breathing
- Pulmonary Embolus
- Decreased Cardiac Output
- Mechanical Ventilation
- Emphysema
What is a shunt and what effect does it have on V/Q?
Shunt = blood passing through capillaries that does not get oxygenated
- V/Q < 1
- bronchial circulation (1-2% CO)
How can you differentiate whether someone has a shunt (no ventilation) versus just low V/Q?
Low V/Q responds to increasing FiO2 (fraction of inspired oxygen)
- Shunt responds minimally to 100% O2
What types of things can cause a shunt? What can cause V/Q mismatch?
Shunt: (things that fill alveolar space
- Pneumonia/ ARDS
- Pulmonary edema
- Heart failure/ congenital heart Dz
- Atelectasis (complete or partial collapse of lung)
V/Q mismatch:
- Bronchitis/ asthma (regional resistance)
- Hypoventilation
- Diffusion defect
- Late emphysema
- ILD- fibrosis
How do you calculate V/Q mismatch?
A-a gradient
- Difference in expected vs. measured arterial O2
- Usually < 10
What is the equation for A-a gradient? (WE NEED TO KNOW THIS ONE)
- Get PaO2 from your blood gas
- Use alveolar gas equation to get PAO2
- PAO2 = [(Pb - PH2O) x FiO2] - PaCO2/R
- In Denver PAO2 = [(630-47)0.21)] - 35/0.8
- PAO2 = 122 - 44 = 78 mmHg
- At sea level PAO2 = [(760-47).21] – 40/0.8
- SL PAO2 = 150– 50 = 100 mmHg
3. A-a gradient = PAO2 - PaO2
How does pulse oximetry work?
What are some problems with pulse oximetry?
SpO2 = Measures ratio of deoxy-Hb and oxy-Hb
- Deoxy-Hb absorbs maximally in visible red band
- Oxy-Hb absorbs maximally in IR band
Trouble:
- Measures only Hb saturation.
- Hb may be bound to something other than O2
- Sensitive to movement and temperature
- May give a falsely high or low reading
- Sensitive to lighting
- Nail polish
- Misread results: Assumes PaO2 or SaO2 instead of SpO2
How does CO-Hb compare to O2-Hb on the pulse oximeter?
=They both have the same absorbance at the visible red band, so you usually can’t tell!
- CO poisoning will show a normal SpO2
- Co-oximetry can pick up four different wavelengths and can differentiate though (see image)
