Physiology-Shunt, V:Q, Hypoxia, Failure Flashcards
What is the expected alveolar PAO2 in a healthy person? How do you calculate someone’s PAO2 if you know their PaCO2? Why would you want to know PAO2?
102 mmHg. You want to know this because you can then calculate the patient’s PaO2.
What is an A-a gradient and what does a normal level tell you?
The A-a gradient is the difference between the expected and actual PaO2 (calculated PAO2 - actual PaO2). If your patient is having trouble breathing and the A-a gradient is normal (<15 mmHg), it tells you that blood O2 transfer is normal and they are hypoxic due to low O2 availability or hypoventilation (high CO2)
What does an abnormal A-a gradient tell you if your patient is experiencing hypoxia?
O2 transfer is abnormal due to diffusion impairment or V/Q inequalities causing shunt
What is the main factor driving diffusion in the lung?
Surface area. Alveolar thickening and changes in driving pressure are less common causes.
How much of a healthy person’s tidal volume is physiologic dead space?
150cc of 500cc.
What are the different zones of the lung? In diseased states where will you first have shunting occur?
Apical = PA> Pa>PV. Middle Pa>PA>Pv. Bottom = Pa>Pv>PA. In diseased states you will have shunt occurring at zone 1 first due to increased pressure needed to peruse that area of the lung.
What is the normal V/Q ratio is the lung? How does this value vary with disease states?
Normal V/Q is 0.8. In some diseased states gravity squishes the airways and you don’t get any ventilation to areas with lots of perfusion (V/Q of 0). In other diseased states people have lots of ventilation but no perfusion (V/Q of infinity).
What is V/Q is dead space? In shunt?
Dead space = infinity. Shunt = 0.
Which West zone is recruitable and is normally 0 in healthy lungs?
West zone 1
What is the level of shunt in healthy lungs?
1-2% due to bronchial circulation and the thebesian vein.
Will 100% O2 help someone with full on shunt? What about V/Q mismatch?
In extreme shunt, 100% O2 will not help because there is no blood flow around the areas of dead space. In V/Q mismatch it will help because it will increase O2 levels to deficient areas and cause the blood vessels to open up around the airway.
A patient comes to see you in the mountains of Afghanistan complaining of difficulty breathing. The pressure at that altitude is about 600 mmHg. What percent shunt might this soldier have?
5%. For every 100mmHg below 700mmHg, you estimate a 5% increase in shunting.
What are causes of V/Q mismatch?
PE, asthma, COPD, CHF and anything that fills the alveoli and decreases ventilation.
What are the two main categories of shunt?
Pulmonary, cardiac and extracardiac (anything that fills the alveoli and inhibits gas exchange)
A 44 year old male presents with hypoxia. His chest x-ray is shown below. What conditions are you thinking about in this patient?
Pneumonia, ARDS, pulmonary edema or interstitial lung disease.
A 44 year old male presents with hypoxia. His chest x-ray is clear. What conditions are you thinking about in this patient?
PE, right to left cardiac shunting of blood and microatelectasis (collapsed alveoli that you can only see on CT)
What are the main causes of hypoxia? Which of these causes will have abnormal A-a gradient?
V/Q mismatch and shunt. You distinguish between the two by placing the patient on 100% FiO2. Hypoventilation, diffusion abnormality and low inspired oxygen content can also cause hypoxia.
What defines hypercarbic/ventilatory failure? What factors determine ventilatory valure?
A PaCO2 > 40 mmHg. CO2 production (VO2*R), alveolar ventilation (1-Vd/Vt) and how much you breath (VE minute ventilation).
Why do narcotics cause hypercarbic states? COPD? Botulism?
Narcotics decrease minute ventilation by decreasing the desire to breath. COPD increases the effort necessary to breath and can decrease minute ventilation as the person gets tired. Botulism causes neuromuscular weakness that decreases the ability to breath and minute ventilation.
Why does bronchiectasis cause hypercarbic states?
It dilates the airways and increases the amount of dead space (1-Vd/Vt)
Why do patients with a high carbohydrate diet coming off ventilators become hypercarbic?
They have increased CO2 production (VO2*R)
