Diffusion and perfusion Flashcards
Normal values of:
CaO2
Minute perfusion
CaO2: 20.7 ml O2/ 100 ml blood,
20.4 of which is bound to Hb
Min Perf: 5-6L
*remember CaO2 = SaO2 x Hb x 1.39
Hypoxemia vs Hypoxia
Hypoxemia: (↓PaO2) aka ↓ [ ] O2 in blood
Hypoxia: ↓ amt of O2 reaching the tissues (PO2
Causes of Hypoxemia when A-a gradient is normal
Low PIO2
- altitude
Low PAO2
- obesitity, central apnea, neuromuscular dis, drugs
What is the normal A-a gradient? Between what two pressures is this A-a gradient?
Normally, there is a 5-10 torr (A-a gradient) difference between PAO2 and PaO2.
What is the A-a gradient for:
diffusion
Shunt
V/Q mismatch?
all ↑
What is the PaCO2 for:
diffusion (interstitial disease)
Shunt (pneumonia)
V/Q mismatch (Mod COPD?
All normal
As long as total ventilation level is normal, your CO2 levels are normal
What value is going to decrease with CO poisoning
↓ SaO2
Causes in increased deadspace
In anatomic deadspace:
1. Rapid shalllow breathing (most TV is in the conducting airways)
In alveolar dead space
- Acute Pulm Embolus
- ↓ CO
Ventilation in excess of perfusion
- Ventilators (+ pressure)
- Emphysema (alveolar septal destruction)
List the major nonanatomic causes of low V/Q (mismatch)
● Regional resistance (bronchitis, asthma)
● Hypoventilation
● Diffusion Defect
- ie: extreme exercise, ILD
List the major causes of shunt
Filled alveolar space
- heart failure (transudate)
- Pneumonia/ARDS (exudate)
List the major anatomic causes of low V/Q (mismatch)
- congenital Heart disease
- Pulm Fistula
- Vascular lung tumor
How can you tell the diff between a very low V/Q and a shunt?
V/Q will respond to increased FiO2
Remember:
● Hypoxemia is ↓ PaO2
● Desaturation is ↓ SaO2
Problems with pulse ox
Normally, pulse ox measures Deox Hb: Ox Hb ratio = SpO2
Problems:
- Hb may be bound to something other than O2
- Sensitive to movement, temp, lighting, nailpolish
- Met-Hb (3+) cant bind O2 and causes O2 to bind better at other sites
