Ventilation Perfusion Inequality And Hypoxemia Flashcards
What is dead space?
Volume of inhaled air that does not participate in gas exchange
Regions 9f no gas exchange
Conducting airways -anatomical dead space(150 ml)
Alveoli with no perfusion (alveolar dead space)
What are shunts?
Imperfect matching of lungs airflow and blood flow
Two causes for physiological shunt (total venous admixture)
- I) 50% comes from anatomical shunt (right to left shunt) (bronchial circulation, thebesian veins)
- II) 50% from low VA/Q e.g. at the base of the lung, partially obstructed airway
What is “wasted air” and “wasted blood”?
- All of the inspired air does not participate in gas exchange, resulting in some “wasted air”- physiologic dead space
- All of the blood entering the lung is not fully oxygenated, leading to some “wasted blood”-physiologic shunt
The lung is a …
Slinky
Top-has most of the weight of the rest of the slinky pulling it down on it- therefore the coils are apart
Middle- has half of the mass of the rest of the slinky pulling down in it - therefore the coils are tighter together than at the top
Bottom- has very little weight of the slinky pulling down on it - therefore the coils are tighter together than at the top
In addition the lung is resting on the contents of the abdomen
What is the pressure volume at FRC?
- The apex of the lung is less compliant than the base
- At rest the lung apex is approximately 70% distended
- At rest the lung base is 15% distended
Describe pressure volume changes during inspiration
During inspiration the change in volume at the apex is significantly less than at the base
The ventilation gradient is aligned with gravity
Summarize regional differences in perfusion
- Pulmonary circulation is a high-flow, low-pressure, low-resistance system
- Since upward flow runs against hydrostatic pressure there is more resistance to blood flow toward the apex of the lung (no hydrostatic pressure to overcome in blood flowing down)
Alveolar pressure also affects pulmonary perfusion- high alveolar volume in the apex reduces blood flow (this restriction is not present when alveolar pressure falls below pulmonary arterial pressure)
What are the ventilation regional differences?
At the top
Intrapleural pressure more negative
Greater transmural pressure gradient, alveoli larger, less compliant
Less ventilation
At the bottom Intrapleural pressure less negative Smaller transmural pressure gradient Alveoli smaller, more compliant More ventilation
What are the perfusion regional differences?
At the top
Lower intra vascular pressures
Less recruitment , distention
Higher resistance, low blood flow
At the bottom Greater vascular pressures More recruitment, distention Lower resistance Greater blood flow
What is the ventilation perfusion ratio gradient?
Ventilation and blood flow are both gravity dependent
Blood flow is proportionally greater than ventilation at the base and vice versa at the apex
Ventilation-perfusion ratio decreases down the lung
Gas exchange is more efficient at the apex of the lung as compared to the base
What is hypoxic vasoconstriction?
For low V/Q ratio (lots of blood or too little ventilation); causes the blood coming into the area to be directed to other parts of the lung
What is bronchoconstriction?
For high V/Q ratio, the bronchi will construct slightly to increase the resistance and decrease the amount of ventilation coming into an area that is not well perfused thus limiting the amount of alveolar dead space
Increases V/Q
What is the effect of embolism and dead space on V/Q?
Perfusion is low in well ventilated areas (embolism) or in the extreme case-perfusion is absent in ventilated areas (dead space-e.g. trachea/bronchi)
What is the effect of a shunt on V/Q ratio?
Shunt- blood is passing through unventilated or poorly areas e.g. COPD
How is A-a O2 gradient measured?
PAO2 without diffusion problems or VA/Q mismatch should ideally be equal to the PaO2
Normal value 5-15 mmHg, this increases with age
A-a O2 gradient= PAO2-PaO2
