What is dead space?
Dead space refers to the areas of the respiratory tract that are ventilated but not perfused and therefore do not undertake gas exchange with the blood. The total dead space is called the physiological dead space. It is composed of anatomical and alveolar dead space.
Anatomical dead space refers to the volume occupied by the conducting airways that supply the alveoli, but don’t undertake gas exchange themselves, and this is generally the first 16 airway generations.
Alveolar dead space refers to the alveoli that are ventilated but do not receive enough blood to undertake gas exchange. This can be physiological, such as in hypoxic pulmonary vasoconstriction, or pathological, as seen in pulmonary embolism.
How can you measure dead space?
Total or physiological dead space is measured using Bohr’s equation.
It is usually around 200-350ml in normal tidal breathing.
We can reasonably assume that a tidal volume is comprised of alveolar volume and dead space volume, and we can also reasonably assume that inspired CO2 is minimal, if there is no rebreathing occurring.
Therefore the entire expired CO2 is only going to be coming from the alveolar ventilation. Therefore we can generate the equations above, and rearrange as demonstrated to form the Bohr equation.
The Enghoff modification simply extends this with another assumption that the alveolar partial pressure of CO2 can reasonably be approximated by the arterial partial pressure of CO2, which makes it all a lot easier to measure.
This works fairly well as long as you remember that this assumption will usually over estimate dead space, as arterial CO2 is slightly higher than alveolar, and will also be affected by:
- Dead space
- Diffusion impairment
Anatomical dead space is measured using Fowler’s method.
It is approximately 2ml/kg.
Step 1 – patient takes a vital capacity breath of pure (100%) oxygen, thereby removing all nitrogen from the anatomical dead space (Remember that the alveoli still receive nitrogen from the blood).
Step 2 – patient exhales all the way to residual volume into a pneumotachograph, which measures flow over time and therefore provides a volume measurement (flow/time = volume).
Step 3 – the concentration of nitrogen detected is plotted against volume to look like the graph below, where Area A = Area B.
To begin with, pure oxygen is exhaled from the dead space, so no nitrogen is detected – this is phase 1.
In phase 2 you’re detecting a gradually increasing concentration of nitrogen, because different alveoli have different time constants. This means that some alveoli will empty their nitrogen before others, so there will be mixing of alveolar and dead space gas.
Towards the end of this phase, most of the dead space oxygen will have already been exhaled, and more alveoli will be now be draining their nitrogen and therefore the nitrogen concentration detected increases.
In phase 3 there is a plateau, representing only alveolar ventilation, as all dead space gas has now been exhaled.
In phase 4 there is an inflection and a sudden increase in the nitrogen concentration, and this occurs at the closing capacity. The reason this occurs is because the basal alveoli are more compliant than the apical alveoli, and as a result receive more oxygen during the initial breath of 100% oxygen, so the concentration of nitrogen is actually higher in the apical alveoli, as less of the nitrogen is washed out in that initial breath.
The basal alveoli drain first during the exhalation, until the closing capacity is reached and the apical alveoli are the only ones left to drain. As a result, the nitrogen concentration detected increases suddenly on the graph
Step 4 (well done keep going!)
Now draw a vertical line approximately half way through phase 2, such that the highlighted areas A and B are equal. This line cuts through the x axis at the anatomical dead space volume
Fowler originally guesstimated this to be a reasonably sensible cut off point, and then tested it with a bunch of rubber models to show that it worked. There is no algebraic explanation that I can find as to exactly why this is the point at which you draw the vertical line.
How is pulmonary vascular resistance calculated?
- PVR = pulmonary vascular resistance (dynes.s^-1.cm^-5)
- The 80 is a conversion coefficient to adjust for the discrepancy between the units used
- MPAP = mean pulmonary arterial pressure
- LAP = left atrial pressure
- CO = cardiac output
What factors affect pulmonary vascular resistance?
|Factors that increase PVR||Factors that decrease PVR|
|Adrenaline and noradrenaline||Volatile anaesthetic agents|
|Serotonin and Histamine||Prostacyclin|
|High or low lung volume||Nitric Oxide|
|High intrathoracic pressures|
|Increased pulmonary venous pressure|
How does pulmonary vascular resistance change with lung volume?
- PVR is at its minimum at Functional Residual Capacity
- As lung volumes decrease, compression of pulmonary vessels increase resistance
- As lung volume increases the vessels are stretched and increases resistance
What are West zones?
When in the upright position, blood distribution to the different areas is divided into zones called ‘West Zones’. Classically there are three zones described, but a fourth has been added to account for low lung volumes.
- Zone 1
- Ventilation is far higher than perfusion as arterial and venous blood vessels are compressed
- Tendency to form dead space
- Zone 2
- Perfusion depends on the difference between arterial and alveolar pressures and varies with both cardiac and respiratory cycles
- Perfusion is higher at the bottom of zone 2 than at the top
- Zone 3
- Given both arterial and venous pressures are higher than alveolar pressure, blood flow is consistent to this zone
- It represents areas of shunt
- Zone 1PA>Pa>Pv
- Alveolar pressure is higher than arterial pressure, meaning there is no perfusion as the blood vessels are compressed closed
- This zone only occurs during positive pressure ventilation
- It represents areas of dead space
- Zone 4
- Similar to zone 3 but with higher resistance
What is normal V/Q matching?
- V represents ventilation, which is usually around 4 – 5 litres per minute
- Q represents perfusion, which is usually around 5 litres per minute
- A normal V/Q ratio is therefore around 0.9
How does V/Q matching vary with zones of the lung?
- Both perfusion and ventilation gradually decrease as you travel from the bottom to the top of the lung, but perfusion decreases to a greater extent
- This means that VQ ratio is lower at the bottom and increases towards the top of the lungs
- Therefore the bottom of the lungs demonstrates shunt and the top demonstrates dead space
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