MA

Alveolar Ventilation and Dead Space

Ventilation: movement of air in and out of the lungs. Gas exchange

Simplified Bohr equation to estimate dead space fraction (VD/VT).

  • Two patients both breathe 12 times per minute each with a tidal volume of 500 mL. Why might one have high CO2 while other does not

    • Not all the air we breathe reaches gas exchange areas.

      • Conducting airways: does not help with gas exchange dead space.

      • Alveoli

  • Types fo Dead space:

    • Anatomic Dead space:

      • Air in teh conductin airwasy the never reaches alveoli nose and bronchioles

      • 150ml in an adult

      • Does not change much unless tubes are added

    • Alveolar dead space

      • Air that reaches alveoli but doesnt exchange gases due to poor blood flow

      • Pulmonary embolism, alveoli get air but no blood.

    • Physiologic dead space    

      • The total of anatomic and alveolar dead space.

      • In healthy people physiologic = anatomic because alveolar dead space is small.

    • Understanding how much of each breath is wasted in dead space is important because it affects how well a person eliminates carbon dioxide. Even if a patient’s total breathing rate looks normal, they can still retain too much carbon dioxide if too much of their breath is trapped in dead space instead of reaching the alveoli.

      For example, a patient with a large endotracheal tube or extra ventilator tubing has more apparatus dead space, which means more of each breath is wasted. Patients with pulmonary embolism or emphysema can also have increased dead space because parts of the lung are not receiving good blood flow or gas exchange.

  • Bohr Equation and VD/VT ratio (%)

    • The Bohr equation estimates the fraction of each breath that’s dead space (doesn’t praticipate in gas exchange)

      • VD/VT=(PaCO2-PeCO2)/PaCO2

        • PaCO₂ = CO₂ in arterial blood (from ABG).

          PeCO₂ = CO₂ in exhaled gas (from capnography).

          VD/VT = fraction of tidal volume that’s wasted.

  • When the carbon dioxide level in the blood paCO2, and the carbon dioxide level in exhaled air PeCO2, are close to each other it means most of the air you breathe out has participated in gas exchange.

    • Indicates low dead space and efficient ventilation.

  • Wen the exhaled CO2 (PeCO2) is much lower that the CO2 in the blood (PaCo2) it means a lot of the air leaving the lungs never took part gas exchange

    • This shows there is a large amount of dead space and that ventilation is less efficient.

  • .20-.40 normal, healthy lungs

  • >.50 inefficient ventilation, pulmonary embolism, ards, emphysema

  • Ventilation efficiency

    • Minute ventilation (VE)

      • Total air mover per minute

      • VE=VT x f(tidal volume x respiratory rate)

      • Some stays in the dead space, which includes airways like the trachea and bronchi that do not participate in oxygen or carbon dioxide exchange

    • Dead space ventilation

      • is the portion that does not participate in gas exchange

    • Alveolar Centilation (VA)

      • The portion of the VE that actually reaches alveoli for gas exchange

        • VA=(VT-VD) x f (F is rate)

    • You can have a normal minute ventilation but poor alveolar ventilation if dead space is large.

    • Think of each breath as a mix of “useful” air and “wasted” air. Minute ventilation tells you the total, while alveolar ventilation tells you how much of that total is actually doing the job. If dead space increases, less of each breath is useful, and CO₂ can build up even if the person’s breathing rate seems normal.

      In conditions such as COPD, pulmonary embolism, or when a patient has long ventilator tubing, the amount of dead space (VD) in the lungs increases. This means more of each breath does not take part in gas exchange, which can lead to carbon dioxide (CO₂) building up in the blood.