A patient has a deadspace to tidal volume ratio of 75%. If the PaCO2 is 40 mm Hg, what is the expected PECO2?

To determine the expected end-tidal CO2 (PECO2) in this scenario, you can use the concept of deadspace ventilation, which refers to areas where ventilation occurs without perfusion. The deadspace to tidal volume ratio (VD/VT) is given as 75%, meaning that a significant portion of the tidal volume does not participate in gas exchange.

In a healthy individual, the relationship between arterial CO2 (PaCO2) and end-tidal CO2 (PECO2) can be used to estimate the expected PECO2. The formula to calculate the expected PECO2 when you know the VD/VT ratio is:

PECO2 = PaCO2 × (1 - VD/VT)

Given the values:

• PaCO2 = 40 mm Hg
• VD/VT = 0.75 (or 75%)

Now, substituting the values into the formula:

PECO2 = 40 mm Hg × (1 - 0.75) PECO2 = 40 mm Hg × 0.25 PECO2 = 10 mm Hg

Thus, the expected end-tidal CO2 (PECO2) is 10 mm Hg, validating the answer. This calculation demonstrates