The following gives you practice using the formulas. I also remind you to check "Review for Final" for additional information on equations and mode descriptions.
An approximately 10 torr change in PaCO2 over an hour is safe. How much would minute ventilation need to change to cause a 10 torr change in PaCO2?
One way to answer this question requires a two-part step and two assumptions. The first equation is PaCO2 = VCO2 / VA. To estimate a person's VCO2, take 3 x the body wt in Kg (Assumption #1). Plug in values for the desired PaCO2 to find VA.
Next we use the formula VA + VD = VE. We guess at the VD / VT ratio. Normally between 0.2 - 0.4, patients with severe COPD requiring mechanical ventilation will be in the 0.6 or more range. If the person does not have COPD, use 0.3.
Applying this to setting ventilator controls, we would then set a tidal volume on the ventilator equal to 10 ml/kg. 10 x 70 = 700 ml. 6 L divided by 0.7 L = 9 breaths/min for a Rate.
Applying this equation to the question, however, we would re-calculate the minute ventilation for this patient that would produce a PaCO2 of 40 torr to find our change in minute ventilation equivalent to a 10 torr change in PaCO2. You do the math now and check your answer.
Another application of these equations is to find the minute ventilation you would use for a CO2 retainer with COPD. The patient's last ABG on discharge was a PaCO2 of 55 torr. He weighs 80 kg. What minute ventilation would you use, ie. tidal volume and rate? You do the math and check your answer.
E-mail me any questions you have.