This equation computes the alveolar partial pressure of CO2.
This equation characterizes a patient's respiratory rate. Alveolar PCO2 is directly proportional to the amount of CO2 produced by metabolism and delivered to the lungs (VCO2). At the same time, the PCO2 is inversely proportional to the alveolar ventilation (VA). The alveolar ventilation is the volume of gas expired from the body (from alveoli) - typically in liters per minute.
It is useful to note that the alveolar and arterial PCO2 can be assumed to be equal.