Ventilation calculations require inspiratory and expiratory times. The inspiratory time is the time taken for inhalation. For ventilators, the inspiratory time is the amount of time it takes to deliver the tidal volume of air to the lung. The ratio of inspiratory time to expiratory time is a vital indication of respiration quality and is directly related to the respiration rate. An increased inspiratory rate leads to more efficient removal of CO2 from the body.

Locate the respiration rate by counting breaths per minute. For this example, take 15 breaths/minute as the rate. The average for adults is 12 to 20 breaths per minute.

Divide 60 by the respiration rate. There are 60 seconds in 1 minute. So, this calculation yields 60/15, or 4 seconds, for each complete breath. One complete breath is one inhale and one exhale.

Locate the expiratory time, which can be measured, or it will be given in seconds. To measure expiratory time, ask the patient to breathe normally into the spirometer. Analyze the graphical output. When the patient inhales, there is a loss in pressure in the device and the graph falls. When there is an exhale, the graph rises due to an increase in pressure. Locate the time interval for which the graph rises for each breathe. Then take the average of those times for the expiratory time. For example, take 2.5 seconds as the expiratory time.

Subtract expiratory time from the value from Step 2. This yields an inspiratory time of 4 - 2.5, or 1.5 seconds.