By Suman Medda
Updated March 24, 2022
Accurate ventilation management relies on precise knowledge of inspiratory and expiratory durations. The inspiratory time is the interval during which the ventilator delivers the tidal volume to the lungs. The ratio of inspiratory to expiratory time (I:E ratio) is a key indicator of respiratory efficiency and directly influences the respiratory rate. A higher inspiratory proportion improves CO₂ clearance.
Count the patient’s breaths per minute. For illustration, we’ll use 15 breaths/min. In adults, a normal range is 12–20 breaths/min.
Divide 60 seconds by the respiratory rate. 60 ÷ 15 = 4 seconds per complete breath (one inhale plus one exhale).
Expiratory time can be measured directly or provided in seconds. To measure, have the patient breathe normally into a spirometer and examine the waveform. The upward portion of the graph corresponds to exhalation. Identify the duration of this upward segment for several breaths and compute the average. For example, an average expiratory time of 2.5 seconds.
Subtract the expiratory time from the total breath cycle: 4 s – 2.5 s = 1.5 seconds. Thus, the inspiratory time is 1.5 seconds.
