What device would a therapist use to measure exhaled tidal volume when calculating a patient's RSBI?

The device a therapist would use to measure exhaled tidal volume, particularly when calculating a patient's Rapid Shallow Breathing Index (RSBI), is a spirometer. The spirometer is specifically designed to measure the volume of air inhaled and exhaled by the lungs, making it ideal for determining tidal volume.

In the context of calculating RSBI, which is the ratio of respiratory frequency to tidal volume, knowing the exhaled tidal volume is crucial. By measuring this volume directly, a therapist can accurately assess the patient's breathing patterns. Devices like vane respirometers can also measure airflow and volume but do not provide the same level of detail required for tidal volume independently.

Flow meters are typically used to measure the flow rate of air, not the volume directly, and humidifiers are employed to moisten inhaled air but have no role in measuring tidal volume. Therefore, a spirometer is the most appropriate choice for this particular task, making it the correct answer.