What parameter indicates the amount of air leaving the lungs during spontaneous breathing?

The correct answer is tidal volume (Vt), which represents the amount of air that is inhaled or exhaled with each breath during spontaneous breathing. Tidal volume is a critical parameter in assessing ventilation because it directly reflects the depth of each breath. In spontaneous breathing, the tidal volume measures how much air is moving in and out of the lungs in a single breath cycle.

Minute ventilation (VE), while related, is calculated by multiplying tidal volume by the respiratory rate, indicating the total volume of air breathed in or out per minute rather than per single breath. The respiratory rate (f) is simply the number of breaths taken per minute and does not provide information on the volume of air per breath. Vital capacity (VC) is the maximum amount of air that can be exhaled after a maximum inhalation, and while it indicates lung capacity, it is not a measure of the amount of air leaving the lungs during normal spontaneous breathing. Therefore, tidal volume is the most direct and relevant measurement for assessing the air leaving the lungs per breath.