Final answer:
Extra tidal volume delivered in mechanical ventilation is calculated by subtracting the set tidal volume from the measured tidal volume.
Step-by-step explanation:
Tidal volume (TV) is the amount of air that is inspired and expired during normal breathing. To calculate extra tidal volume (also known as ventilatory support), one must subtract the set tidal volume (VT) from the measured tidal volume during mechanical ventilation. This difference represents the additional volume of air delivered to the patient through the mechanical ventilator. Therefore, the correct option is D, 'By subtracting set tidal volume from measured tidal volume'.