A mechanical ventilator delivers a tidal volume of 500 mL at 10 breaths/min with an I:E ratio of 1:2 and fresh gas flow of 6 L/min. With a closed inspiratory valve, what is the actual tidal volume delivered to the patient?

A mechanical ventilator delivers a tidal volume of 500 mL at 10 breaths/min with an I:E ratio of 1:2 and fresh gas flow of 6 L/min. With a closed inspiratory valve, what is the actual tidal volume delivered to the patient?

• A. 400 mL
• B. 500 mL
• C. 600 mL
• D. 700 mL

Answer: (D)

Fresh gas flow can augment the volume delivered to the patient during inspiration in a circle system when the inspiratory valve is closed. With 10 breaths per minute, each breath cycle lasts 6 seconds. An I:E ratio of 1:2 means the inspiratory time is 2 seconds and the expiratory time is 4 seconds. The fresh gas flow is 6 L/min, which is 6000 mL per 60 seconds, or 100 mL per second. During the 2-second inspiratory phase, 100 mL/s × 2 s = 200 mL of fresh gas enters the circuit and is inspired. The ventilator is set to deliver 500 mL per tidal breath, so the actual tidal volume delivered to the patient becomes 500 + 200 = 700 mL.

Fresh gas flow can augment the volume delivered to the patient during inspiration in a circle system when the inspiratory valve is closed. With 10 breaths per minute, each breath cycle lasts 6 seconds. An I:E ratio of 1:2 means the inspiratory time is 2 seconds and the expiratory time is 4 seconds. The fresh gas flow is 6 L/min, which is 6000 mL per 60 seconds, or 100 mL per second. During the 2-second inspiratory phase, 100 mL/s × 2 s = 200 mL of fresh gas enters the circuit and is inspired. The ventilator is set to deliver 500 mL per tidal breath, so the actual tidal volume delivered to the patient becomes 500 + 200 = 700 mL.