A mechanically ventilated patient is transported with a portable ventilator using 2 L/min of oxygen to run valves and drive the ventilator. If an E cylinder gauge reads 2000 psi, and the patient’s VT is 500 mL at 10 breaths/min, how long could ventilation continue if the ventilator requires 200 psi to operate and the cylinder holds 625 L?

A mechanically ventilated patient is transported with a portable ventilator using 2 L/min of oxygen to run valves and drive the ventilator. If an E cylinder gauge reads 2000 psi, and the patient’s VT is 500 mL at 10 breaths/min, how long could ventilation continue if the ventilator requires 200 psi to operate and the cylinder holds 625 L?

• A. 20 min
• B. 40 min
• C. 60 min
• D. 80 min

Answer: (D)

When measuring how long an oxygen cylinder will last for a portable ventilator, you must account for two things: how much usable gas remains in the cylinder as pressure falls from the starting value to the minimum operating pressure, and how quickly that gas is consumed. In this scenario, the cylinder starts at 2000 psi and you can use gas until 200 psi, so the usable portion of gas represents a 1800 psi drop. The cylinder is rated to hold 625 L of gas at 2000 psi, so the usable volume is 625 L × (2000 - 200)/2000 = 625 × 0.9 = 562.5 L. Next, determine how fast that gas is consumed. The patient’s minute ventilation is VT × rate = 0.5 L × 10 breaths/min = 5 L/min. The ventilator itself also requires oxygen to operate, which is given as 2 L/min. Therefore, the total oxygen flow drawn from the cylinder is 5 + 2 = 7 L/min. Divide the usable gas by the total consumption rate to get the duration: 562.5 L / 7 L/min ≈ 80 minutes. So ventilation could continue for about 80 minutes under these conditions.

When measuring how long an oxygen cylinder will last for a portable ventilator, you must account for two things: how much usable gas remains in the cylinder as pressure falls from the starting value to the minimum operating pressure, and how quickly that gas is consumed. In this scenario, the cylinder starts at 2000 psi and you can use gas until 200 psi, so the usable portion of gas represents a 1800 psi drop. The cylinder is rated to hold 625 L of gas at 2000 psi, so the usable volume is 625 L × (2000 - 200)/2000 = 625 × 0.9 = 562.5 L.

Next, determine how fast that gas is consumed. The patient’s minute ventilation is VT × rate = 0.5 L × 10 breaths/min = 5 L/min. The ventilator itself also requires oxygen to operate, which is given as 2 L/min. Therefore, the total oxygen flow drawn from the cylinder is 5 + 2 = 7 L/min.

Divide the usable gas by the total consumption rate to get the duration: 562.5 L / 7 L/min ≈ 80 minutes. So ventilation could continue for about 80 minutes under these conditions.