Oxygen Supply and Therapy: Sources, Devices, and Delivery Math
Sources and hardware
Oxygen reaches the patient from compressed cylinders, liquid systems (1 L liquid ≈ 860 L gas), concentrators, or a wall pipeline at roughly 50 psig. A pressure-reducing regulator lowers cylinder pressure, and a Thorpe-tube flowmeter meters flow; read the ball at its center. Humidification is needed at cannula flows above 4–6 L/min, with thick secretions, or when the upper airway is bypassed.
Devices and realistic FiO2
Nasal cannula: 1–6 L/min, roughly 24–44% (about 4% per liter). Simple mask: 5–10 L/min, 35–55%. Venturi mask: air entrainment gives a precise 24–50%, provided the flow is set at or above the labeled value. Nonrebreather: 10–15 L/min, 80–100%, with the reservoir bag kept inflated. High-flow nasal cannula delivers heated, humidified flow up to 60 L/min at a set FiO2 and meets inspiratory demand better than low-flow devices.
Traps
Low-flow device percentages are estimates, not prescriptions; a tachypneic patient entrains room air and receives less than the chart says. Never withhold oxygen from a COPD patient over the "hypoxic drive" - titrate to SpO2 88–92% if the patient is a CO2 retainer. Watch for absorption atelectasis and toxicity (FiO2 >0.5 for 24–48 hours). No petroleum products, no smoking.
Worked example
Cylinder duration (minutes) = (gauge pressure × cylinder factor) / flow. An E cylinder (factor 0.28) at 1500 psig delivering 6 L/min: 1500 × 0.28 = 420; 420 / 6 = 70 minutes. Never plan to drain the tank to zero.