Nonrebreathing

Nonrebreathing refers to a class of breathing circuits and masks designed to minimize rebreathed gases during respiratory support. The core idea is to deliver fresh gas to the patient while directing exhaled gas away, so that inhaled gas contains little or no gas previously exhaled by the patient. This arrangement allows higher and more predictable oxygen delivery and reduces the buildup of carbon dioxide from prior breaths.

Nonrebreathing masks consist of a tight-sealing face mask, a reservoir bag, and one-way valves that prevent expired gas from entering the bag or escaping back into the inspiratory limb. Oxygen flows into the reservoir; during inhalation the patient draws gas from the reservoir. Exhaled gas exits through expiratory ports; the valves block backflow. Adequate flow rates are required to keep inhaled gas rich in oxygen; insufficient flow allows room air to mix in and lowers delivered oxygen concentration.

In anesthesia, nonrebreathing circuits use unidirectional inspiratory and expiratory valves and a high fresh gas flow to minimize rebreathed gas. Exhaled gas is vented to the scavenging system. This configuration is useful for short procedures or when rapid adjustment of gas composition is needed and avoids the need for carbon dioxide absorbers that are used in some rebreathing circuits.

Nonrebreathing is contrasted with rebreathing systems that conserve part of exhaled gas and require carbon dioxide removal. The choice depends on clinical goals, equipment, and the balance between oxygen efficiency, carbon dioxide management, and gas usage.