Anaesthesia Breathing Systems
                     Anaesthesia 
Breathing Systems
Introduction
The function of breathing systems is to 
deliver oxygen and anaesthetic gases to 
patients and eliminate carbon dioxide. All 
breathing systems are composed of similar 
components but are configured differently.
Source: resources.wfsahq.org
Common Components
The common components include: fresh gas 
flow, tubing to direct gas flow, an adjustable 
pressure limiting valve to control pressure 
within the system & allow scavenging of 
waste gas and a reservoir bag to store gas and 
assist with ventilation.
Source: resources.wfsahq.org
Breathing System
Each breathing system receives three sources 
of gas: fresh gas, exhaled dead space gas and 
exhaled gas from the alveoli. The proportions 
of each within the system are most greatly 
influenced by fresh gas delivery. 
Source: resources.wfsahq.org
COMPONENTS OF BREATHING SYSTEMS
Adjustable Pressure Limiting (APL)
The Adjustable Pressure Limiting (APL) valve 
allows a variable pressure within the 
anaesthetic system using a one-way, spring-
loaded valve. At a pressure above the 
opening pressure of the valve, a controlled 
leak of gas is allowed from the system, which 
enables control of the patient’s airway 
pressure. 
Source: resources.wfsahq.org
Reservoir Bag
The reservoir bag allows collection of fresh 
gas flow during expiration, which in turn 
minimizes the amount of fresh gas required 
to prevent rebreathing. In addition, it allows 
the anaesthetist to monitor the breathing 
pattern of a spontaneously breathing patient. 
Source: resources.wfsahq.org
Circle System
1. closed (fresh gas inflow exactly equal to 
patient uptake, complete rebreathing 
after carbon dioxide absorbed, and APL 
closed)
2. semi-closed (some rebreathing occurs, 
FGF and APL settings at intermediate 
values), or
3. semi-open (no rebreathing, high fresh gas 
flow [higher than minute ventilation])
Source: resources.wfsahq.org
Non-rebreathing (Mapleson) breathing 
circuits
• All non-rebreathing (NRB) circuits lack 
unidirectional valves and soda lime 
carbon dioxide absorption.
• The amount of rebreathing is highly 
dependent on dilution of expirations with 
high fresh gas flow (FGF) in all. Work of 
breathing is low in all (no unidirectional 
valves or soda lime granules to create 
resistance).
Source: resources.wfsahq.org
The Pethick Test for the Bain Circuit
• A unique hazard of the use of the Bain 
circuit is occult disconnection or kinking 
of the inner, fresh gas delivery hose. 
• If this occurs, the entire corrugated limb 
becomes dead space. This results in 
respiratory acidosis which is unresponsive 
to increased minute ventilation.
Source: resources.wfsahq.org
Circle Breathing Circuit
• The circle is the most popular breathing 
system in the US. 
• It cleanses carbon dioxide from the 
patient’s exhalations chemically, which 
allows rebreathing of all other exhaled 
gases (a unique breathing arrangement in 
health care, but rebreathing is used 
extensively in other environments e.g. 
space, submarine).
Source: resources.wfsahq.org