CLASSIFICATION OF BREATHING SYSTEMS
An anesthesia breathing circuit is a system of tubing, reservoir bag, and valves used to deliver a precise mixture of oxygen and anesthetic gases from the anesthesia machine to the patient and removal of carbon dioxide. Breathing systems may be best classified in a number of different ways:
Open—Open systems have no valves, tubing, or reservoir bag: for example, insufflation or open-drop ether. In either, the patient has access to atmospheric gases.
Semi-open—A semi-open system has a reservoir such as a breathing bag and there is no rebreathing. For example, a Mapleson circuit or a circle at high fresh gas flow (FGF) (> minute ventilation [VE]).
Semi-closed—A semi-closed system has a reservoir such as a breathing bag and allows for partial rebreathing. For example, a Mapleson circuit or a circle at low FGF (< VE), the most commonly used method today.
Closed—A closed system has a reservoir such as a breathing bag and allows for complete rebreathing, and CO2 is absorbed. For example, a circle with pop-off (adjustable pressure-limiting [APL] valve) valve closed and a very low FGF that equals oxygen uptake by the patient.
Two factors must be considered in the breathing systems:
Dead space—In the circle systems, the tubing (mechanical) dead space ends at the point where inspired and expired gas streams meet at the Y-connector, resulting in loss of tidal volume (VT) from the compliance of the distensible corrugated inspiratory and expiratory tubing and from gas compression. The elbow, the heat and moisture exchanger (HME), and the D-lite sensor contribute to real apparatus dead space where part of VT does not participate in gas exchange. Increasing the dead space increases rebreathing of carbon dioxide. Hence, to avoid hypercarbia in the face of an acute increase in dead space, a patient must increase minute ventilation.
Resistance to breathing—Resistance is always high with turbulent flow, hence narrow diameter tubing and orifices, sharp bends, increasing circuit length, and eliminating unnecessary valves that produce this should be avoided in the apparatus. Circle system resistance is increased by unidirectional valves, the absorber, and high respiratory rates and tidal volumes.
Insufflation is an open system and depending on the respiratory pattern, depth of anesthesia is unpredictable and air entrainment in varying degrees occurs. Ventilation cannot be assisted and fire and toxicity risks exist. Oxygen and/or gases are insufflated over the face during a child’s induction or via a catheter/tube placed in the airway, laryngoscope, or trachea during endoscopic procedures or to prevent rebreathing of CO2 during ophthalmic surgery. One may use spontaneous ventilation (SV) or controlled ventilation (CV) with brief periods of apnea.
No longer used, this open system ...