Breathing Circuits and Systems

Quick Answer

Anaesthetic breathing circuits deliver fresh gas to patient and remove expired CO₂, classified by rebreathing characteristics and presence of CO₂ absorption. Mapleson classification (non-rebreathing): A (Magill): optimal for spontaneous ventilation (FGF 70-100 mL/kg/min); B, C: little clinical use; D (Bain): coaxial circuit, optimal for controlled ventilation (FGF 200-300 mL/kg/min), requires high flows to prevent rebreathing; E (Ayre's T-piece): pediatric use, minimal dead space; F (Jackson-Rees modification): adds reservoir bag for assisted ventilation. Circle system (rebreathing with CO₂ absorption): Unidirectional valves, CO₂ absorber (soda lime: Ca(OH)₂ 75%, NaOH 20%, KOH 5%, water 15-20%; or Amsorb/Ca(OH)₂ without NaOH), reservoir bag, APL valve, fresh gas inlet; advantages include low FGF (0.5-1 L/min), conservation of heat and humidity, reduced OR pollution. CO₂ absorption: 1 kg soda lime absorbs 100-140 L CO₂; exhausted when color change (ethyl violet: white → purple; phenolphthalein: pink → white) or capnography shows inspired CO₂ >5 mmHg. Fresh gas flow requirements: Non-rebreathing: 2-3× minute ventilation; Circle system: 0.5-1 L/min (closed circuit) to 3-6 L/min (semi-closed). Resistance: Minimal with modern systems (<5 cm H₂O at 30 L/min flow). Dead space: Apparatus dead space (y-piece, connectors) + anatomical dead space; should be minimized especially in pediatric patients. Work of breathing: Higher in Mapleson systems due to resistance; compensated by reservoir bag compliance. Humidity: Circle systems conserve humidity (saturated at 37°C, 43 mg/L); Mapleson systems require active humidification if prolonged use. ANZCA standards: PS41 mandates continuous monitoring of airway pressure, expired CO₂, and oxygen concentration in all breathing systems. [1-10]