Breathing Circuits — Circle, Mapleson, Bain

🫀Allows rebreathing exhaled gas after CO₂ removal — efficient agent use, heat + humidity conservation.

Most common adult OR circuit.

Compact integrated versions exist (Datex-Ohmeda ABS) with smaller compliance volume.

Failure modes: stuck open (allows backflow, increases dead space, raises ETCO₂), stuck closed (occludes flow, severely impaired ventilation).

Diagnose with capnography — incompetent expiratory valve shows elevated inspired CO₂ baseline; incompetent inspiratory valve shows phase III slope abnormalities.

Verify pre-use by squeezing bag + watching flow direction.

Mapleson A (Magill): most efficient for spontaneous ventilation, FGF ≈ minute ventilation.

Mapleson D: most efficient for controlled ventilation, FGF ≈ alveolar minute ventilation.

Mapleson E + F (Jackson-Rees): T-piece pediatric standard pre-circle era.

No CO₂ absorbent in any Mapleson — efficiency comes from positioning of fresh gas + reservoir relative to patient.

Co-axial geometry warms incoming fresh gas via counter-current heat exchange + reduces dead space at Y-piece.

Lightweight, ideal for MRI + transport.

FGF requirements: ~70 mL/kg/min controlled ventilation, 150-200 mL/kg/min spontaneous.

Disadvantage: kinking or disconnection of inner tube goes undetected — produces massive dead space.

👶Low-resistance circuit critical in small children — minimizing work of breathing under sevoflurane induction.

Modern pediatric circle systems with low-volume corrugated tubing now perform comparably; T-piece largely replaced.

Still used in NICU transport + some smaller pediatric centers.

FGF 2-3× minute ventilation required.

MAPLESON A spontaneous: ~70 mL/kg/min (most efficient for spontaneous breathing).

MAPLESON D controlled: ~70 mL/kg/min.

MAPLESON D spontaneous: 2-3× minute ventilation.

Always verify capnography — inspired CO₂ >0 indicates rebreathing (exhausted absorbent in circle, inadequate FGF in Mapleson).

Higher FGF wastes agent + produces more environmental release; balance economy vs ventilation needs.

KINK (in tubing or under patient/drape) — increased peak pressure + decreased exhaled volume + abnormal capnogram phase IV.

STUCK VALVE (failed unidirectional valve) — abnormal capnogram (rising baseline if expiratory valve incompetent; phase II/III abnormalities if inspiratory valve incompetent).

LEAK (cuff under-inflated, circuit connection loose) — slow phase IV, audible escape, low exhaled volume.

Lighter + ergonomic for transport + MRI + intubation positioning.

Pre-use check: occlude the inner tube briefly with O2 flush; bag should NOT collapse (if it does, inner tube leaking).

PARALLEL (separate inspiratory + expiratory limbs): more visible + easier to detect kinks; standard in OR circles.

Pediatric vs adult: pediatric circuits use smaller-diameter tubing (less compressible volume — important for delivering accurate small tidal volumes); adult tubing too compliant for small TVs.