Breathing Systems Basics - Circuit Central
- Core Components:
- Fresh Gas Source (FGS): Gases/O2 input.
- Delivery Tubes: Inspiratory/expiratory.
- Reservoir Bag: Tidal volume, manual/spontaneous ventilation.
- APL Valve: Pressure limit, excess gas vent.
- Patient Connection: Mask/ETT/LMA.
and classification overview)
- Classification:
| System Type | Rebreathing | CO2 Absorption | Example(s) |
|---|---|---|---|
| Open | No | No | Open drop |
| Semi-Open | Minimal/No | No | Mapleson A,D |
| Semi-Closed | Partial | Yes | Circle (low) |
| Closed | Total | Yes | Circle (min) |
Mapleson Circuits - Alphabet Soup Fun
| Circuit | Components | FGF (SV) | FGF (CV) | Efficiency (SV) | Efficiency (CV) | Best Use | Notes |
|---|---|---|---|---|---|---|---|
| Mapleson A (Magill) | FGF, CT, RB, APL (pt end) | $FGF = MV$ | High, 2-3x MV | Most efficient | Least efficient | Adults, SV | APL near patient. |
| Mapleson D | FGF (pt), CT, RB, APL (mach end) | 2-3x MV | 1-2x MV ($70 \text{ ml/kg/min}$) | Good | Most efficient | All ages, SV & CV | Bain: coaxial D. Test inner tube. |
| Mapleson E (Ayre's T) | FGF (pt), CT (exp limb) | 2-3x MV | 2-3x MV | Good | Good | Peds < 25 kg, SV | No RB/APL. Low dead space/resistance. |
| Mapleson F (Jackson-Rees) | FGF (pt), CT, open RB | 2-3x MV | 2-3x MV | Good | Good | Peds, SV & CV | Mod of E. Allows CPAP/PEEP. |
- 📌 Efficiency (CV): 'Dead Babies Can't Assist' (D > B > C > A)
⭐ The Bain circuit, a coaxial Mapleson D, requires integrity testing of the inner fresh gas delivery tube to prevent rebreathing of exhaled gases if it becomes kinked or disconnected.
Circle System - Round & Round We Go
Most common system; allows rebreathing of gases after CO₂ removal.
Components:
- CO₂ absorber (Soda lime)
- Unidirectional inspiratory & expiratory valves
- APL (Adjustable Pressure Limiting) valve
- Reservoir bag
- Y-piece
- Fresh Gas Flow (FGF) inlet
- Corrugated breathing tubes
CO₂ Absorption (Soda Lime):
- Reactions: $CO_2 + 2NaOH \rightarrow Na_2CO_3 + H_2O + Heat$; then $Na_2CO_3 + Ca(OH)_2 \rightarrow CaCO_3 \text{ (s)} + 2NaOH$.
- Granule size: 4-8 mesh.
- Indicator: Ethyl violet (Pink → Violet). 📌 "Violet is Violating" (CO₂ rebreathing).
Advantages:
- Low FGF: Conserves gases & agents.
- Conserves heat & humidity.
- ↓ Operating room pollution.
- Suitable for all patient ages.
Disadvantages:
- Complex; many parts, ↑ failure risk.
- ↑ Resistance to breathing.
- Rebreathing risk (valve/absorber failure).
- Compound A formation (Sevoflurane + dry absorbent).
Simplified Pre-Use Check:
⭐ Channeling in the CO₂ absorber canister can lead to premature exhaustion and rebreathing of CO₂ despite color change indicating otherwise in parts of the absorbent.
Safety & Monitoring - Vigilant Vents
- Hazards: Leaks, disconnections, obstructions, barotrauma, volutrauma, hypoxia, hypercarbia.
- Key Monitoring:
- Airway pressures (Peak, Plateau, PEEP).
- Capnography (ETCO2: normal 35-45 mmHg).
- Inspired O2 (FiO2).
- Spirometry (VT, MV).
- Alarms: Low/High pressure, Apnea.
- Scavenging: Active/passive systems for Waste Anesthetic Gas Disposal (WAGD).
⭐ A sudden drop in ETCO2 to near zero can indicate esophageal intubation, circuit disconnection, or cardiac arrest.
High‑Yield Points - ⚡ Biggest Takeaways
- Mapleson A (Magill) is best for spontaneous ventilation; Mapleson D for controlled ventilation.
- The Bain circuit is a coaxial Mapleson D; check inner tube integrity.
- Circle systems enable low FGF, conserve heat/humidity, and need CO2 absorption.
- Key circle components: unidirectional valves, APL valve, CO2 absorber.
- Minimize apparatus dead space for efficient ventilation and to prevent rebreathing.
- Soda lime exhaustion: color change (ethyl violet), ↑ETCO2, ↑HR, ↑BP.
- Adequate Fresh Gas Flow (FGF) prevents rebreathing in Mapleson systems.
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