11.3 Breathing Systems, Vaporizer Mechanics, and Scavenging Systems

11.3 Breathing Systems, Vaporizer Mechanics, and Scavenging Systems

Anaesthetic breathing systems deliver fresh gases and inhaled anaesthetics from the workstation to the patient's airway while removing exhaled carbon dioxide. Selecting the optimal breathing circuit and understanding carbon dioxide absorption chemistry, vaporizer thermodynamics, and scavenging systems are core competencies in anaesthetic physics.

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Classification of Breathing Systems: The Mapleson Hierarchy

In 1954, W.W. Mapleson categorized semi-closed breathing systems into types A through E based on the relative spatial arrangements of three primary components: the Fresh Gas Flow (FGF) inlet, the Adjustable Pressure-Limiting (APL) spill valve, and the Reservoir Bag. In 1975, Willis added the Jackson-Rees modification as Mapleson F.

The Efficiency Mnemonic: "All Dogs Can Bite"

To memorize the relative gas efficiency (lowest required FGF to prevent rebreathing of carbon dioxide):

• Spontaneous Ventilation: A > D > F = E > C > B ("All Dogs Feel Every Cold Breeze" or "A is best for Awake")
• Controlled Mechanical Ventilation: D > F = E > B > C > A ("Dogs Can Bite All" or "D is best for Dead / sedated")

Detailed Mapleson Circuit Mechanics

Mapleson A (Magill):  [FGF] ====== (Reservoir Bag) ====== [Corrugated Limb] ====== [APL Valve] --- (Patient)
Mapleson D (Bain):    [Machine End: Bag & APL] ====== [Outer Tube / Inner FGF Tube] ====== (Patient)
Mapleson E (Ayre's):  [Machine End: Open Tube] ====== [T-Piece with FGF] ====== (Patient)
Mapleson F (J-Rees):  [Open-tail Bag] ====== [Expiratory Limb with T-Piece FGF] ====== (Patient)

1. Mapleson A (Magill and Lack Systems)

• Configuration: Fresh gas enters at the machine end near the reservoir bag; the APL valve is positioned at the patient end of the corrugated tubing.
• Spontaneous Ventilation Mechanics: During exhalation, the first gas leaving the patient is anatomical dead space gas (which contains $100\%$ fresh gas, high $O_2$, zero $CO_2$). This dead space gas travels back up the corrugated tubing into the reservoir bag. As exhalation continues, alveolar gas ($CO_2$-rich) enters the tubing; simultaneously, pressure in the system rises, opening the APL valve near the patient and venting the late alveolar gas to the scavenging system. During the next inspiration, the patient inhales the unvitiated dead space gas stored in the tubing, followed by incoming fresh gas.
• Spontaneous Efficiency: Most efficient system for spontaneous breathing. Requires an FGF equal to only $0.7\text{ to }1.0 \times \text{minute volume (MV)}$ to prevent rebreathing.
• Controlled Ventilation Mechanics: Completely inefficient. When the anaesthetist squeezes the bag during positive-pressure inspiration, the APL valve opens under pressure, dumping fresh gas out into the scavenger before it reaches the patient. During expiration, alveolar gas enters the corrugated limb. Preventing rebreathing during controlled ventilation requires astronomical flows ($>2.5-3.0 \times MV$).
• The Lack System: A coaxial version of Mapleson A where the APL valve is relocated to the machine end via an internal expiratory tube, avoiding heavy valves near the patient's face.

2. Mapleson D and the Bain Coaxial Circuit

• Configuration: Fresh gas enters at the patient connector; the APL valve and reservoir bag are situated at the distal machine end.
• The Bain Circuit: A coaxial Mapleson D where fresh gas travels through a narrow internal tube ($7\text{ mm}$), while expired gas flows through the outer corrugated transparent tube ($22\text{ mm}$) to the bag and APL valve.
• Controlled Ventilation Mechanics: Most efficient system for controlled ventilation. During expiration, continuous high-flow fresh gas delivered directly at the patient connector flushes exhaled alveolar gas down the outer corrugated tube away from the patient toward the machine-end APL valve and bag. During mechanical inspiration, the patient receives incoming fresh gas mixed with gas from the outer limb. Required FGF to maintain normocarbia is only $1.5\text{ to }2.0 \times MV$ (or $70-100\text{ mL/kg/min}$).
• Spontaneous Ventilation Mechanics: Requires high fresh gas flows of $2.5\text{ to }3.0 \times MV$ because during exhalation, alveolar gas mixes with fresh gas in the outer tube and must be washed out by pure flow volume.
• Bain Circuit Hazard and the Pethick Test: If the inner fresh gas tube becomes disconnected or cracked at the machine end, fresh gas never reaches the patient port; instead, the entire length of the outer tube becomes massive mechanical dead space, producing catastrophic, lethal hypercapnia without any external leak. The Pethick Test: Occlude the patient end of the circuit with a thumb and depress the oxygen flush valve while observing the reservoir bag. The high-speed flow through the intact inner tube creates a Venturi effect that draws gas out of the outer tube, causing the reservoir bag to collapse flat. If the inner tube is disconnected or torn, the bag inflates or remains distended.

3. Mapleson E (Ayre's T-Piece) and Mapleson F (Jackson-Rees Modification)

• Configuration: Valveless circuits with an open expiratory limb (Mapleson E) or an open-ended reservoir bag with an adjustable tail-bleed valve (Mapleson F).
• Paediatric Advantages: Because there are no internal valves and no heavy corrugated hoses, these circuits exert extremely low resistance to breathing and minimal mechanical dead space, making them ideal for neonates and infants weighing $<20\text{ kg}$.
• Fresh Gas Requirements: Valveless design means ambient air can be entrained if flow is inadequate; requires high flows of $2.5\text{ to }3.0 \times MV$ to prevent rebreathing.

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The Circle System: Components and Carbon Dioxide Absorption Chemistry

The Circle System is the standard breathing system in modern anaesthetic workstations. It allows complete rebreathing of expired gases after chemical absorption of carbon dioxide, enabling low-flow anaesthesia ($FGF < 1\text{ L/min}$) and closed-circuit techniques.

Components of the Circle System

1. Fresh gas inlet (downstream of absorber, upstream of inspiratory valve)
2. Unidirectional inspiratory valve
3. Corrugated inspiratory breathing hose
4. Patient Y-piece (the only point of mechanical dead space in the circuit!)
5. Corrugated expiratory breathing hose
6. Unidirectional expiratory valve
7. Adjustable pressure-limiting (APL) valve / ventilator spill valve
8. Reservoir bag / mechanical ventilator bellows
9. Carbon dioxide absorption canister

Safety Feature: The unidirectional valves (lightweight mica, plastic, or ceramic discs) ensure that gas flows in one direction only. If an expiratory or inspiratory valve warps or sticks open with moisture, gas moves bidirectionally, converting the entire corrugated limb into mechanical dead space. This results in immediate rebreathing of $CO_2$, diagnosed on capnography by an elevated Phase I baseline ($>0\text{ mmHg}$).

Carbon Dioxide Absorption Chemistry: Soda Lime

Soda lime absorbs $CO_2$ through an exothermic acid-base neutralization cascade.

• Composition:
  • Calcium Hydroxide ($Ca(OH)_2$): $76-80\%$ (the primary absorbing substrate)
  • Water ($H_2O$): $13-19\%$ (critical; moisture dissolves gaseous $CO_2$ into carbonic acid)
  • Sodium Hydroxide ($NaOH$): $3-4\%$ (catalytic strong-base activator)
  • Potassium Hydroxide ($KOH$): $1\%$ (omitted in modern formulations due to toxicity)
  • Silica / Kieselguhr: $0.2\%$ (added for granule hardness to prevent caustic dust formation)

The Chemical Neutralization Cascade

1. Gaseous $CO_2$ dissolves in the thin aqueous film on the granules to form carbonic acid: $CO_2 + H_2O \rightleftharpoons H_2CO_3$
2. Carbonic acid reacts rapidly with the strong-base catalyst ($NaOH$) in an exothermic neutralization: $H_2CO_3 + 2NaOH \rightarrow Na_2CO_3 + 2H_2O + \text{heat}$
3. Sodium carbonate reacts with calcium hydroxide to precipitate insoluble calcium carbonate, regenerating the catalyst: $Na_2CO_3 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + 2NaOH$

• Heat and Moisture Generation: Neutralization produces $1\text{ mole of } H_2O$ and $\approx 13{,}700\text{ calories}$ ($\approx 57\text{ kJ}$) of heat per mole of $CO_2$ absorbed. A warm canister ($45-55°\text{C}$) confirms active, functional absorption.
• Exhaustion Indicator: Ethyl violet dye is incorporated into the granules. Under alkaline conditions (pH > 10.3), ethyl violet is completely colorless. As absorption exhausts the hydroxyl ions and accumulation of carbonic acid lowers $\text{pH}$ below $10.3$, ethyl violet turns deep purple/violet.
• Indicator Regeneration Trap: Exhausted soda lime left overnight can temporarily revert from purple to white as unreacted base from the center of granules diffuses slowly to the surface. However, upon re-exposure to $CO_2$, it turns purple again within minutes.

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Toxic Degradation Hazards: Compound A and Carbon Monoxide

When volatile halogenated anaesthetics interact with strong alkaline bases ($KOH$ and $NaOH$) in carbon dioxide absorbents, dangerous degradation reactions can occur.

1. Compound A Formation (Sevoflurane)

Sevoflurane undergoes base-catalyzed dehydrofluorination when exposed to strong-base absorbents ($KOH > NaOH$), forming Compound A (fluoromethyl-2,2-difluoro-1-[trifluoromethyl]vinyl ether).

• Toxicity: Nephrotoxic in Wistar rats, causing proximal renal tubular necrosis (toxic threshold $\approx 150-300\text{ ppm}\cdot\text{hours}$). Although human clinical nephrotoxicity remains controversial, clinical guidelines mandate caution.
• Conditions Favoring Compound A Production:
  • Very low fresh gas flows ($<1-2\text{ L/min}$, allowing accumulation in the circle)
  • High concentrations of sevoflurane
  • Warm absorbent temperatures
  • Dry, desiccated absorbent
  • Absorbents containing high concentrations of $KOH$ or $NaOH$
• Clinical Prevention: The US product label states that fresh gas flows below $1\text{ L/min}$ are not recommended and that flows of $1$ to $<2\text{ L/min}$ should not exceed $2\text{ MAC-hours}$; many European labels permit lower flows, so follow local product information. Absorbents without strong bases minimise production.

2. Carbon Monoxide ($CO$) Production

Volatile anaesthetics containing a difluoromethoxy group ($-\text{OCHF}_2$) react with completely desiccated strong-base absorbents ($KOH \gg NaOH$) to produce lethal concentrations of Carbon Monoxide ($CO$).

• Magnitude of Production: Desflurane $\gg$ Enflurane $>$ Isoflurane $\gg$ Halothane $\approx$ Sevoflurane. (Desflurane produces the greatest quantities of $CO$ by a massive margin).
• Clinical Impact: Inhaled $CO$ binds avidly to hemoglobin (affinity $>200$ times higher than $O_2$), generating dangerous carboxyhemoglobinemia ($COHb > 30\%$), shifting the oxyhemoglobin dissociation curve to the left and causing severe tissue hypoxia.
• The "Monday Morning Phenomenon": If high fresh gas flow is left running through an anaesthetic machine over a weekend, dry gas desiccates the soda lime. The first patient on Monday morning receiving desflurane suffers acute $CO$ poisoning.
• Modern Prevention: Transition to calcium hydroxide-only absorbents (such as Amsorb Plus, which contains $Ca(OH)_2$ and $CaCl_2$, with zero $KOH$ and zero $NaOH$). These absorbents do not produce Compound A or Carbon Monoxide, even when completely dry.

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Vaporizer Engineering: Variable-Bypass vs. Heated-Pressurized (Tec 6)

Vaporizers transform liquid volatile anaesthetics into calibrated, predictable vapor concentrations.

Physical Properties of Volatile Anaesthetics at 20°C

Variable-Bypass (Plenum) Vaporizers (Tec 4, Tec 5, Tec 7, Dräger Vapor 2000)

• Mechanism: Fresh gas entering the vaporizer divides into two streams:
  1. Bypass Stream: The majority ($>80\%$) passes straight through the bypass channel without contacting liquid agent.
  2. Vaporizing Chamber Stream: A smaller fraction enters the chamber, passing over wicks and baffles that ensure $100\%$ saturated vapour equilibrium ($SVP$). The two streams recombine at the outlet. The concentration dial adjusts a precision rotary valve that alters the splitting ratio.
• Temperature Compensation: Evaporation consumes latent heat of vaporization, causing the liquid to cool and lowering its $SVP$. To prevent output drop, a bimetallic strip (two metals with differing thermal expansion coefficients bonded together) or a thermal expansion cone bends as temperature drops. This automatically widens the vaporizing chamber orifice, increasing the fraction of gas diverted through the chamber to maintain a constant output concentration across operating temperatures ($15-35°\text{C}$).
• Flow and Pressure Compensation: Wicks maintain saturation at flows from $0.25$ to $15\text{ L/min}$. Internal baffles prevent liquid splashing into the bypass stream during transport. Keyed filler systems prevent filling with the wrong agent.

Heated-Pressurized Vaporizer (Tec 6 for Desflurane)

• The Desflurane Challenge: Desflurane has a boiling point of $22.8°\text{C}$ (virtually ambient room temperature) and an extremely high $SVP$ of $88.5\text{ kPa}$ ($664\text{ mmHg}$) at $20°\text{C}$. In a variable-bypass vaporizer on a warm day, desflurane would boil uncontrollably, resulting in massive, lethal surges in delivered concentration.
• Tec 6 Design:
  • The vaporizer is an electrically heated, pressurized, measured-flow injector.
  • It is thermostatically heated to $39°\text{C}$, well above desflurane's boiling point, generating a stable saturated vapour pressure of approximately $200\text{ kPa}$ ($2\text{ bar}$ / $1500\text{ mmHg}$).
  • Pure desflurane vapour is metered directly into the fresh gas stream through a variable resistance valve regulated by the concentration dial and an electronic differential pressure sensor matching fresh gas flow rate.
  • Electrical Requirement: Requires continuous mains electricity; incorporates battery backup for alarms.

The Altitude Effect on Vaporizers

Critical Distinction: Clinical depth of anaesthesia is governed by the partial pressure of volatile agent in the brain, NOT by volume percentage ($P_{\text{agent}} = \text{Vol}\% \times P_{\text{barometric}}$).

1. Variable-Bypass Vaporizers at Altitude: At high altitude (low barometric pressure), liquid volatile agent evaporates more readily, increasing the delivered volume percentage. However, because ambient barometric pressure is lower, the delivered partial pressure remains virtually unchanged. Thus, variable-bypass vaporizers require no dial adjustment at altitude.
2. Tec 6 Vaporizer at Altitude: The Tec 6 meters pure vapour to deliver a constant volume percentage ($Vol\%$), regardless of barometric pressure! Because barometric pressure ($P_{\text{barometric}}$) drops at altitude, the delivered partial pressure decreases significantly, risking intraoperative awareness.
   • Dial Correction Formula for Tec 6 at Altitude: $\text{Required Dial Setting} = \frac{\text{Desired Sea-Level Dial Setting} \times 760\text{ mmHg}}{P_{\text{barometric}} (\text{mmHg})}$ Example: At an altitude of $2500\text{ m}$ ($P_{\text{barometric}} \approx 570\text{ mmHg}$), to deliver a clinical depth equivalent to $6\%$ desflurane at sea level, the dial must be set to $\frac{6 \times 760}{570} = 8.0\%$.

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Anaesthetic Gas Scavenging Systems (AGSS)

Anaesthetic Gas Scavenging Systems (AGSS) capture and remove Waste Anaesthetic Gases (WAGs) discharged from breathing systems (APL valves and ventilator relief valves) to protect healthcare personnel from chronic exposure risks.

• Occupational Standards: NIOSH limits atmospheric exposure to $<25\text{ ppm}$ for nitrous oxide and $<2\text{ ppm}$ for halogenated agents ($<0.5\text{ ppm}$ when used with $N_2O$).

The Four Components of an AGSS

1. Collecting System: Shrouds enclosing the APL valve and ventilator exhaust port. Mandates a $30\text{ mm}$ conical connector (different from the $22\text{ mm}$ breathing circuit and $15\text{ mm}$ airway connectors) to physically prevent accidental connection of scavenging lines to the patient breathing circuit.
2. Transfer System: Tubing that routes collected gas to the receiving block.
3. Receiving System (Reservoir Block): Accommodates flow surges from exhalation.
   • Open Systems: Contain ports open to atmospheric air. Ambient air is entrained if suction exceeds gas production; gas spills out if suction fails. Requires no relief valves and cannot transmit positive or negative pressures to the patient.
   • Closed Systems: Require spring-loaded safety relief valves: a positive-pressure relief valve ($+5\text{ cmH}_2\text{O}$) to prevent circuit barotrauma if suction fails, and a negative-pressure relief valve ($-0.5\text{ cmH}_2\text{O}$) to prevent subatmospheric suction from evacuating gas from the patient's lungs.
4. Disposal System:
   • Active Disposal: Dedicated vacuum pump generating extraction flows of $75-130\text{ L/min}$ to an external roof vent.
   • Passive Disposal: Gas flows down a wide-bore hose venting through an external hospital wall via natural pressure gradients.