2.4 Non-Rebreathing Systems & Waste Gas Scavenging

Classification and Mechanics of Mapleson Non-Rebreathing Circuits

While the circle rebreathing system dominates adult surgical suites, non-rebreathing (semi-closed) breathing circuits remain vital in pediatric anesthesia, patient transport, magnetic resonance imaging (MRI) units, and emergency resuscitation. In 1954, British physicist W.W. Mapleson systematically classified these circuits into six configurations, designated Mapleson A through F.

Core Engineering Principles

Unlike circle systems, Mapleson circuits:

• Contain no chemical carbon dioxide absorbent canister.
• Contain no unidirectional check valves.
• Rely entirely on high fresh gas flow (FGF) to sweep expired alveolar gas out through an adjustable pressure limiting (APL) pop-off valve before the patient takes the next inspiration.
• Offer minimal airway resistance and low mechanical apparatus dead space, making them well-suited for neonates and infants whose small respiratory musculature cannot overcome the resistance of heavy circle system valves and absorbent canisters.
• Suffer from high running costs and operating room pollution because they require massive fresh gas flows (frequently 2 to 3 times the patient's minute ventilation, V̇E) to prevent hypercapnic rebreathing.

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Comparative Efficiency: Spontaneous vs. Controlled Ventilation

The relative arrangement of the three mobile components—the fresh gas inlet (FGI), the APL valve, and the reservoir bag—determines how efficiently each Mapleson system clears carbon dioxide.

A standard memory aid summarizes the ranking: spontaneous ventilation A > DFE > CB and controlled ventilation DFE > BC > A.

The Physiological Mechanism of Mapleson A Efficiency

In the Mapleson A, fresh gas enters at the reservoir bag end, while the APL valve sits directly adjacent to the patient's mask:

1. During spontaneous expiration, anatomical dead space gas (which filled the conducting airways and contains zero CO₂) travels back into the corrugated limb and reservoir bag first.
2. As the patient continues to exhale alveolar gas (rich in CO₂), the reservoir bag fills completely. Pressure rises, opening the APL valve near the patient, venting the alveolar gas out of the circuit.
3. During the subsequent inspiration, the patient inhales the dead space gas stored in the tubing, followed immediately by incoming fresh gas. Because alveolar gas was vented first, rebreathing is eliminated at a fresh gas flow equal to minute ventilation (FGF = V̇E).
4. During controlled ventilation, squeezing the reservoir bag forces fresh gas out through the open APL valve while driving alveolar gas into the patient's lungs. Thus, Mapleson A is terribly inefficient during controlled ventilation.

The Inverse Efficiency of Mapleson D

In the Mapleson D, fresh gas enters right at the patient connection, while the APL valve and reservoir bag are at the machine end. During controlled ventilation, high-pressure bag compression forces mixed gas out through the distal APL valve while continuous fresh gas flow at the patient port guarantees that only pure, fresh gas enters the lungs on inspiration. Hence, Mapleson D is the most efficient non-rebreathing circuit for mechanical ventilation.

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Coaxial Mapleson D (Bain Circuit) & The Pethick Safety Test

In 1972, J.A. Bain modified the Mapleson D into a compact, coaxial ("tube-within-a-tube") assembly.

THE COAXIAL BAIN CIRCUIT (MAPLESON D):

[ Machine End ]                                                      [ Patient End ]
-----------------+                                                  +-----------------
Outer Corrugated |   Expiratory Gas Flow (Outer 22mm Tube)          | Corrugated Limb
Limb to APL &    | <=============================================== | to Mask / ETT
Reservoir Bag    |                                                  |
                 |      [ Inner 7mm Fresh Gas Tube ]                |
                 +====> Fresh Gas Flow ===========================> +
-----------------+                                                  +-----------------

Mechanical Architecture

• Inner Tube: A narrow, flexible plastic tube (7 mm diameter) carrying cool, dry fresh gas from the anesthesia machine flowmeters directly to the patient's airway.
• Outer Tube: A standard corrugated breathing hose (22 mm diameter) surrounding the inner tube, carrying warm, humid exhaled gas away from the patient toward the APL valve and reservoir bag.
• Clinical Advantages: Lightweight, eliminates surgical field clutter in head and neck procedures, and provides countercurrent thermal exchange, where warm exhaled gas in the outer tube transfers heat and humidity to the cool incoming fresh gas in the inner tube.

The Fatal Hazard: Inner Tube Disconnection

If the inner fresh gas tube kinks, disconnects from its machine mount, or ruptures internally, fresh gas never reaches the patient port. Instead, fresh gas discharges into the machine end of the outer tube. The outer corrugated tube (often about 1.8 m long) becomes a large bidirectional apparatus dead space. The patient rebreathes their own alveolar gas on every cycle, developing lethal hypercapnia, severe respiratory acidosis, and progressive hypoxemia that cannot be detected by standard pressure monitors.

The Pethick Safety Test Protocol

To confirm inner tube integrity prior to patient connection, the anesthesia technologist must perform the Pethick Test:

+-------------------------------------------------------------------------+
|                        THE PETHICK SAFETY TEST                          |
|                                                                         |
|   STEP 1: Occlude the patient connection port (thumb or mask plug).     |
|   STEP 2: Close the APL valve completely.                               |
|   STEP 3: Press O2 Flush to fill the reservoir bag until distended.     |
|   STEP 4: Unplug the patient port while continuing high-flow O2 flush.  |
|                                                                         |
|   ---> PASS: Reservoir bag FLATTENS / COLLAPSES (Venturi Effect)        |
|   ---> FAIL: Reservoir bag REMAINS DISTENDED (Inner tube disconnected!) |
+-------------------------------------------------------------------------+

• Fluid Dynamics Rationale: When high-velocity oxygen (35–75 L/min) flushes through an intact inner tube and discharges through the open patient port, it acts as an ejector / aspirator. By the Bernoulli principle and Venturi effect, the high-speed jet creates a localized zone of sub-atmospheric (negative) pressure at the patient end of the outer tube. This negative pressure suctions gas out of the outer corrugated tube, causing the reservoir bag to instantly collapse.
• Failure Interpretation: If the inner tube is detached, broken, or retracted, the high-pressure oxygen jet discharges directly into the outer limb, pressurizing the outer tube and causing the reservoir bag to remain distended or expand. If the bag fails to collapse, the circuit must be immediately removed from service.

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Pediatric Non-Rebreathing: Jackson-Rees (Mapleson F) Mechanics

The Jackson-Rees modification of Ayre's T-piece (Mapleson F) is the premier circuit for pediatric anesthesia induction, emergence, and transport:

• Architecture: A lightweight T-piece connected to the endotracheal tube or mask. Fresh gas enters at the patient port. The expiratory limb terminates in an open-ended pediatric reservoir bag equipped with an adjustable tail-bleed valve or side-hole pop-off mechanism.
• Clinical Function: Allows the practitioner to switch instantaneously between spontaneous breathing (feeling the infant's rapid, shallow tidal excursions through the delicate latex bag) and manual assisted/controlled ventilation (occluding the tail mechanism with a thumb while compressing the bag).
• Flow Requirements: Requires a fresh gas flow of about 2 to 3 times the child's minute ventilation to prevent CO₂ rebreathing.

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Waste Anesthetic Gas Scavenging Systems (WAGS) & NIOSH Thresholds

Studies of personnel chronically exposed to trace anesthetic gases have raised concerns about effects such as spontaneous abortion, although the evidence is mixed. In 1977, the National Institute for Occupational Safety and Health (NIOSH) published recommended exposure limits (RELs).

NIOSH Occupational Exposure Thresholds

+-------------------------------------------------------------------------+
|                   NIOSH OCCUPATIONAL EXPOSURE LIMITS                    |
|                                                                         |
|   - Halogenated Volatile Agents ALONE:                                  |
|     2 ppm ceiling (sampling period of no more than 1 hour)              |
|                                                                         |
|   - Halogenated Agents COMBINED WITH Nitrous Oxide:                     |
|     0.5 ppm ceiling for the halogenated agent                           |
|                                                                         |
|   - Nitrous Oxide (N2O):                                                |
|     25 ppm time-weighted average during anesthetic administration       |
+-------------------------------------------------------------------------+

The Five Essential Components of WAGS

Every complete scavenging circuit comprises five sequential elements:

1. Gas-Collecting Assembly: Collects waste gas from the breathing circuit APL valve (during manual/spontaneous breathing) and the anesthesia ventilator spill/exhaust valve (during mechanical ventilation).
2. Transfer Tubing: Carries gas from collecting ports to the scavenging interface. Uses rigid or reinforced 19-mm or 30-mm hose to avoid kinking.
3. Scavenging Interface: The safety core of the system that balances gas volume variations and protects the patient from barotrauma or excessive vacuum.
4. Gas Disposal Tubing: Carries gas from the interface to the disposal outlet.
5. Disposal System: Removes waste gas from the hospital building entirely.

Fitting Geometry and Non-Interchangeability

To prevent the lethal catastrophe of connecting the high-vacuum scavenging line into a patient's endotracheal tube or breathing circuit:

• Standard breathing circuit fittings utilize 15-mm internal / 22-mm external conical tapers.
• Scavenging transfer ports and fittings utilize dedicated 19-mm or 30-mm fittings (ISO standards).
• Under no circumstances can a 19-mm or 30-mm scavenging tube mechanically mate with a 15-mm or 22-mm breathing circuit port.

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Scavenging Interfaces: Open (Valveless) vs. Closed (Valved) Architectures

The scavenging interface prevents pressure fluctuations in the disposal system from transmitting backward into the patient breathing circuit.

OPEN (VALVELESS) INTERFACE               CLOSED (VALVED) INTERFACE

       [ Open Atmospheric Ports ]                [ Sealed Canister Housing ]
                 |                                         |
                 v                                         v
+---------------------------------+      +---------------------------------+
|  Canister Open to Room Air      |      |  Positive-Pressure Relief (+5)  |
|  Waste Gas Inlet                |      |  Negative-Pressure Relief (-0.5)|
|  Suction Flow Indicator Bobbin  |      |  Dedicated Scavenging Bag (3L)  |
|  Active Suction Disposal Line   |      |  Active OR Passive Disposal     |
+---------------------------------+      +---------------------------------+
(ACTIVE VACUUM ONLY!)                    (ACTIVE VACUUM OR PASSIVE DUCTING)

Open Scavenging Interface (Valveless)

• Architecture: A clear cylindrical canister open directly to room atmosphere through continuous relief ports at its top rim. It contains no mechanical valves.
• Operating Principle: Requires an active disposal system (hospital dedicated medical vacuum). Waste gas enters the bottom of the canister. Active vacuum draws gas out through the disposal line.
• Safety Mechanisms:
  • Positive Pressure Protection: If the vacuum fails or disposal line kinks, waste gas spills harmlessly out through the open top ports into the operating room, preventing backpressure buildup in the patient's lungs.
  • Negative Pressure Protection: If active vacuum suction exceeds waste gas flow, the vacuum simply entrains room air through the open top ports. It cannot draw gas from the patient circuit.
• Technologist Operational Window: The technologist adjusts a vacuum needle valve until an internal indicator bobbin floats within a marked "green window." If set too low, anesthetic gas spills into the OR; if set too high, room air is drawn in wastefully.
• Strict Limitation: Can only be used with active suction systems. It cannot function with passive disposal.

Closed Scavenging Interface (Valved)

• Architecture: A sealed canister communicating with the atmosphere exclusively through spring-loaded or weighted mechanical relief valves, coupled with a dedicated scavenging reservoir bag (typically 3 L).
• Relief Valve Thresholds:
  1. Positive-Pressure Relief Valve (Opens at +5.0 cmH₂O): If the disposal line occludes or vacuum fails, pressure inside the interface rises. At +5 cmH₂O, this valve pops open, venting waste gas into the OR and preventing pulmonary barotrauma, tension pneumothorax, and cardiovascular collapse.
  2. Negative-Pressure Relief Valve (Opens at -0.5 cmH₂O): If active vacuum is excessive and empties the scavenging reservoir bag, sub-atmospheric pressure opens this valve at -0.5 cmH₂O, entraining room air into the interface. This prevents negative pressure from transmitting into the breathing circuit, which would evacuate the ventilator bellows, collapse the patient's lungs, and induce negative-pressure pulmonary edema (NPPE).

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Disposal Pathways: Active Vacuum vs. Passive Ventilation

Technologist Pre-Use Inspection Checklist for WAGS

During morning setup, the anesthesia technologist must verify:

1. Scavenging transfer hoses are securely locked into the 19-mm / 30-mm APL and ventilator exhaust ports.
2. On open interfaces, hospital vacuum line is connected and the flowmeter bobbin floats within the marked green operational window.
3. On closed interfaces, the dedicated scavenging reservoir bag is moderately compliant (neither sucked flat against its mount nor tautly overdistended).
4. Relief valves are free of sticky residues, lint, or mechanical obstruction.
5. Active vacuum hose is connected to a dedicated WAGS wall suction terminal, never to the high-vacuum patient surgical suction cannister.