Anesthesia Machine Circuit Week 4-5

Anesthesia Machine Circuit

Overview

• The anesthesia machine circuit is crucial for delivering anesthetic and oxygen to patients, while also removing expired gases.

• Two main categories of circuits: Rebreathing System and Non-rebreathing System.

Functions of Anesthetic Circuits

• Carries anesthetic and oxygen from the fresh gas inlet to the patient.

• Conveys expired gases away from the patient.

• Provides manual support for ventilation.

Rebreathing System (Circle System)

Characteristics

• Closed system commonly used for patients over 7 kg.

• CO2 removal: exhaled carbon dioxide is removed from the air.

• Exhaled gases can flow back to the patient, facilitating gas recycling.

• Unidirectional gas flow: gases move in only one direction.

Parts of a Rebreathing System

• Inhalation unidirectional flutter valve

• Pop-off (pressure relief) valve

• Exhalation unidirectional flutter valve

• Pressure manometer

• Reservoir bag

• Carbon dioxide absorber canister

• Corrugated breathing tubes

Unidirectional Valves (Flutter Valves)

• Composed of a disc in a transparent dome that moves with breathing:

  • Inspiratory valve directs gas toward the patient during inhalation.

  • Expiratory valve directs gas away from the patient during exhalation.

  • Allows for monitoring of respiratory rate and depth as well as proper ET tube placement.

Carbon Dioxide Absorber Canister

• Contains absorbent granules to extract CO2 from exhaled gases.

• Essential when recirculating expired gases.

• Common absorbents include soda lime and carbolyme.

• Must be replaced upon depletion to prevent hypercapnia.

• May feature ethyl violet indicator: turns purple when depleted.

Pop-off Valve (Pressure Relief Valve)

• Manages gas exit from the breathing circuit to the scavenging system.

• Prevents excessive pressure buildup, avoiding barotrauma.

• Should remain open except during manual/mechanical ventilation.

Scavenging System

• Collects and eliminates waste anesthetic gases (WAG) from the breathing system.

• Comprises a hose linked to the pop-off valve that interfaces with the scavenging system.

Types of Scavenging Systems

• Active System: Uses a dedicated vacuum pump and ducting.

• Passive System: Utilizes natural airflow to dispose of WAGs.

Activated Charcoal Canisters (F-air Canisters)

• Absorb exhaled anesthetic gases.

• Replace every 12 hours or when weight increases by 50g.

• Must maintain an upright position for effective absorption.

Reservoir Bag

• Rubber bag used to store oxygen and inhalant.

• Acts as an indicator for respiratory rate and depth.

• Utilized for manual ventilation and confirming ET tube placement.

Reservoir Bag Size

• Should be 2/3 full during respiration.

• Check O2 flow rate, pop-off valve, and scavenging system regularly.

Pressure Manometer

• Indicates pressure in the breathing circuit in cmH2O.

• Do not exceed 20 cmH2O for small animals and 40 cmH2O for large animals.

• Ensures safety during manual ventilation.

Manual Ventilation (Bagging the Patient)

• Involves closing the pop-off valve.

• Squeeze the reservoir bag to maintain pressure of 15-20 cmH2O for dogs and 10-15 cmH2O for cats.

• Constantly monitor the pressure manometer.

Air Intake Valve (Negative Pressure Relief Valve)

• Not present on all machines; located on the dome of the inspiratory valve.

• Allows room air (21% oxygen) into the circuit to prevent hypoxia.

Breathing Tubes and Y-Piece

• Made from corrugated rubber or plastic.

• Connects inspiratory and expiratory valves; Y-piece connects to the ET tube.

• Comes in various sizes: 50 mm (large), 22 mm (standard), 15 mm (pediatric).

Universal F-Circuit

• Designed to conserve heat and moisture.

• Cold oxygen delivered via the inner tube; warm gases through the outer tube.

Non-Rebreathing System (Bain or Jackson-Rees Circuit)

Characteristics

• Recommended for patients <7 kg.

• Fresh gas routed directly from the flowmeter and vaporizer.

• High fresh gas flow rates needed to clear CO2.

• No CO2 absorber, pressure manometer, or unidirectional valves.

Advantages of Non-Rebreathing Systems

• Reduced resistance to breathing.

• Allows for rapid adjustments in anesthetic depth.

Calculating Oxygen Flow Rates

For Rebreathing System (Patients ≥7 kg)

• Flow rate: 20-40 mL/kg/min (use 30 mL/kg/min midrange for lab/exams).

• 50-100 mL/kg/min after induction/recovery.

• Never set flowmeter below 500 mL/min.

For Non-Rebreathing System (Patients <7 kg)

• Requires 200-400 mL/kg/min (use 300 mL/kg/min midrange for lab/exams).

Rounding Rules for Oxygen Flow Rate

• Round to the nearest 100 mL.

• Examples:

  • 778.2 mL/min → 800 mL/min

  • 1459 mL/min → 1500 mL/min

  • 1966 mL/min → 2000 mL/min

  • 239.7 mL/min → 300 mL/min

Flowmeter Settings for Sample Patients

• 48.4 lb (22 kg) dog: 660 mL/min (22 kg × 30 mL/kg/min).

• 9.3 lb (4.2 kg) cat: 1260 mL/min (4.2 kg × 300 mL/kg/min).

• 19.5 lb (8.8 kg) puppy: 2640 mL/min (8.8 kg × 300 mL/kg/min).

• 4.6 lb (2.1 kg) kitten: 630 mL/min (2.1 kg × 300 mL/kg/min).