Classifications of Breathing Circuits in Anesthesia

Classifications of Breathing Circuits

• Breathing circuits are traditionally classified into four categories: Open, Semi-open, Semi-closed, and Closed.
• The classification is based on the presence of a reservoir and/or rebreathing.

Open Breathing Circuit

• Rebreathing is not possible in open breathing circuits.
• Examples of open breathing circuits include:
  • Insufflation
  • Simple face mask
  • Nasal cannula
  • Open drop

Semi-open Breathing Circuit

• Rebreathing is not possible in semi-open breathing circuits.
• A reservoir is present in semi-open breathing circuits.
• Examples of semi-open breathing circuits include:
  • Mapleson circuit (FGF dependent on design)
  • Circle system (if FGF > Minute ventilation)

Semi-closed Breathing Circuit

• Partial rebreathing is possible in semi-closed breathing circuits.
• A reservoir is present in semi-closed breathing circuits.
• Examples of semi-closed breathing circuits include:
  • Circle system (FGF < Minute ventilation)

Closed Breathing Circuit

• Complete rebreathing is possible in closed breathing circuits.
• A reservoir is present in closed breathing circuits.
• Examples of closed breathing circuits include:
  • Circle system with very low FGF and APL closed
• Most anesthesia machines use a circle breathing circuit.

Classifications of Breathing Circuits

• Breathing circuits are traditionally classified into four categories: Open, Semi-open, Semi-closed, and Closed.
• The classification is based on the presence of a reservoir and/or rebreathing.

Open Breathing Circuit

• Rebreathing is not possible in open breathing circuits.
• Examples of open breathing circuits include:
  • Insufflation
  • Simple face mask
  • Nasal cannula
  • Open drop

Semi-open Breathing Circuit

• Rebreathing is not possible in semi-open breathing circuits.
• A reservoir is present in semi-open breathing circuits.
• Examples of semi-open breathing circuits include:
  • Mapleson circuit (FGF dependent on design)
  • Circle system (if FGF > Minute ventilation)

Semi-closed Breathing Circuit

• Partial rebreathing is possible in semi-closed breathing circuits.
• A reservoir is present in semi-closed breathing circuits.
• Examples of semi-closed breathing circuits include:
  • Circle system (FGF < Minute ventilation)

Closed Breathing Circuit

• Complete rebreathing is possible in closed breathing circuits.
• A reservoir is present in closed breathing circuits.
• Examples of closed breathing circuits include:
  • Circle system with very low FGF and APL closed
• Most anesthesia machines use a circle breathing circuit.

Chemical Absorption of Carbon Dioxide

• Used in semiclosed and closed breathing systems, such as circle and to-and-fro systems, to remove exhaled carbon dioxide.

Process of Carbon Dioxide Removal

• Exhaled carbon dioxide is absorbed by a chemical agent within the circuit.
• Other exhaled gases, including oxygen and anesthetic agents, are rebreathed.

Fresh Gas Flow

• Replenishes oxygen and anesthetic agents lost due to:
  • Uptake by the body
  • Metabolism
  • Leaks in the circuit

Dilution with Fresh Gas

• Carbon dioxide is excreted intermittently (exhalation), while fresh gas inflow is continuous.
• Carbon dioxide removal efficiency is affected by:
  • Fresh gas flow rate
  • Inflow site location
  • Reservoir bag location
  • Pop-off valve location

Elimination of Carbon Dioxide

• When fresh gas flows are 1 to 1.5 times the minute volume (around 10 L/min in an adult), dilution alone is enough to eliminate carbon dioxide.
• In this scenario, the system functions like a nonrebreathing system.

Chemical Absorption of Carbon Dioxide

• Used in semiclosed and closed breathing systems, such as circle and to-and-fro systems, to remove exhaled carbon dioxide.

Process

• Exhaled carbon dioxide is absorbed by a chemical agent within the circuit.
• Other exhaled gases, including oxygen and anesthetic agents, are rebreathed.
• Fresh gas flow replenishes oxygen and anesthetic agents lost due to:
  • Uptake by the body
  • Metabolism
  • Leaks in the circuit

Dilution with Fresh Gas

• Carbon dioxide is excreted intermittently through exhalation, whereas fresh gas inflow is continuous.
• Carbon dioxide removal efficiency is affected by factors such as:
  • Fresh gas flow rate
  • Inflow site location
  • Reservoir bag location
  • Pop-off valve location
• When fresh gas flows are 1 to 1.5 times the minute volume (around 10 L/min in an adult), dilution alone is enough to eliminate carbon dioxide.
• In this scenario, the system functions like a nonrebreathing system.

Valve Function in Breathing Circuits

• Nonrebreathing valves are used to separate exhaled gases from inhaled gases in breathing circuits.
• The primary purpose of these valves is to remove exhaled carbon dioxide without relying on chemical absorption.
• High flow rate circuits that achieve a similar effect are not considered nonrebreathing circuits.

Open-Drop Ether and T-Piece Without Reservoir

• Open-drop ether is a method where anesthetic liquid (ether) is poured over gauze held near the patient's face.
• In open-drop ether, carbon dioxide is removed through diffusion into the air.
• A T-piece without a reservoir is a simple tube with fresh gas flowing in, allowing exhaled gases to escape into the atmosphere.
• In a T-piece without a reservoir, carbon dioxide is removed through dilution with Fresh Gas Flow (FGF).

Open-Drop Ether and T-Piece Without Reservoir

• Open-drop ether is a method where anesthetic liquid (ether) is poured over gauze held near the patient's face.
• In open-drop ether, carbon dioxide is removed through diffusion into the air.
• A T-piece without a reservoir is a simple tube with fresh gas flowing in, allowing exhaled gases to escape into the atmosphere.
• In a T-piece without a reservoir, carbon dioxide is removed through dilution with Fresh Gas Flow (FGF).

Components of a Breathing Circuit

• Patient connection is facilitated through a face mask, laryngeal mask, or tracheal tube, which is adapted to the circuit through a Y-piece or elbow.

Valve Components

• Valves may be present in the circuit to control gas flow direction.

Reservoir Bag

• The reservoir bag is nearly always present in the breathing circuit.
• Its primary function is to facilitate manual ventilation.

Fresh Gas Supply

• The fresh gas supply is an essential component for replenishing oxygen and anesthetic gases.

Excess Gas Release Mechanism

• The excess gas release mechanism allows for the venting of excess gases.

Optional Components

• APL carbon dioxide absorption may be included as an optional component for carbon dioxide removal.
• Other ancillary devices that may be included are humidifiers, spirometers, and pressure gauges.

Components of a Breathing Circuit

• Patient connection is facilitated through a face mask, laryngeal mask, or tracheal tube, which is adapted to the circuit through a Y-piece or elbow.

Valve Components

• Valves may be present in the circuit to control gas flow direction.

Reservoir Bag

• The reservoir bag is nearly always present in the breathing circuit.
• Its primary function is to facilitate manual ventilation.

Fresh Gas Supply

• The fresh gas supply is an essential component for replenishing oxygen and anesthetic gases.

Excess Gas Release Mechanism

• The excess gas release mechanism allows for the venting of excess gases.

Optional Components

• APL carbon dioxide absorption may be included as an optional component for carbon dioxide removal.
• Other ancillary devices that may be included are humidifiers, spirometers, and pressure gauges.

Connection of the Patient to the Breathing Circuit

• Methods to connect a patient to the breathing circuit include anesthesia masks, supraglottic devices (e.g., laryngeal mask), and tracheal tubes.

Anesthesia Masks

• Made of rubber or clear plastic materials.
• Features include inflatable or inflated cuffs for sealing and come in various sizes and styles.
• Proper fitting is crucial, covering the nose and fitting in the groove between the mental protuberance and alveolar ridge.
• Poor fitting can lead to potential issues such as leaks, trauma, or nerve damage.

Connection to Circuit

• Anesthesia masks have a 22-mm (⅞-inch) female connection to the Y-piece.

Connection of the Patient to the Breathing Circuit

• Methods to connect a patient to the breathing circuit include anesthesia masks, supraglottic devices (e.g., laryngeal mask), and tracheal tubes.

Anesthesia Masks

• Made of rubber or clear plastic materials.
• Features include inflatable or inflated cuffs for sealing and come in various sizes and styles.
• Proper fitting is crucial, covering the nose and fitting in the groove between the mental protuberance and alveolar ridge.
• Poor fitting can lead to potential issues such as leaks, trauma, or nerve damage.

Connection to Circuit

• Anesthesia masks have a 22-mm (⅞-inch) female connection to the Y-piece.

Breathing Tubing Characteristics

• Breathing tubing has a 1-meter length and a 22 mm large bore, which minimizes resistance to flow.
• The tubing has corrugated or spiral reinforcement, making it flexible.

Material and Sterility

• Breathing tubing is primarily made of disposable plastic, which is lightweight but not biodegradable.
• The tubing is supplied sterile, despite the lack of strong evidence supporting the necessity of sterility.

Connections and Dimensions

• The tubing has 22 mm internal diameter ends.
• Pediatric circuits use smaller diameter tubing for infants and children.

Compliance and Distensibility

• Modern plastic tubing has lower compliance values than rubber.
• The tubing has higher distensibility due to gas compression under pressure, leading to apparatus dead space.

Resistance to Flow and Gas Flow Pattern

• The resistance to flow in standard corrugated tubes is exceedingly small.
• The gas flow pattern in the tubing is turbulent due to corrugations, promoting mixing and facilitating changes in gas composition.

Additional Features and Precautions

• Scavenging devices have specific diameters to prevent misconnections.
• Reusable tubing connects to the mask/tube via a separate Y-piece, while disposable sets may have an integrated Y-piece.
• Swivel joints in connectors increase the risk of leaks.
• The circuit should be tested before use to ensure there are no leaks.

Breathing Tubing Characteristics

• Breathing tubing has a 1-meter length and a 22 mm large bore to minimize resistance.
• The tubing has corrugated or spiral reinforcement for flexibility.
• The internal volume of the tubing is 400-500 mL per meter.

Material and Sterility

• Breathing tubing is mostly made of disposable plastic.
• The plastic is lightweight but not biodegradable.
• The tubing is supplied sterile, despite the lack of strong evidence supporting the necessity.

Connections and Compliance

• The tubing has 22 mm internal diameter ends.
• The compliance of modern plastic tubing is lower than that of rubber.
• Distensibility is higher due to gas compression under pressure, creating apparatus dead space.

Resistance to Flow and Gas Flow Pattern

• The resistance to flow in standard corrugated tubes is exceedingly small.
• Multiple tubes or extra-long tubing can be used for distance.
• The gas flow pattern is turbulent due to corrugations, promoting mixing and allowing changes in gas composition to quickly reach the patient connection.

Additional Info and Pediatric Circuits

• Scavenging devices have specific diameters to prevent misconnections.
• Pediatric circuits use smaller diameter tubing for infants and children.

Connections and Testing

• Reusable tubing connects to mask/tube via a separate Y-piece.
• Disposable sets may have an integrated Y-piece.
• Swivel joints in connectors increase the risk of leaks.
• The circuit should be tested before use for leaks.

Unidirectional Valves

• Purpose: Direct respiratory gas flow in breathing circuits to prevent rebreathing.

Types of Unidirectional Valves

• Disc on knife edges
• Rubber flaps
• Sleeves

Essential Characteristics

• Low resistance to ensure easy opening
• High competence to ensure quick and complete closing

Usage in Breathing Systems

• Circle systems: Two valves (inspiratory and expiratory) are used to prevent rebreathing
• Nonrebreathing systems: Two valves ensure patient inhales fresh gas and exhales to the room or scavenger

Location in Breathing Systems

• Circle systems: Valves can be located anywhere in inspiratory/expiratory limbs, but one must be between the patient and reservoir bag in each limb
• Modern anesthesia machines: Valves are often located near or within the carbon dioxide absorber canister casing

Design Features

• Common type: Dome valves with a light disk on a knife edge
• Disk material: Hydrophobic plastic
• Orientation: Usually vertical with a horizontal disk, but can be vertical (e.g., GE-Datex-Ohmeda)

Nonrebreathing Valves

• Early designs required manual occlusion for assisted/controlled ventilation
• Modern designs automatically close the expiratory valve during controlled respiration

Breathing Bags

• Function as a reservoir for anesthetic gases and/or oxygen.
• Allow for visual assessment of ventilation presence and volume.
• Provide means for manual ventilation.

Necessity

• Anesthesia machines cannot provide sufficient peak inspiratory flow rates for spontaneous breathing solely from FGF.

Visual Assessment

• Low fresh gas flows: Bag excursion reflects tidal volume.
• High fresh gas flows: Bag excursion is minimal, not reflective of tidal volume.

Characteristics

• Ellipsoid shape for easy grasping.
• Made of plastic or latex.
• Sizes range from 0.5 L to 6 L.
• Ideally, size should exceed patient's inspiratory capacity.

Location

• In circle systems: usually near the carbon dioxide absorber canister or at the end of a corrugated tube.

Pressure-Volume Considerations

• Overinflated bags can become problematic if the pop-off valve is closed.
• Rubber bags: pressure-limiting, reaching 40-50 cm H2O.
• Disposable bags: can reach higher pressures than rubber before rupturing.

Breathing Bags

• Function as a reservoir for anesthetic gases and/or oxygen.
• Allow for visual assessment of ventilation presence and volume.
• Provide means for manual ventilation.

Necessity

• Anesthesia machines cannot provide sufficient peak inspiratory flow rates for spontaneous breathing solely from FGF.

Visual Assessment

• Low fresh gas flows: Bag excursion reflects tidal volume.
• High fresh gas flows: Bag excursion is minimal, not reflective of tidal volume.

Characteristics

• Ellipsoid shape for easy grasping.
• Made of plastic or latex.
• Sizes range from 0.5 L to 6 L.
• Ideally, size should exceed patient's inspiratory capacity.

Location

• In circle systems: usually near the carbon dioxide absorber canister or at the end of a corrugated tube.

Pressure-Volume Considerations

• Overinflated bags can become problematic if the pop-off valve is closed.
• Rubber bags: pressure-limiting, reaching 40-50 cm H2O.
• Disposable bags: can reach higher pressures than rubber before rupturing.

Pop-Off Valves

• Also known as overflow, outflow, relief, and spill valves.
• Function is to allow excess gas to leave the circuit, thereby matching inflow.
• Efficiency of the valve depends on the placement of fresh gas inflow.
• Most pop-off valve designs are based on spring-loaded dome valves.
• The valve should open at a pressure less than 1 cm H2O.
• The pressure needed to open the valve can be increased by tightening the screw cap, which allows for Positive End-Expiratory Pressure (PEEP).
• The exhaust gas can be collected by a scavenging system.

Carbon Dioxide Absorption

• In partial rebreathing and nonrebreathing systems, carbon dioxide is vented to room air.

Closed Anesthesia Systems

• In closed anesthesia systems, exhaled carbon dioxide must be removed rather than vented.
• Carbon dioxide removal relies on a chemical reaction involving a neutralization reaction.
• The reaction occurs between carbonic acid (formed from CO2 and water) and metal hydroxides.

Composition of Soda Lime

• Soda lime is primarily composed of calcium hydroxide.
• It also contains sodium and/or potassium hydroxide, water, and inert substances.

Function of Soda Lime

• Hydroxides in soda lime react with carbonic acid to form carbonates and water.

Exhaustion of Soda Lime

• Exhaustion occurs when all hydroxides in soda lime are converted to carbonates.

Absorption Capacity of Soda Lime

• 100 grams of soda lime has the capacity to absorb approximately 26 liters of CO2.

Calcium Hydroxide Lime (Amsorb)

• A newer alternative absorbent.
• Composed of calcium hydroxide, calcium chloride (humectant), setting agents, and water.
• Offers the advantage of eliminating the need for strong alkali (sodium/potassium hydroxide) in its composition.
• Reduces the potential for harmful byproducts and agent degradation.
• Decreases the formation of Compound A when used with Sevoflurane (Sevo).

Indicator Dyes

• Provide visual indication of soda lime exhaustion through a color change that occurs as pH shifts due to carbonate formation.
• Common indicator dyes include:
  • Ethyl violet (changes from white to blue-violet)
  • Ethyl orange (changes from orange to yellow)
  • Cresyl yellow (changes from red to yellow)
• Limitations of indicator dyes include:
  • Being bleached by intense light (specifically ethyl violet)
  • False "regeneration" of color due to small amount of hydroxide regeneration
  • Channeling affecting color distribution

Gold Standard for CO₂ Removal Assessment

• Capnography (monitoring of inspired CO₂) remains the most reliable way to assess CO₂ removal

Mesh Size and Channeling

• Mesh size of soda lime granules is 4 to 8 mesh, which allows them to pass through a strainer with 4-8 wires per inch, maximizing surface area for efficient CO₂ absorption
• Channeling is the preferential flow of gas along the canister walls or within the absorbent, reducing soda lime effectiveness
• Methods to minimize channeling include:
  • Using baffles
  • Ensuring vertical gas flow
  • Avoiding frequent canister movement
  • Using prepackaged cylinders
  • Avoiding overly tight packing
  • Shaking the canister before use

Indicator Dyes

• Provide visual indication of soda lime exhaustion by undergoing a color change as pH shifts due to carbonate formation.
• Common indicator dyes include:
  • Ethyl violet (shifts from white to blue-violet)
  • Ethyl orange (shifts from orange to yellow)
  • Cresyl yellow (shifts from red to yellow)
• Limitations of indicator dyes include:
  • Ethyl violet can be bleached by intense light

Compound A

• Formed when sevoflurane reacts with certain alkalis in absorbents
• Can be nephrotoxic in high concentrations
• Factors that increase production of Compound A include:
  • Low fresh gas flow rates
  • High absorbent temperatures
  • Desiccation
• Current guidelines recommend against using low fresh gas flows to minimize Compound A production

Bacterial Filters

• Protect against contamination in breathing systems
• Highly efficient at filtering bacteria (>99.9%) and viruses (96.43% to 99.84%)
• Come in various shapes and sizes

Placement and Types

• Usually placed on the expiratory limb
• HME (Heat and Moisture Exchanger) filters are placed at the Y-piece

Complications

• Obstruction due to:
  • Humidity
  • Sputum
  • Fluid
  • Aerosols
  • Malpositioning
• Leakage
• Can be expensive

Risks and Balance

• Balance between the known risk of filter complications and the unknown risk of viral contamination

Circle Breathing Systems

• Basic components include inspiratory and expiratory limbs, unidirectional valves in each limb, and a reservoir bag.
• Additional components involve a carbon dioxide absorber, fresh gas inflow site, and a pop-off valve.

Component Placement

• Multiple configurations of components are possible.
• Optimal configurations differ depending on whether spontaneous or controlled ventilation is being used.

Common Misconception

• The reservoir bag, not the carbon dioxide absorber, is on the opposite side of the circle from the patient.

Placement of Components in Anesthesia Circle

• Carbon Dioxide Absorber: • Typically placed in the inspiratory limb on the bag side • Necessary in low fresh gas flow techniques

Placement of Fresh Gas Inflow

• Recommended placement is on the bag side of the inspiratory limb, between the inspiratory valve and absorber
• This allows for continuous delivery and storage of fresh gas during exhalation
• Improper placement on the patient side of valves can lead to: • Inaccuracies in spirometry • Rebreathing of exhaled gases

Placement of Pop-Off Valve

• Can be placed anywhere in the circle, but some locations are more logical
• Typically placed: • Between the expiratory valve and bag mount • Opposite the bag mount before the absorber

Mapleson Circuits

• Reservoir can be filled by fresh gas, exhaled gas, or a combination of both
• May or may not have a pop-off valve for pressure regulation
• CO2 elimination occurs through dilution with fresh gas and specific circuit arrangement
• There is no CO2 absorber in Mapleson circuits
• No unidirectional valves are used in Mapleson circuits

Factors Affecting CO2 Elimination

• Fresh gas flow rate has a significant impact on CO2 elimination
• Respiratory pattern factors influencing CO2 elimination include:
  • Respiratory rate
  • Tidal volume
  • Dead space
  • I:E ratio
  • Flow patterns

Mapleson Classifications

• Mapleson circuits are classified into A, B, C, D, E, and F types
• The most efficient Mapleson circuit for spontaneous ventilation is Type A
• For assisted/controlled ventilation, the most efficient circuits are:
  • Type D
  • Type E
  • Type F (in descending order of efficiency)

Mapleson A Configuration

• Spontaneous breathing occurs with an open APL (Adjustable Pressure Limiting) valve.
• During exhalation, fresh gas flow flushes CO2 from the circuit.
• A fresh gas flow exceeding 55% of minute ventilation is typically required to prevent rebreathing.

Assisted/Controlled Ventilation

• In assisted or controlled ventilation, the exhaled tidal volume and fresh gas flow enter the breathing tube.
• During the next compression, some alveolar gas may re-enter the airway.
• Rebreathing depends on various factors, including inspiratory flow, compliance, and resistance.

Proprietary Semiclosed Systems

• Bain Circuit:
  • Coaxial Mapleson D design with fresh gas delivered at patient end and pop-off valve at bag end
  • Requires fresh gas flow of 70 mL/kg/min for controlled ventilation
  • Can malfunction if central tube disconnects

Lack Circuit

• Coaxial Mapleson A design with fresh gas flowing between outer and inner tubes to patient
• Optimal for spontaneous breathing
• Requires fresh gas flow of 70% of minute volume to prevent rebreathing

Mera F and Universal F Systems

• Coaxial Bain-type circuit mounted on circle system hardware
• Functions similarly to standard Bain circuit at 100 mL/kg/min fresh gas flow
• Universal F can be used as a transport system or part of a nonrebreathing system

Enclosed Afferent Reservoir (EAR) System

• Retains efficiency during both spontaneous and controlled ventilation
• Prevents venting of gas during controlled inspiration
• Requires 70 mL/kg/min fresh gas flow in adults for controlled ventilation
• Fresh gas flow for spontaneous ventilation determined by clinical assessment

Jackson-Rees Circuit

• Modification of Ayre's T-piece with corrugated tube, bag, and sometimes valve to expiratory limb
• Functions like Mapleson D, E, or F during spontaneous breathing with fresh gas flow of 100 mL/kg/min or more
• Can be used with controlled ventilation with fresh gas flow of 70 mL/kg/min if assembled with spring-loaded valve

Proprietary Semiclosed Systems

• Bain Circuit:
  • Coaxial Mapleson D design with fresh gas delivered at patient end and pop-off valve at bag end
  • Requires fresh gas flow of 70 mL/kg/min for controlled ventilation
  • Can malfunction if central tube disconnects

Lack Circuit

• Coaxial Mapleson A design with fresh gas flowing between outer and inner tubes to patient
• Optimal for spontaneous breathing
• Requires fresh gas flow of 70% of minute volume to prevent rebreathing

Mera F and Universal F Systems

• Coaxial Bain-type circuit mounted on circle system hardware
• Functions similarly to standard Bain circuit at 100 mL/kg/min fresh gas flow
• Universal F can be used as a transport system or part of a nonrebreathing system

Enclosed Afferent Reservoir (EAR) System

• Retains efficiency during both spontaneous and controlled ventilation
• Prevents venting of gas during controlled inspiration
• Requires 70 mL/kg/min fresh gas flow in adults for controlled ventilation
• Fresh gas flow for spontaneous ventilation determined by clinical assessment

Jackson-Rees Circuit

• Modification of Ayre's T-piece with corrugated tube, bag, and sometimes valve to expiratory limb
• Functions like Mapleson D, E, or F during spontaneous breathing with fresh gas flow of 100 mL/kg/min or more
• Can be used with controlled ventilation with fresh gas flow of 70 mL/kg/min if assembled with spring-loaded valve

Positive End-Expiratory Pressure (PEEP)

• Improves oxygenation in breathing systems
• Achieved through various methods in a circle system, controlled electronically by the anesthesia machine on the ventilator

Circuit Malfunction and Safety

• Breathing circuits are a common source of injury in anesthesia despite improvements in machine design
• Most claims related to vaporizers, supplemental oxygen, and breathing systems
• Causes of breathing circuit problems:
  • Misconnections
  • Disconnections
  • Leaks
  • Valve failure
  • Filter issues
• Most claims involve provider error and can be prevented with:
  • Pre-anesthesia machine checks
  • Familiarity with equipment
  • Routine equipment checks
  • Simple and user-friendly equipment design
  • Vigilance to identify and manage potential malfunctions

Positive End-Expiratory Pressure (PEEP)

• Improves oxygenation in breathing systems
• Achieved through various methods in a circle system, controlled electronically by the anesthesia machine on the ventilator

Circuit Malfunction and Safety

• Breathing circuits are a common source of injury in anesthesia despite improvements in machine design
• Most claims related to vaporizers, supplemental oxygen, and breathing systems
• Causes of breathing circuit problems:
  • Misconnections
  • Disconnections
  • Leaks
  • Valve failure
  • Filter issues
• Most claims involve provider error and can be prevented with:
  • Pre-anesthesia machine checks
  • Familiarity with equipment
  • Routine equipment checks
  • Simple and user-friendly equipment design
  • Vigilance to identify and manage potential malfunctions

Humidification in the Airway

• Nasal turbinates create turbulent airflow to maximize contact with mucosa, trapping particles of all sizes and facilitating heat and moisture transfer.

Humidification Process

• Inspired gas reaches 34°C with an absolute humidity of 34-38 mg/L (95-100% relative humidity) by the mid-trachea.
• The gas is further humidified to 44 mg/L at 100% relative humidity in the distal airway.

Countercurrent Heat and Moisture Exchange (HME)

• Exhaled air cools and condenses, releasing heat and moisture to rewarm and rehydrate mucosa.
• HME retains heat and moisture to condition gas for the alveoli.
• Daily net loss of ~250 mL water and 250 kcal in adults.

Mucociliary Elevator

• Location: Found in the nose, trachea, and bronchi (not in the nasopharynx, pharynx, or larynx).
• Function: Responsible for the majority of humidification.
• Mucus glands keep the mucosa moist through secretion and transudation.

Mucus Composition and Layers

• Composition: Secreted by goblet cells in response to irritation.
• Forms a 10-μm film around respiratory cilia.
• Two layers:
  • Superficial: Viscous, traps inspired particles.
  • Deep: Watery, acidic, contains antibacterial compounds.

Humidification in the Airway

• Nasal turbinates create turbulent airflow to maximize contact with mucosa, trapping particles of all sizes and facilitating heat and moisture transfer.

Humidification Process

• Inspired gas reaches 34°C with an absolute humidity of 34-38 mg/L (95-100% relative humidity) by the mid-trachea.
• The gas is further humidified to 44 mg/L at 100% relative humidity in the distal airway.

Countercurrent Heat and Moisture Exchange (HME)

• Exhaled air cools and condenses, releasing heat and moisture to rewarm and rehydrate mucosa.
• HME retains heat and moisture to condition gas for the alveoli.
• Daily net loss of ~250 mL water and 250 kcal in adults.

Mucociliary Elevator

• Location: Found in the nose, trachea, and bronchi (not in the nasopharynx, pharynx, or larynx).
• Function: Responsible for the majority of humidification.
• Mucus glands keep the mucosa moist through secretion and transudation.

Mucus Composition and Layers

• Composition: Secreted by goblet cells in response to irritation.
• Forms a 10-μm film around respiratory cilia.
• Two layers:
  • Superficial: Viscous, traps inspired particles.
  • Deep: Watery, acidic, contains antibacterial compounds.

Consequences of Underhumidification

• Viscous mucus hinders ciliary movement
• Reduced humidification capacity of dry membranes
• Ciliary loss, mucosal inflammation, ulceration, and necrosis
• Bronchial obstruction, encrustation, and sputum retention
• Infection, atelectasis, reduced functional capacity, V/Q mismatch, and reduced compliance
• Exacerbation of hypothermia, especially in neonates/children

Consequences of Overhumidification

• Condensation of water in the airway
• Reduced mucosal viscosity and risk of water intoxication
• Inefficient mucociliary transport
• Increased airway resistance
• Risk of pulmonary infection, surfactant dilution, atelectasis, and V/Q mismatch
• Inaccurate volume readings and potential workstation complications due to water obstruction
• Thermal injury to the epithelium (tracheitis)

Humidification Devices

• Goal: Achieve 30-35 mg/L absolute humidity in the trachea
• Ideal characteristics: humidify and warm gases to physiologic conditions, no added resistance or dead space, no increased risk of infection, safe, easy to use, and cost-effective

Heat and Moisture Exchangers (HMEs)

• Function: act as an "artificial nose" by retaining moisture and heat from exhaled gases
• Placement: between Y-piece and endotracheal tube/LMA
• Types: hygroscopic (use moisture-retaining chemicals), hydrophobic (contain pleated membranes), and steady state
• ISO 9360 Standard: specifies testing parameters for HMEs

Potential Hazards and Limitations of Heat and Moisture Exchangers

• Increased resistance: HMEs introduce resistance to the breathing circuit, potentially increasing the work of breathing and causing hypercapnia
• Obstruction: HMEs can become progressively or acutely obstructed by sputum, blood, or other fluids, requiring regular visual inspection and replacement after 24 hours of use
• Unpredictable performance with active humidification: combining HMEs with active humidifiers is generally not recommended
• Pediatric considerations: infants and neonates are more vulnerable to respiratory water and heat loss, and low dead space HMEs are preferred, but often have lower moisture retention.

Consequences of Underhumidification

• Viscous mucus hinders ciliary movement
• Reduced humidification capacity of dry membranes
• Ciliary loss, mucosal inflammation, ulceration, and necrosis
• Bronchial obstruction, encrustation, and sputum retention
• Infection, atelectasis, reduced functional capacity, V/Q mismatch, and reduced compliance
• Exacerbation of hypothermia, especially in neonates/children

Consequences of Overhumidification

• Condensation of water in the airway
• Reduced mucosal viscosity and risk of water intoxication
• Inefficient mucociliary transport
• Increased airway resistance
• Risk of pulmonary infection, surfactant dilution, atelectasis, and V/Q mismatch
• Inaccurate volume readings and potential workstation complications due to water obstruction
• Thermal injury to the epithelium (tracheitis)

Humidification Devices

• Goal: Achieve 30-35 mg/L absolute humidity in the trachea
• Ideal characteristics: humidify and warm gases to physiologic conditions, no added resistance or dead space, no increased risk of infection, safe, easy to use, and cost-effective

Heat and Moisture Exchangers (HMEs)

• Function: act as an "artificial nose" by retaining moisture and heat from exhaled gases
• Placement: between Y-piece and endotracheal tube/LMA
• Types: hygroscopic (use moisture-retaining chemicals), hydrophobic (contain pleated membranes), and steady state
• ISO 9360 Standard: specifies testing parameters for HMEs

Potential Hazards and Limitations of Heat and Moisture Exchangers

• Increased resistance: HMEs introduce resistance to the breathing circuit, potentially increasing the work of breathing and causing hypercapnia
• Obstruction: HMEs can become progressively or acutely obstructed by sputum, blood, or other fluids, requiring regular visual inspection and replacement after 24 hours of use
• Unpredictable performance with active humidification: combining HMEs with active humidifiers is generally not recommended
• Pediatric considerations: infants and neonates are more vulnerable to respiratory water and heat loss, and low dead space HMEs are preferred, but often have lower moisture retention.

Consequences of Underhumidification

• Viscous mucus hinders ciliary movement
• Reduced humidification capacity of dry membranes
• Ciliary loss, mucosal inflammation, ulceration, and necrosis
• Bronchial obstruction, encrustation, and sputum retention
• Infection, atelectasis, reduced functional capacity, V/Q mismatch, and reduced compliance
• Exacerbation of hypothermia, especially in neonates/children

Consequences of Overhumidification

• Condensation of water in the airway
• Reduced mucosal viscosity and risk of water intoxication
• Inefficient mucociliary transport
• Increased airway resistance
• Risk of pulmonary infection, surfactant dilution, atelectasis, and V/Q mismatch
• Inaccurate volume readings and potential workstation complications due to water obstruction
• Thermal injury to the epithelium (tracheitis)

Humidification Devices

• Goal: Achieve 30-35 mg/L absolute humidity in the trachea
• Ideal characteristics: humidify and warm gases to physiologic conditions, no added resistance or dead space, no increased risk of infection, safe, easy to use, and cost-effective

Heat and Moisture Exchangers (HMEs)

• Function: act as an "artificial nose" by retaining moisture and heat from exhaled gases
• Placement: between Y-piece and endotracheal tube/LMA
• Types: hygroscopic (use moisture-retaining chemicals), hydrophobic (contain pleated membranes), and steady state
• ISO 9360 Standard: specifies testing parameters for HMEs

Potential Hazards and Limitations of Heat and Moisture Exchangers

• Increased resistance: HMEs introduce resistance to the breathing circuit, potentially increasing the work of breathing and causing hypercapnia
• Obstruction: HMEs can become progressively or acutely obstructed by sputum, blood, or other fluids, requiring regular visual inspection and replacement after 24 hours of use
• Unpredictable performance with active humidification: combining HMEs with active humidifiers is generally not recommended
• Pediatric considerations: infants and neonates are more vulnerable to respiratory water and heat loss, and low dead space HMEs are preferred, but often have lower moisture retention.