Breathing Circuits

Calcium Chloride and Calcium Hydroxide

• Calcium chloride is used as a humectant to keep calcium hydroxide damp, eliminating the need for sodium or potassium hydroxide.

Anesthesia Valves

• Valves in anesthesia circuits are typically dome valves with a circular knife edge occluded by a light, rigid disk.
• The disk lifts off the knife edge when flow is initiated by the patient's inspiratory effort or positive pressure from the reservoir bag or ventilator.
• Disks are usually made of hydrophobic plastic to prevent water condensation from causing sticking.

Unidirectional Valves

• Unidirectional valves are used to prevent backflow in anesthesia circuits.
• Valves are typically mounted vertically, with the disk oriented horizontally, to ensure proper sealing.
• Failure to seal can result in rebreathing and increased apparatus dead space.

Nonrebreathing Systems

• Nonrebreathing systems require two one-way respiratory valves to permit inspiration of fresh gas and exhalation of alveolar gas.

Special Valves

• Special types of pop-off valves permit spontaneous or assisted respiration without tedious adjustment.
• Examples include the Steen valve, which is essentially two inverted knife-edge valves sharing a common disk.
• The Georgia valve adds a light spring loading to the Steen valve design, increasing the range of gas flows it can exhaust.

Soda Lime and Baralyme

• Wet soda lime is composed of calcium hydroxide (~80%), sodium hydroxide and potassium hydroxide (~5%), water (~15%), and inert substances.
• Baralyme, no longer produced, replaced sodium and potassium hydroxide with barium hydroxide in the chemical reactions.

Gas Inflow and Pop-off Valves

• In circle systems, the gas inflow nipple is incorporated with the inspiratory unidirectional valve or the carbon dioxide-absorbent canister housing.
• The preferred fresh gas inflow site is between the carbon dioxide absorber and the inspiratory valve.
• The location for other circuits depends on whether breathing is spontaneous, assisted, or controlled.
• During controlled breathing, a fresh gas flow of 70% of total minute ventilation is required in Mapleson D circuits.

Mapleson Circuits

• The Mapleson A circuit is less efficient during assisted ventilation than during spontaneous ventilation in terms of preventing rebreathing.
• If two-thirds of the fresh gas flow is washed into the lungs, it will provide 4 L/min of carbon dioxide-free alveolar ventilation, and the PAco2 will be normal despite obvious rebreathing of some carbon dioxide.

Proprietary Semiclosed Systems

• The Bain circuit may malfunction if the central tube (fresh gas delivery) becomes disconnected, either where it enters the corrugated outer tube or from its retaining spider at the patient end.
• Either disconnection effectively increases the apparatus dead space during the subsequent inspiration.

Breathing Circuits

• A variety of ancillary devices may be present in breathing circuits, including humidifiers, spirometers, pressure gauges, filters, gas analyzers, PEEP devices, and waste gas scavengers.
• Breathing circuits can be organized by method of carbon dioxide elimination, which includes semiclosed and closed systems, such as circle and to-and-fro systems.

Semiclosed and Closed Systems

• Semiclosed and closed systems rely on chemical absorption of carbon dioxide.
• Circle breathing systems have a carbon dioxide absorber, fresh gas inflow, and pop-off valve.
• The placement of these components affects the flow and concentration of gases.

History of Device Development

• Breathing circuits have been an important concern in anesthesia since the start of the field.
• The earliest circuits were mechanically simple, with differences related to the characteristics of the primary anesthetic agent.
• The development of breathing circuits has been shaped by factors such as convenience, economy, and the need to control heat and humidity, measure patient variables, and control contamination of the operating room environment.

Connection of the Patient to the Breathing Circuit

• The patient is connected to the breathing circuit using an anesthesia mask, supraglottic device, or tracheal tube.
• Anesthesia masks are available in a variety of sizes and styles to accommodate different facial contours and have an inflatable or inflated cuff to seal to the face.

Carbon Dioxide Elimination

• Methods of carbon dioxide elimination include chemical absorption, nonrebreathing valves, and open-drop ether or a T-piece without a reservoir.
• Nonrebreathing valves separate exhaled gases from inhaled gases, and circuits that use high flows can behave as if they were nonrebreathing.
• Open-drop ether or a T-piece without a reservoir releases exhaled carbon dioxide into the atmosphere.

Circuit Malfunction and Safety

• Understanding the functional characteristics of each circuit is vital for the anesthesiologist to prevent circuit malfunction and ensure safety.
• Increasing fresh gas flow, for example, can markedly increase the work of breathing.

Breathing Circuits

• A variety of ancillary devices may be present in breathing circuits, including humidifiers, spirometers, pressure gauges, filters, gas analyzers, PEEP devices, and waste gas scavengers.
• Breathing circuits can be organized by method of carbon dioxide elimination, which includes semiclosed and closed systems, such as circle and to-and-fro systems.

Semiclosed and Closed Systems

• Semiclosed and closed systems rely on chemical absorption of carbon dioxide.
• Circle breathing systems have a carbon dioxide absorber, fresh gas inflow, and pop-off valve.
• The placement of these components affects the flow and concentration of gases.

History of Device Development

• Breathing circuits have been an important concern in anesthesia since the start of the field.
• The earliest circuits were mechanically simple, with differences related to the characteristics of the primary anesthetic agent.
• The development of breathing circuits has been shaped by factors such as convenience, economy, and the need to control heat and humidity, measure patient variables, and control contamination of the operating room environment.

Connection of the Patient to the Breathing Circuit

• The patient is connected to the breathing circuit using an anesthesia mask, supraglottic device, or tracheal tube.
• Anesthesia masks are available in a variety of sizes and styles to accommodate different facial contours and have an inflatable or inflated cuff to seal to the face.

Carbon Dioxide Elimination

• Methods of carbon dioxide elimination include chemical absorption, nonrebreathing valves, and open-drop ether or a T-piece without a reservoir.
• Nonrebreathing valves separate exhaled gases from inhaled gases, and circuits that use high flows can behave as if they were nonrebreathing.
• Open-drop ether or a T-piece without a reservoir releases exhaled carbon dioxide into the atmosphere.

Circuit Malfunction and Safety

• Understanding the functional characteristics of each circuit is vital for the anesthesiologist to prevent circuit malfunction and ensure safety.
• Increasing fresh gas flow, for example, can markedly increase the work of breathing.

Breathing Systems Classification

• Mapleson classification is used to categorize breathing systems, with systems classified as Mapleson A (Magill attachment, Lack, and Humphrey A) being the most efficient for spontaneous, unassisted ventilation.

Limitations of Model Studies

• Model studies have several limitations, including:
  • Embodiment of nonphysiologic states, such as a lack of responsiveness to carbon dioxide and simplified flow patterns.
  • Studies being conducted in awake volunteers, whose metabolic rates and physiologic responses differ from those of anesthetized patients.
  • Imprecise endpoints being used, such as rebreathing of carbon dioxide identified by capnography rather than by an increase in alveolar or arterial carbon dioxide concentration.

Mapleson Circuits

• Mapleson A circuit is optimal for spontaneous respiration.
• Mapleson C system is a simple bag and mask.
• Moving the pop-off valve to the bag tail (Cʹ) is a major improvement, as it permits more mixing of fresh and exhaled gas.
• Mapleson D, E, or F (Jackson-Rees) circuits are most efficient for assisted or controlled ventilation.

Universal F System

• The Universal F system (King Systems, Noblesville, IN) is essentially equivalent in design and function to the Japanese Mera F system.
• It is manufactured and available in the United States.
• It uses a coaxial arrangement and mounts on standard circle system hardware.
• It has larger diameter inspiratory and expiratory tubing to decrease resistance, corrugation of the inspiratory tube to prevent kinking, and the ability to be used as a transport system.

Criticism of Published Analyses

• A general criticism of published analyses of breathing circuits is the failure to distinguish between the quantity of re-inspired carbon dioxide and minimum inspired carbon dioxide tension.
• Capnography is often used to measure airway concentration, but it can be misleading if airway carbon dioxide falls slowly with inspiration, as it may be interpreted as no rebreathing, even though a significant amount of carbon dioxide has been re-inspired.