Anesthesia Breathing Systems

Introduction

• Anesthesia breathing systems are made up of components that connect the patient to the anesthesia machine to deliver oxygen and anesthetic gases to the patient and eliminate CO₂. Most breathing systems have similar components but are configured differently.^1,2
• The adjustable pressure limiting (APL) valve is a one-way, spring-loaded valve that allows the maintenance of variable pressures within the anesthesia breathing system and control of the patient’s airway pressures.¹
• The reservoir bag allows the collection of fresh gas flow (FGF) during expiration and monitoring of the breathing pattern in a spontaneously ventilating patient. They are usually made of rubber or plastic and come in a range of sizes from 0.5 to 3 liters. Most adult systems use a 2-liter reservoir bag.¹
• The inspiratory limb allows passage of FGF to the patient and the expiratory limb allows passage of expired gas from the patient. While the length of the tubing varies, the diameter of the tubing is standard across all systems: 22 mm for adults and 18 mm for children.¹
• The position of the various components and the presence of valves determine the functional characteristics and resistance of the different anesthesia breathing systems.
• Circle or rebreathing systems include a CO₂ absorber and unidirectional valves, while nonrebreathing systems do not include a CO₂ absorber.

Circle Systems

• The circle system is the most commonly used anesthesia breathing system. The components of a circle system are (Figure 1)^1-3
  • CO₂ absorbent (i.e., soda lime)
  • Two unidirectional valves (inspiratory and expiratory)
  • Fresh gas entry
  • Y-piece connecting to the patient
  • Reservoir bag
  • APL valve
  • Low resistance interconnecting tubing
• The defining design element of a circle system is a CO2 absorber that effectively eliminates CO₂ from exhaled gases, allowing rebreathing. This capability reduces the need for high FGF and enables the conservation of heat and inhaled anesthetic agents (Figure 1).
• A Y-piece septation prevents the mixing of fresh and exhaled gases and minimizes dead space in the circuit. Exhaled gas travels through the Y-piece to an APL valve and reservoir bag before being filtered and returned to the patient.^1-3
• The apparatus dead space in a circle system is minimal and is limited to the area distal to the point of inspiratory and expiratory gas mixing at the Y-piece.
• The unidirectional valves in circle systems function as check valves and increase the resistance to breathing. The expiratory valve can also become stuck by water vapor, leading to an increase in dead space. Malfunction of either unidirectional valve may allow rebreathing of CO₂, resulting in hypercarbia.
• During the initial 5-10 minutes of use, a high FGF is required to equilibrate the breathing system with the desired mixture, after which low FGF of 0.5 liters per minute can be used.

• Advantages of a circle system^3,4
  • Respiratory support can be adjusted to optimize the FGF rate and volume of volatile anesthetic agents consumed.
  • Reduction in operating room pollution compared to other systems
  • Conservation of moisture, heat, and anesthetic
• Disadvantages of a circle system^3,4
  • Increased complexity
  • Multiple components with the potential for disconnection
  • Increased resistance
  • Potential for difficulty assessing fractional exhalation composition
  • Potential to rebreathe CO₂ in the event of exhausted CO₂ absorption components
  • Subclassifications
  • A circle system can be closed or semi-closed. In a closed circle system, the APL valve is completely closed. while in a semi-closed system, the APL valve is open to allow excess gas to be removed from the system.¹
  • Closed circle systems allow for total rebreathing. These systems have demonstrated greater control in maintaining constant alveolar concentrations of anesthetic but require a deeper understanding of the uptake and distribution to operate. When low-flow anesthetic gases are used, the inflow of fresh gas can be titrated to match the patient’s oxygen consumption and anesthetic uptake.^2,5
  • Semi-closed systems allow for partial rebreathing and are the most common breathing systems in the United States.^2,5

Noncircle Systems

• Noncircle systems include insufflation, open-drop anesthesia, and semi-open systems.
• Insufflation is analogous to “blow by,” where oxygen with or without anesthetic is blown into the patient’s airway without direct contact, such as via a face mask. This breathing system is primarily used in pediatric patients where mask placement may be resisted. Insufflation can also occur directly into the airway/ trachea via a device such as a bronchoscope.
• Open-drop anesthesia is the simplest breathing circuit and is no longer in use. Gauze soaked in anesthetic liquid, historically ether or chloroform is placed over a patient’s face. Inhalation causes vaporization of the anesthetic as air moves through the gauze, delivering highly unpredictable levels of anesthetic to the patient.²
• Semi-open systems or Mapleson circuits:
  • The patient is connected to the system via a mask linked by corrugated tubing to a reservoir bag and an FGF inlet where mixed gas will be delivered without the presence of a CO₂ absorber.
  • The movement of fresh gas through the FGF inlet forces exhaled gas into the environment.
  • These systems are generally referred to as Mapleson circuits and are separated into classes A-F depending on the position of the FGF inlet and APL valve.^2,5 (Figure 2)
  • While less complex than closed circuits, FGF requirements are higher which leads to greater gas waste, operating room pollution, and loss of heat and/or moisture.^2,4,5

• • The APL valve is close to the patient in Mapleson A, B, and C circuits, and the reservoir bag is on the opposite side of the circuit. Interchanging the position of the APL valve and the FGF inlet converts a Mapleson A circuit to a Mapleson D circuit. Mapleson E and F circuits do not have an APL valve.
  • The Mapleson A circuit is the most efficient Mapleson circuit for spontaneous ventilation because an FGF equal to minute ventilation is sufficient to prevent rebreathing.¹ The efficiency of Mapleson circuits (measured as the lowest FGF that prevents rebreathing) for spontaneous ventilation is A > D, F, E > C, B (mnemonic: A dog can bite)
  • The Mapleson D circuit is most efficient for controlled ventilation because FGF forces alveolar air away from the patient and towards the APL valve. The efficiency of Mapleson circuits (measured as the lowest FGF that prevents rebreathing) for controlled ventilation is D, F, E > B, C > A (mnemonic: dog bite can ache)
  • A commonly used configuration is the coaxial Mapleson D circuit or Bain system, where the fresh gas inlet tubing is inside the breathing circuit. FGF is delivered via the inner tube and the waste gases are removed via the outer tube. (Figure 3).¹ This modification decreases the circuit’s bulk and retains heat and humidity better than a conventional Mapleson D circuit. A disadvantage of this coaxial system is the possibility of kinking or disconnection of the inner tubing resulting in the delivery of a hypoxic gas mixture.
  • A Mapleson D circuit is often used for manual ventilation while transporting patients. A modified Mapleson D circuit can also be used to deliver continuous positive airway (CPAP) pressure to the nonventilated during one-lung ventilation.

• • Advantages of Mapleson circuits
    • Inexpensive, lightweight, and simple to use
    • Low resistance circuits
    • Changes in FGF result in rapid changes in inspiratory gas concentrations
  • Disadvantages of Mapleson circuits
    • Higher FGF needed to prevent rebreathing
    • Loss of heat and humidity unless a coaxial system used
    • Greater environmental pollution

Portable Ventilation Devices

Self-Reinflating Portable Ventilation Devices (i.e., Ambu bags)

• Also known as bag-valve-mask devices, these include a self-inflating bag and a nonrebreathing valve that allows air within the bag to pass to the patient while blocking the expiratory port⁶ (Figure 4).

• The nonrebreathing valve prevents mixture of expired and fresh gases in a self-inflating bag by allowing gas to escape into the atmosphere upon expiration. The valve closes during inspiration to prevent fresh gas from escaping, ensuring the patient receives the contents of only the self-inflating bag (Figure 5).

• Manual compression of the self-inflating bag is needed to deliver oxygen during positive pressure ventilation.
• Passive oxygenation methods, such as administering oxygen through blowby or using a mask seal during spontaneous ventilation, cannot be achieved with self-inflating systems. This limitation arises because oxygen flow in self-inflating systems necessitates manual compression of the self-inflating bag.
• Rigidity gives the reservoir bag “memory,” and allows for spontaneous reexpansion during expiration.
• An external supplemental oxygen source can be used to deliver oxygen at different concentrations depending on the status of the patient. An oxygen flow of above 15 L/minutes is needed to attain close to 100% oxygen.⁶ The fraction of inspired concentration delivered to the patient is directly proportional to the oxygen inflow concentration and flow rates and inversely proportional to the minute ventilation delivered to the patient.
• A tight seal to a patient’s face during self-reinflation is imperative to ensuring pure oxygen is delivered on the next administered breath as to not entrain room air and dilute the inspired oxygen concentration.
• While ventilating a patient with an endotracheal tube, the squeezing of the bag should be synchronized to the inspiratory efforts of the patient.
• This device cannot be used to deliver CPAP to the patient.⁶ However, an adjustable, spring-loaded disk valve attached at the expiratory port can be used to provide positive end-expiratory pressure up to 20 cm H₂O (Figure 4). This is only effective with a tight-fitting facemask or a cuffed endotracheal tube.
• Self-inflating bags can facilitate resuscitation using room air, even when a medical gas oxygen source is unavailable.

Non-Self-Reinflating Breathing Devices

• Also called a hyperinflation system
• Must receive a constant flow from an oxygen source to maintain inflation
• A summary of the different anesthesia breathing systems is listed in Table 1.