Anesthetic breathing circuits/system deliver oxygen and anesthetic gases to the subject. They also assist in ventilation and removing exhaled carbon dioxide. These systems can be divided into two main categories: re-breathing and non-rebreathing systems. Re-breathing systems involve passing of expired gases through soda lime canister or other chemical absorbers to eliminate carbon dioxide. The subject then rebreathes this filtered gas. However, the use of rebreathing systems is recommended in subjects that weigh over 7 kg since the unidirectional valves used in the maintenance of flow direction leads to increased resistance of breathing.
On the other hand, non-rebreathing circuits are generally used for subjects under 7 kgs. These systems deliver oxygen and anesthetic gases with less resistance to breathing. Unlike, the rebreathing systems, non-rebreathing systems rely on specially designed circuits and higher gas flow to exhaust carbon dioxide.
While selecting a breathing system, it is important to consider that different types of breathing systems yield different degrees of resistance to breathing and vary in dead space volumes. The respiratory efforts put in by the subjects is highly influenced by the degree of resistance offered by these systems. Greater the resistance to the gas flow, greater the respiratory effort by the subject; excessive effort can lead to respiratory muscles fatigue, depress respiration and increase the oxygen need of the animal. The volume of dead space determines the amount of expired carbon-rich gas that remains in the breathing system. Significant re-inhalation of this gas by the subject can result in the rise of blood carbon dioxide concentration and other adverse effects.
Other factors to be considered when selecting a breathing system, apart from the ease and efficiency of carrying out assisted ventilation, are tidal volume and minute volume of the subject’s respiratory system. Tidal volume refers to the volume of gas drawn into the respiratory tract with each breath, while minute volume is calculated as the product of the tidal volume and the respiratory rate. However, the latter measure may not always be equivalent to the flow of gas delivered by the breathing system. Usually, the gas flow must be three times the subject’s minute volume. Regardless of the choice of anesthetic breathing system, a face mask, nasal tube or an endotracheal tube will be required to connect it to the subject. Many breathing systems are equipped with a reservoir for unused gas to reduce the required fresh gas flows by anesthetic machine.
The following table shows the recommended fresh gas flow rates for different anesthetic breathing systems,
|Body weight (kg)||Estimated tidal volume (ml)||Minute volume (l)||Flow rate(l/min)|
|Open system||T-piece or Bain’s system||Magill||Closed circuit|
Open Breathing Systems
The open face mask is the most widely used breathing system, which provides a simple and convenient way of delivering anesthetic agents to the subject. For these systems, the gas flow should be sufficiently high to avoid rebreathing of exhaled gases and the dilution of anesthetic gases. The expired gases pass around the edges of the face mask.
This system relies on a relatively high gas flow for larger animals. In smaller subjects, the concentric face mask may be used, however, in currently available masks, the gas extraction rate is usually quite high resulting in dilution of fresh gas. Alternatively, low flow masks combined with passive scavenging system or down-draft table may be used to remove WAGs. Further, open breathing systems only allow manual compression in case assisted ventilation is needed.
Ayre (1937) first described the T-piece system as a low-resistance, low-dead-space breathing system for use in infants and young children. The system uses a T-shaped tubing; one end is connected to the subject while the other end is attached to a length of tubing to serve as a small reservoir for anesthetic gases. This system reduces the required gas flow to about twice the subject’s minute volume without rebreathing.
The volume of the reservoir limb should exceed one-third of the subject’s tidal volume. During expiration, exhaled gas fills this limb which is then washed out by fresh gas from the sidearm during the pause between the next inspiration. Ventilation can be controlled using an open-ended reservoir bag to the expiratory limb or with the help of a mechanical ventilator attached to the reservoir limb.
Effective usage of this system can be done by directly connecting the T-piece to the endotracheal tube or a close-fitting face mask. Measures to reduce dead space should be taken to extract the full potential of this system. The T-piece system is ideal for use in small subjects since it offers low resistance to breathing and can be easily constructed using Luer adaptor ‘Y’ and plastic tubing.
Bain Coaxial Breathing System
The Bain system has the fresh gas inflow tubing running inside the reservoir limb and functions similarly as the T-piece system. The system has a light-weight design that reduces the pull on the endotracheal tube and the possibility of accidental extubating. Further, since the positioning of breathing valves and bags are away from the subject, this allows assisted ventilation without disturbing the subject’s environment or surgical procedures. The system has a low dead space and allows easy scavenging of waste anesthetic gases.
The system has two available modifications; the expiratory limb can be terminated with a pop-off valve and a reservoir bag, or the limb can be mounted with an open-ended reservoir bag. The pop-off value may not be a suitable addition to the system when used for small subjects since the valve increases the resistance of breathing. Mechanical ventilators can also be connected to the reservoir limb as in the T-piece system.
Magill Breathing System
The Magill system uses a reservoir bag that is connected to the subject via a length of corrugated tubing. The system uses an expiratory pop-off valve as close to the subject possible to reduce equipment dead space. However, the system is not suitable for use in subjects with a body mass under 10 kg. For effective use of the system, it is recommended that the system is attached to the subject by an endotracheal tube or a close-fitting face mass. On correct usage, the WAGs can be scavenged by a suitable attachment to the expiratory valve.
The Magill breathing system is economical due to its preferential elimination of carbon dioxide-rich alveolar gas. When the subject first exhales, the expired gases are from the subject’s anatomical dead space making it carbon dioxide free. This gas travels up the corrugated tubes into the reservoir bag. As the pressure increases, the remaining expired gases are exhausted through the expiratory valve. The continuous flow of fresh gas flushes any remaining carbon dioxide-rich alveolar gas during the pause before the next inspiration, down the breathing system.
However, in small subjects, the system imposes a significant resistance to expiration. Additionally, the typical dead space of the system represents a significant portion of the tidal volume of the subject.
Closed Breathing System
The Closed breathing systems usually employ soda lime canisters for the absorption of carbon dioxide. The system is usually used in the anesthetization of large animals (>20 to 30kg) due to considerably lower fresh gas flows. The circle system is the most popular variety of closed breathing system in use. However, light-weight disposable systems with low-resistance values have gained popularity in use for small subjects.
The use of nitrous oxide should be avoided if possible due to the build-up in the concentration of this gas in the breathing system which consequently affects the concentration of oxygen. Another important aspect to remember while using closed systems is that the concentration of anesthetic shown by the vaporizer will not reflect the actual value. Apart from these considerations, an advantage of the closed system is that they conserve the heat and moisture.
A semi-closed breathing system allows some rebreathing of expired gases and does not use carbon dioxide absorption.
Rebreathing circuits permit recirculation and reuse of expired oxygen and anesthetic vapors, making them more economical than non-rebreathing systems. The humidification of the inspired gas and the heat generated from the soda lime during absorption of CO2 helps preserve heat and moisture of the subject. However, the one-way values of the system, soda lime canister, and pressure relief valve lead to resistance to gas flow.
Non-rebreathing systems offer less resistance and less mechanical dead space. These systems allow rapid manipulation of the depth of anesthetic by the adjustment of the fresh gas inflow. However, these systems produce significantly greater waste of carrier gas and anesthetic agent making them less economical than the rebreathing systems. Further, the high flow dry cool gas can impact the heat and humidity loss in the subject.
Fish, R. E. (2008). Anesthesia and analgesia in laboratory animals. Amsterdam: Elsevier.
Flecknel, P. (2009). Laboratory Animal Anaesthesia. Elsevier.