Anesthetic circuits deliver anesthetic gases to the patient and may or may not allow rebreathing of expired gases. Rebreathing anesthetic circuits allow some expired gases to mix with fresh gases thus providing economy of inhaled anesthetic while limiting evaporative moisture loss from the lungs. Rebreathing circuits direct the expired gases through soda lime for absorption of expired carbon dioxide. Unidirectional valves are required to properly direct gas flow through the circuit. Soda lime and unidirectional valves increase the resistance to breathing through the circuit. It is recommended that patients weighing less than 7 kg be maintained on a non-rebreathing circuit to minimize the work of breathing during anesthesia. Non-rebreathing circuits are described later in this chapter.
Soda lime (calcium hydroxide) chemically removes carbon dioxide from the rebreathing circuit with the aid of activators such as sodium, potassium, and barium hydroxide (Figure 1). Soda lime also has a pH-sensitive color indicator such as phenolphthalein or ethyl violet. As the acidic carbon dioxide is absorbed by the soda lime, the granules become less alkaline and change color from white to blue, purple, or pink. A small amount of regeneration of soda lime may occur when the absorber is not in use, but the color change quickly returns in the presence of carbon dioxide. The soda lime granules should be changed when the color change has occurred for 3/4 of the visible area of the canister, after about 6 hours of use, or when the inspired carbon dioxide tension exceeds 1%. In the process of absorbing carbon dioxide, the soda lime granules become calcium carbonate and are difficult to crush between one's fingers. Fresh soda lime granules are soft and easily crushed.
The chemical reaction is exothermic and the soda lime canister will warm while the granules are absorbing carbon dioxide. However, the detection of heat production by placing one's hand on the canister may be difficult to appreciate when anesthetizing small animals such as a domestic cat.
When filling the soda lime canister, the granules should not be packed too tightly and should reach to within about 2.5 cm from the top of the canister (Figure 2). These measures will help to ensure adequate air space for contact with the surface area of the granules and will aid in preventing uneven channelling through the granules. The air space in between the granules is approximately half the volume of the soda lime canister. For efficient carbon dioxide absorption, the volume of the canister should be at least twice the patient's tidal volume.
Unidirectional valves are placed at the ends of the inspiratory and expiratory limbs of the rebreathing circuit. The valves should be rinsed and dried following use to prevent condensate build-up which may affect their function. The valve covers should be carefully hand-tightened to prevent leakage, yet not overly tightened. Clear valve covers allow visual confirmation of proper valve function (Figure 3).
The pressure relief valve (pop-off valve) is usually placed near the exhalation unidirectional valve (Figure 4). This valve is necessary to allow escape of excess gas delivered to the circuit. At the same time, a side port on the pressure relief valve allows conductance of the waste anesthetic gases to a scavenging system. Scavenging systems are described later in this chapter.
The pressure relief valve allows variable amounts of pressure to be placed within the breathing circuit. After closing the pressure relief valve, the animal's lungs can be effectively ventilated by manually squeezing the reservoir bag. Generally, the pressure relief valve remains in the open position at all other times. In some instances (ie. managing patients with pulmonary edema) the pressure relief valve may be intentionally left partially closed to allow 5-15 cm of water pressure to build up during the expiratory cycle (positive end-expiratory pressure, PEEP). Excessive pressure in the breathing circuit due to a closed pressure relief valve may cause barotrauma to the pulmonary structures, resulting in ruptured lung, ruptured airway, pneumothorax, hemothorax, hypotension, hypoxia, or death.
The reservoir bag should be appropriate for the size of the patient. The bag volume should be 2-5 times the patient's tidal volume. The reservoir bag allows accumulation of gas during exhalation to accommodate the patient's next inspiration. The reservoir bag also allows the anesthetist to assist or control ventilation. In addition, the reservoir bag protects against sudden build-up of pressure within the breathing circuit and provides a visual and tactile monitor of the patient's respiration. A reservoir bag that is too large may not allow adequate visualization of tidal motion of the bag. A bag that is too small may not contain adequate gas for the animal's normal inspiration.
Rebreathing circuits for large animals
Animals weighing more than 150 kg usually produce more carbon dioxide than can be eliminated by standard soda lime canisters in anesthesia machines designed for small animals or humans. In addition, the gas delivery hoses in the circuit may be too restrictive, leading to increased work of breathing and hypoventilation. Anesthesia for certain species weighing less than 150 kg may be best managed using a large animal anesthesia machine. For example, inhalation anesthesia for ostriches weighing more than 50 kg should be maintained using a large animal machine.
Anesthetic machines designed for large animal use are commercially available (Figure 5, Figure 6 & Figure 7). Large animal anesthetic machines function similar to small animal machines. Because of the large volume contained in the breathing circuit of large animal machines, induction of inhalation anesthesia may occur slowly unless the circuit is "primed". The large animal anesthetic circuit is primed by turning on the oxygen flow meter and setting the vaporizer dial to 4 or 5 % for several minutes prior to attachment to the endotracheal tube. Priming of the anesthetic machine is usually done prior to induction of anesthesia using an injectable agent. When priming an anesthetic machine, the "Y" piece orifice for the endotracheal tube connection must be occluded to prevent room pollution by anesthetic. High oxygen flow rates (eg. 8-10 l/min) will decrease the time required for priming the anesthetic machine.
Large animal anesthetic machines usually have a drain cock located on the bottom of the tubing leading from the soda lime canister. The drain is opened after use of the machine to aid in drying the condensed water vapor on the inner surfaces of the machine. Failure to replace the drain cock will result in entrainment of room air into the circuit and inability to maintain adequate anesthetic depth of the patient.
Closed vs. semi-closed rebreathing circuits
Circle rebreathing circuits are used as either closed or semi-closed circuits based on the fresh gas flow rate delivered to the circuit. When the fresh gas (oxygen) flow rate is equal to the patient's metabolic oxygen consumption (approximately 10 ml/kg/min), the circuit is functionally termed "closed". Operation of the closed circuit results in less consumption of inhaled anesthetic due to less waste gas production. Although the closed circuit is more economical to use than the semi-closed circuit, it is more sensitive to leaks in the circuit that may result in inadequate anesthetic depth, hypoxia, or both.
Oxygen consumption may be underestimated in febrile animals and in patients with hypermetabolic conditions such as hyperthyroidism, tachypnea, tachycardia, or malignant hyperthermia. Monitoring of patient oxygenation is suggested when using closed circuit flow rates.
Higher oxygen flow rates at the initiation of inhalation anesthesia will enhance the transition from injectable to gas anesthetic maintenance providing a more stable patient early in the procedure. Higher oxygen flow rates will also aid in prevention of inadequate anesthetic depth that occurs when there are leaks at anesthetic machine connections, gaskets, soda lime canister, or around the endotracheal tube cuff. Many precision vaporizers will not produce linear output of anesthetic at fresh gas flow rates less than 500 ml/min. Thus, there are certain instances when a higher, ie. semi-closed, oxygen flow rate may be more suitable.
A semi-closed oxygen flow rate means that greater than 2 to 3 times the patient's metabolic oxygen consumption is delivered to the circuit. Use of a semi-closed oxygen flow rate with a minimum flow of 500 ml/min will accommodate nearly all patients while avoiding problems common to the use of the closed circuit flow rate.
Non-rebreathing circuits are less complicated and less bulky than the rebreathing circuits described above. They are designed to provide oxygen and inhaled anesthetics with less resistance to breathing to small patients (< 7 kg) . These circuits decrease resistance to breathing by eliminating the soda lime canister and, in many cases, unidirectional valves. Removal of exhaled carbon dioxide from the breathing circuit is accomplished by using properly designed circuits and relatively high (compared to rebreathing circuits) fresh gas flow rates. Non-rebreathing circuits are less economical to use than rebreathing circuits. Greater heat loss and drying of the respiratory tract are also associated with the use of non-rebreathing circuits when compared to rebreathing circuits.
A system for classification of the many varieties of non-rebreathing circuits was devised by Mapleson (Figure 8). The most common non-rebreathing circuits used for anesthesia of small pets include the Bain circuit (modified Mapleson type D), the Ayre's T-piece (Mapleson type E), and the Jackson-Rees or Norman elbow (Mapleson type F).
The Bain non-rebreathing circuit is a coaxial system that delivers fresh gases through a small diameter tube within a larger diameter tube that carries exhaled gas to the rebreathing bag (Figure 9). The design allows the exhaled gases to warm the inspired gases, decreasing heat loss from the patient. The fresh gas flow rate must be greater than twice the minute ventilation rate to prevent rebreathing of carbon dioxide. Minute ventilation is calculated by multiplying the respiratory rate by the tidal volume (eg. 15 breaths per minute X 10 ml/kg = 150 ml/kg/min). Most anesthetists use an oxygen flow rate of 100-200 ml/kg/min, with 500 ml/min as a minimum. An adapter can be used with the Bain circuit to provide a reservoir bag port, a pressure relief valve, circuit pressure gauge, and a waste gas scavenger port (Figure 10).
The Mapleson type E systems include the Ayre's T-piece and the modified Ayre's T-piece. The expiratory limb of the metal T-piece can be extended with a rubber tube to prevent breathing of room air (Figure 11). The fresh gas flow rate to prevent rebreathing of carbon dioxide is the same as for the Bain circuit. A disadvantage of this circuit is that controlled or manual ventilation of the patient is difficult without causing a sudden increase in airway pressure. Addition of a reservoir bag to this system changes the classification to a Mapleson type F system.
The Mapleson type F systems (eg. Norman elbow) are basically Mapleson type E systems to which a reservoir bag has been added to allow improved manual ventilation (Figure 11). The tip of the reservoir bag is cut off allowing insertion of a plastic or aluminum cylindrical pressure relief valve. Waste gases can be collected and scavenged from the valve. A reservoir bag with a pressure relief valve in the center of the bag allowing both controlled ventilation and scavenging of waste anesthetic gases is also available (Figure 12). The fresh gas flow rate for the Mapleson type F systems is also 2 to 3 times the patient's minute ventilation (see calculations above).
Container and mask inductions
Closed containers such as a modified aquarium may be used to induce anesthesia for small animals (Figure 13). The oxygen flow rate should be 3-5 liters/min. There should be an inlet for the fresh gases as well as an exit port to allow scavenging of the waste anesthetic gases. The container should be regularly checked to insure that leakage around the lid does not occur. Once the animal is removed from the container, the scavenging system should be allowed to evacuate the anesthetic gases remaining in the container. Mask administration of anesthetic should be used as soon as the patient is manageable outside of the container.
Many animals are readily mask-induced using halothane or isoflurane if gentle restraint can be provided (Figure 14). Anesthesia can also be maintained with the mask. Controlled ventilation should be avoided in those patients in which anesthesia is maintained via a mask since gastric distention is a common sequelae of this practice. Compared to maintenance of anesthesia using a facemask, endotracheal intubation is associated with less room pollution from anesthetic, better control of anesthetic depth, and improved ability to manually ventilate the patient.
Most animals respond best to mask induction of anesthesia if they are held by a handler while the mask is applied. A clear plastic mask allows the animal to see and seems to be preferred by most species. When beginning the mask induction, it is recommended to let the animal breathe pure oxygen for 3-5 minutes as long as it is tolerant of the mask. Gradual increases in the anesthetic vaporizer concentration by 0.5% increments every 6 breathes usually results in smooth, excitement-free, anesthetic inductions.
The Norman elbow mask circuit may be used for anesthetizing rodents and similar small animals. Anesthetic systems incorporating an induction chamber with the Norman elbow mask circuit are useful when procedures requiring anesthesia for multiple animals at the same time are performed (Figure 15). Specialized systems for rodents utilizing a work platform for securing the anesthetized animal are ideal for tedious procedures (Figure 16).
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