Anesthesia Principles and Monitoring

Chapter 23


Anesthesia Principles and Monitoring




Anesthesia Principles and Monitoring


Command of the pharmacologic principles of uptake and distribution is essential for understanding the concepts of anesthetic action, effects, and interaction. In its simplest form, general anesthetic action occurs when a critical dose of drug reaches areas of the brain or spinal cord, causing a decrease in the amount and strength of output from these centers and allowing the animal to enter into a controlled anesthetic state. By the same token, when drug levels fall below these critical levels, the return of consciousness begins, and the animal displays a lighter anesthetic plane. Maintenance of a stable anesthetic plane depends on preservation of this critical level of anesthesia for the duration of the procedure. Regardless of the method of administration (intravenous, inhalational), this concept remains the same.


Two methods may be used to initiate and maintain a steady state concentration of drug at the level of the brain. One involves administration of a loading dose, usually a multiple of the target dose, followed by a constant rate infusion of the drug at a lower dose (maintenance dose). The other method is to start the infusion at the maintenance dose, over time (approximately 5 half-lives) building the blood concentration to the maintenance level. Administration of any drug into the body is balanced by two factors: redistribution to other areas of the body, and metabolism and excretion of the drug. These factors define the requirements for both the loading dose and the maintenance dose (which balances the drug infused into the body against the decrease in central nervous system concentration by redistribution and metabolism [i.e., clearance]). Drugs that are more lipid soluble take longer to achieve a steady state concentration, as they readily redistribute to many areas of the body (i.e., they have a larger volume of distribution).


Administration of an intravenous drug by constant rate infusion (CRI) allows more precise dosing to a target drug level or effect. It also eliminates the peaks and troughs of serum drug concentration that occur with bolus dosing. Many drugs used in anesthesia, including benzodiazepines, opioids, propofol, ketamine, and alpha-2 agonists, may be administered as a CRI. In addition, many vasoactive drugs used during anesthesia are administered as a CRI because of their short half-lives. For this reason, rapid calculation of CRI dosages is a necessary skill for the advanced anesthetist. CRIs may be administered via a syringe pump, or by preparation of a dilution of drug in normal saline or other appropriate isotonic fluid, which then can be administered by pump or drip set. Only diazepam is unable to be diluted in this manner, because of its adsorption to plastics and its insolubility in water. Some schemata for calculating CRI rates are provided in Box 23-1. CRIs are useful in the postanesthetic period as well, allowing the veterinarian to tailor a specific rate and dose of drug to the patient’s needs.



Box • 23-1   Dilution for Use With Syringe Pump or Intravenous Fluid Bag




1. Select a desired rate of fluid administration (e.g., 10 mL/hr). This should be based on the overall rate of fluids that the patient is receiving and should be at least 1 mL/hr to ensure accurate delivery.


2. Select a desired volume of infusion; this may be based on the size of the syringe or fluid bag, or on the amount of drug that will be required to make the dilution (e.g., for controlled substances).


3. Given the fluid rate and total volume to be administered, calculate the number of hours of infusion by dividing the total volume by the rate.


4. Knowing the duration of the infusion and the desired dose of drug in mg/kg/hr, first calculate the mg/hr of drug by multiplying the mg/kg/hr by the animal’s weight.


5. Multiply the mg/hr of drug by the total duration of infusion (calculated in step 3) to determine the total mg of drug to be added to the solution. For higher volumes of drug (>10 mL), an equal amount of diluent should be removed from the bag or syringe before the drug is added.


6. Example:


    Formulate a dilution of morphine to be administered at a rate of 5 mL per hour to a 25 kg dog. The diluent will be a 100 mL bag of saline, and the dose is 0.1 mg/kg/hr.



The rule of 6 technique:



1. Using this technique, drugs to be given at 1 mcg/kg/hr will be administered at 1 mL/hr.


2. The body weight of the animal in kg is multiplied by 6 to obtain the number of mg that is added to 100 mL of diluent.


3. Once diluted, the solution is administered at 1 mL/hr to give a dose of 1 mcg/kg/min; to increase the dose, just increase the rate by the equivalent amount (i.e., 5 mcg/kg/min is 5 mL/hr).


4. Using a multiplier of 60 for step 2 results in a 1 mL/hr rate equivalent to 10 mcg/kg/min.


5. Example:



From the perspective of general anesthetic action, the organs and tissues of the body may be divided into three groups: the vessel-rich group, which are organs that consistently receive a large percentage (up to 75%) of the cardiac output and frequently have a high rate of oxygen consumption (e.g., heart, brain, kidney); the muscle group; and the vessel-poor group, consisting of tissues with low metabolic rates (e.g., fat). Because of the high rate of blood flow, the administered dose of drug is rapidly delivered to the vessel-rich group after administration. In the context of anesthesia, this represents the loading or induction dose of anesthetic. Swift delivery of the critical dose of anesthetic to the brain allows for a rapid and smooth induction. This rapid onset of anesthesia (if administered only as a single dose) is balanced by rapid redistribution of the drug to the other areas of the body, which causes the central nervous system anesthetic levels to drop and the patient to begin to awaken. It is important to note that this awakening is caused by redistribution of drug, rather than by metabolism or excretion of drug. This is the scenario for most drugs that are relatively soluble in blood and tissue. Some drugs (e.g., thiobarbiturates) redistribute quickly but have a high solubility and can accumulate in the fatty tissues of the body, especially after prolonged administration. Drugs such as the newer inhalant anesthetics (notably desflurane) are not very soluble in blood and have minimal redistribution. In this case, exhalation (excretion) of the drug is the primary method by which brain levels of drug decrease. Anesthetic drug delivery and redistribution are dependent on cardiac output, and anesthetic action or offset may be delayed in patients with decreased cardiac output (and vice versa).


Administration of volatile inhalant anesthetics parallels the intravenous CRI model. The dose of inhalant to be administered is related to the minimum alveolar concentration value for the species and the physical properties of the inhalant used (e.g., blood/gas solubility). The necessary multiple of minimum alveolar concentration to produce anesthesia in a given patient will vary, depending on the use of other anesthetic or analgesic drugs (e.g., opioids), as well as on physical parameters (e.g., body temperature, age). Minimum alveolar concentration value is delineated in volume percent of inhalant vaporized in a carrier gas (usually oxygen or an air/oxygen mixture). The concentration of inhalant in the alveoli parallels the concentration of inhalant in the brain, as long as cardiac output is adequate. The time necessary to achieve a steady state of anesthetic will depend on similar parameters to the CRI model: the minute ventilation (which is functionally the rate of delivery of drug into the body), the cardiac output (delivery of drug to the target organs), the speed of redistribution (away from the target organs), and the amount of drug that is eliminated from the circulation (by metabolism or exhalation). The speed of inhalant action between different anesthetics will thus depend on the solubility of the inhalant in blood and tissue, in addition to the delivered concentration, minute ventilation, and cardiac output.


If the inhalant is delivered at minimum alveolar concentration at the initiation of anesthesia, it will take some time for the anesthetic concentration in the brain to build up to the critical level for anesthesia, because of the kinetics of redistribution and elimination. This is analogous to starting an injectable drug at a constant rate infusion maintenance dose, without a loading dose. By contrast, if a higher percentage of inhalant is delivered at the start of anesthesia, the drug level in the brain will increase more rapidly, resulting in a faster time to effect. Once at the effective level, the delivered percentage may be decreased and maintained at a steady state. Inhalants that are less soluble in tissue (e.g., desflurane) will be less dissolved in the blood and will have less redistribution (less soluble in tissues) and thus a faster equilibrium to the delivered percentage in the lungs and target organ. The opposite is true for inhalant anesthetics with high blood/gas solubility (e.g., methoxyflurane). The use of adjunctive analgesic and anesthetic drugs decreases minimum alveolar concentration requirements, and adjunctive drugs (e.g., premedication) should be taken into account when an inhalant loading dose is chosen.


At the induction of anesthesia, a relatively high concentration of inhalant is present in the arterial blood, with a relatively low concentration of anesthetic in the venous blood, which reflects uptake of the inhalant by the tissues of the body; exhaled gas after the first breaths will contain little to no anesthetic agent. Because the transfer from blood to tissue follows a concentration gradient, as anesthesia continues, less anesthetic will be taken up by the tissues (their concentration begins to equal that of the arterial blood) and the venous anesthetic concentration rises. As the amount of anesthetic returned to the lungs increases, anesthetic levels in the exhaled gas rise. Once an equilibrium exists between arterial and venous anesthetic concentrations, relatively little additional anesthetic needs to be supplied to the circuit (assuming rebreathing of exhaled gases, which contain an equal percentage of inhalant).


This phenomenon is the concept behind low-flow (closed-circuit) anesthesia; at equilibrium, only enough inhalant must be supplied to make up for that lost from the system by redistribution or metabolism, and only enough oxygen must be supplied to meet the animal’s metabolic demands (approximately 5 mL oxygen/kg/min, depending on temperature and metabolic factors). Higher inhalant concentrations may have to be delivered at low flows, although the total amount of agent delivered into the circuit is significantly less than that delivered at moderate to high flows. Closed-circuit anesthesia in human beings and animals has been reviewed elsewhere.18,120,170



Anesthetic Drug Delivery


For inhalant anesthetics to have clinical effect, the agent(s) must be delivered from a tank or vaporizer through the anesthesia machine, into the patient’s lungs and bloodstream, and thereby to the brain and spinal cord. The inhalant must be delivered at an accurate, constant concentration in a manner that balances maintenance of anesthesia with support of the patient’s physiologic functions, including oxygenation, ventilation, and cardiovascular stability. Anesthesia machines are designed to deliver a fresh gas flow of oxygen with or without additional gases through a calibrated vaporizer and into an anesthesia circuit. Although some machines are able to deliver only 100% oxygen as a carrier gas, other machines have flowmeters that allow blending of oxygen with medical air or nitrous oxide to decrease the fractional inspired oxygen concentration.



Vaporizers


With the exception of nitrous oxide, inhalant anesthetics are liquid at standard temperature and pressure (STP). Contained in the vaporizer reservoir, the volatile agents reach equilibrium between the liquid and gas phases within the reservoir. The pressure at the point where the gas of the anesthetic agent is in dynamic equilibrium with the liquid agent is the saturated vapor pressure. The concentration of inhalant at the saturated vapor pressure is too high to be clinically useful, and the time needed to achieve this concentration is too long. A vaporizer is required to produce a clinically useful concentration of inhalant vapor rapidly, so that changes to the vaporizer dial are accurately reflected by vaporizer output.


Vaporizers are designed and calibrated to deliver a constant concentration of anesthetic vapor over a wide variety of carrier gas flows and ambient temperatures. The carrier gas used for initial calibration of some vaporizers ranges from 21% oxygen (medical air) to 100% oxygen, depending on the manufacturer. Therefore, if N2O or medical air (primarily nitrogen) mixed with oxygen is used as the carrier gas, the output may be different than that indicated by the dial.251 Also, as carrier gas flow increases above 5 L/min, or below 0.1 L/min, the concentration of vapor delivered from the vaporizer may be significantly different from the dial setting.9 Most vaporizers are designed for the ambient temperature range typically encountered in the operating room setting. However, extreme temperatures, such as those that may be encountered in an outdoor field situation, may affect vaporizer output, usually with warmer weather increasing output and colder weather decreasing output.114 At high altitudes, because of the relatively lower atmospheric pressure, vaporizer output may be slightly higher than the dialed concentration for most vaporizers, with the exception of the desflurane vaporizer.


Vaporizers are classified by their regulation of vapor output, method of vaporization, vaporizer location in the anesthetic circuit, temperature compensation, and agent specificity. Most vaporizers in use today are out of circuit, variable-bypass, flow-over, temperature compensated, and agent specific.



Vaporizer Output: Output from vaporizers may be variable-bypass or measured flow. Variable-bypass is made by splitting the incoming fresh gas flow to direct a variable portion through the vaporizing chamber and the remainder through a bypass chamber. The two flows are merged before exiting the vaporizer, producing the concentration set by the dial (Figure 23-1). Variable-bypass vaporizers are the most common type of vaporizer in clinical use today. Measured-flow vaporizers use two different flowmeters, one that passes a small amount of carrier gas through the vaporizer, fully saturating this gas with anesthetic vapor, and a larger diluent flow, which does not contact any inhalant and dilutes the saturated carrier gas to the appropriate concentration for inhalation by the patient. The flows for this type of anesthetic machine are set independently, and calculations are necessary to determine the gas flows necessary to result in specific anesthetic concentrations. For these reasons, measured-flow vaporizers are rarely used in modern anesthetic practice.




Methods of Vaporization: Methods of vaporization for inhalant anesthetics include flow-over, bubble-through (both of these require a vaporizer chamber), and direct injection. In flow-over vaporizers (see Figure 23-1), the carrier gas passes over a reservoir of inhalant and picks up the anesthetic vapor as it does so (the concentration of vapor being relative to the specific vapor pressure of the agent). Often, wicks are incorporated into the vaporizer to increase surface area for contact between the inhalant and the carrier gas. These are the vaporizers most commonly used in current clinical practice. Bubble-through vaporizers bubble the carrier gas through the bottom of the reservoir of anesthetic to pick up the vapor. This type of vaporizer is used in measured-flow machines such as the “copper kettle,” and is rarely used in modern anesthetic practice. Injection-type vaporizers inject an atomized spray of inhalant into the stream of the carrier gas, rapidly vaporizing the inhalant, as is done in desflurane vaporizers. Liquid anesthetic agent may also be directly injected into the anesthetic circuit by the anesthetist.30



Circle Systems: Vaporizers may be located in the anesthetic circuit or out of the circuit. In circuit vaporizers have highly variable output depending on ambient temperature, patient ventilation, and the volatility of the selected agent. They do not have temperature compensation and may incorporate wicks for the vaporization of agents with low vapor pressures (e.g., methoxyflurane). The wicks should be removed for use with more volatile agents such as isoflurane and sevoflurane.197 With an increase in respiratory minute volume, the patient will draw a greater volume of carrier gas over the vaporizer, resulting in an increase in the concentration of inhalant in the circuit. In practice, this may result in a variable anesthetic plane. In circuit vaporizers are not often used for veterinary anesthesia.


Out of circuit vaporizers are located before the common gas outlet of the machine and are not part of the patient circuit. Consequently, out of circuit vaporizers deliver a constant dose of anesthetic agent to the circuit, regardless of the patient’s respiratory minute volume. Out of circuit vaporizers are the most commonly used type in veterinary medicine.




Vaporizer Agent Specificity: Most out of circuit precision vaporizers are designed and calibrated for use with single agents, taking into account the vapor pressure of the specific agent. Safety mechanisms such as keyed filling devices are available on some models to ensure that they are filled with only the appropriate agent for the vaporizer. If an incorrect agent is used in an agent-specific vaporizer, this may result in a higher or lower output concentration.


Vaporizers designed for multiagent use are often in circuit vaporizers, and thus have generic settings. As indicated previously, these vaporizers may have unpredictable output. They should be used with an anesthetic gas analyzer to verify output concentration.


The vaporizer for desflurane is unique from the other vaporizers in common use today. The saturated vapor pressure for desflurane is very high, making desflurane very nearly a gas at standard temperature and pressure (STP) (the boiling point of desflurane is 74.3° F [23.5° C]), and it is stored in a pressurized bottle. A standard vaporizer would create too high of a desflurane concentration for safe use and might result in unpredictable amounts of vapor output with fluctuations in temperature. The solution has been to create an externally warmed vaporizer, which turns the desflurane from a liquid/vapor fully into a gas. The desflurane gas is then injected into the carrier gas, creating a clinically useful concentration. The vaporizer relies on external power for its function, so it must be plugged in to a power source. In addition, because desflurane vaporizes so readily, the refilling mechanism ensures a tight seal between the refilling bottle and the vaporizer to prevent any desflurane vapor from escaping.11



Anesthetic Circuits


Once the carrier gas and the inhalant anesthetic exit the vaporizer, they flow through the common gas outlet and subsequently to the anesthesia circuit for delivery to the patient. This circuit supports patient ventilation with mechanisms that allow positive-pressure ventilation and prevent rebreathing of carbon dioxide (CO2), which is constantly exhaled by the patient. Rebreathing of CO2 over time would rapidly lead to hypercarbia. Anesthesia circuits are classified as rebreathing systems (also known as circle systems) or nonrebreathing systems, depending on the method by which CO2 is eliminated from the system.



Rebreathing System


The rebreathing circuit is defined by the use of a CO2 absorbent to remove CO2 from the system. This absorbent is supplied in granule form (from 4 to 8 mesh in size) and typically contains a strong base in a proprietary formula that also incorporates water into the granules. For example, soda lime is a combination of sodium hydroxide, potassium hydroxide, water, and calcium hydroxide. CO2 exhaled by the patient reacts with water to form carbonic acid on the surface of the absorbent granules. The carbonic acid then dissociates to free protons and carbonate. These then associate with the strong bases to form water and calcium carbonate. Heat and water are created in the reactions. An indicator dye (ethyl violet) similar to that used with litmus paper, changes color on reaction with acid. The absorbent granules will take on a purple color as they absorb CO2 and will gradually lose their ability to absorb additional CO2. With soda lime, some regeneration of CO2 absorbent capacity may occur over time. This may allow granules that had previously changed color to lose their color change, but the regeneration is minimal, and these granules will rapidly become saturated and change color again once exposed to CO2. As a general rule, the CO2 absorbent should be changed whenever rebreathing of CO2 by the patient is seen on a capnograph. Other guidelines will vary, depending on the degree of machine usage and the average fresh gas flows used. At higher fresh gas flows, the absorbent may lose water to evaporation and become less effective for CO2 removal (this may also result in the production of other byproducts, such as carbon monoxide [CO]).97


In addition to the CO2 absorbent, the circle system (Figure 23-2) incorporates one-way valves to prevent rebreathing of expired CO2, a reservoir bag to allow positive-pressure ventilation, a pressure gauge, an adjustable pressure limiting valve (commonly called the APL, or “pop-off” valve), and breathing tubes. The common gas inlet enters the system upstream from the inspiratory valve. From there, the gas flows into the inspiratory limb of the breathing circuit, into the patient, out through the expiratory limb of the breathing circuit, through the expiratory valve, past the reservoir bag and pop-off, through the absorbent (where the CO2 is removed), and finally back through the inspiratory valve to the patient. The inspiratory and expiratory valves ensure unidirectional flow through the system. If the inspiratory valve becomes stuck open when the patient breathes out, the expired air can travel up the inspiratory limb of the circuit. On the next inspiration, the patient will rebreathe this exhaled gas, which contains CO2. If the expiratory valve becomes stuck open, the expired gas in the expiratory limb of the circuit can be inhaled on the next inspiration, causing the patient to rebreathe gas containing CO2.



When the rebreathing circuit is used, fresh gas containing inhalant enters the circuit. At the beginning of anesthesia, the circuit concentration of inhalant is 0%. As fresh gas containing inhalant enters the circuit, the concentration rises. However, because of dilution of the incoming gas with gas already in the circuit, the concentration of inhalant in the circuit will be lower than that entering via the common gas inlet. Consequently, the concentration of inhalant the patient inspires will be less than that dialed on the vaporizer. A higher carrier gas flow will bring a greater amount of gas with the desired inhalant concentration into the circuit and will raise the circuit inhalant concentration more quickly. Thus, to increase the patient’s inspired inhalant anesthetic concentration, the vaporizer setting can be increased or the carrier gas flow can be increased.



Closed and Semi-Closed Circuit Rebreathing System


The rebreathing systems require relatively low fresh gas flows compared with the nonrebreathing system. The minimum oxygen flow for the rebreathing system is equal to the patient’s metabolic oxygen demand (usually estimated as 10 × body weight (kg)0.75). If the carrier gas is only oxygen and is set to equal the patient’s metabolic oxygen demand, the system is said to be functioning as a “closed” circuit, since the pop-off valve may be closed and the pressure inside the system will not change because all incoming oxygen is being metabolized. This is a highly economical system, because relatively little inhalant anesthetic will be vaporized from the vaporizer, and little oxygen will be used. Alternatively, use of a higher, standard fresh gas flow (e.g., 1 L/min), creating a semi-closed rebreathing system, allows the clinician to calculate the time constant, or the amount of time necessary to effect a change in inspired anesthetic amount with a change in vaporizer setting.229


Dead space is produced in the rebreathing system where the inspiratory and expiratory breathing tubes meet at the patient (the Y piece). In a small patient, this dead space may be sufficient to cause rebreathing of CO2. Therefore, rebreathing systems are typically reserved for patients larger than 5 kg.206 In practical terms, this is the only system that works for very large animals, as the nonrebreathing system cannot eliminate CO2 rapidly enough for very large patients.



Nonrebreathing Systems


Nonrebreathing systems prevent rebreathing of CO2 by using high fresh gas flow rates. The terminology arises from the fact that rebreathing of expired gases is minimized or eliminated by high incoming fresh gas flow. As a patient breathes in, inspired gas is supplied by the common gas outlet continuously throughout the inspiratory cycle. Upon exhalation, the expired gas passes into a reservoir bag or out of the system via the adjustable pressure-limiting valve. The recommended fresh gas flow rate for an nonrebreathing system is at least three times the patient’s respiratory minute volume (MV).205


MV = Respiratory rate × Tidal volume; estimated tidal volume = 15 mL/kg.205


If the common gas outlet enters the nonrebreathing circuit close to the patient, the gas from the common gas outlet is delivered directly to the patient. The exact way that gas flows in a nonrebreathing system is determined by the organization of the system. Most nonrebreathing systems represent one of the schemata described by Mapleson.218 Because no dilution of this gas occurs, as happens in rebreathing systems, the inhalant concentration set on the vaporizer represents the inspired concentration of inhalant that is delivered to the patient. Fresh gas flows into the patient with inspiration. On exhalation, expired gases are exhausted from the system. On the next inspiration, a small amount of expired gas may enter the patient before the incoming fresh gas gets to the patient, particularly if the fresh gas flow rate is too low.


Nonrebreathing circuits (Figure 23-3) offer minimal resistance to airflow and minimal dead space, making them most suitable for small patients. They also have fewer components so are technically simpler and easy to clean.205 However, nonrebreathing circuits may not be compatible with some ventilators and are wasteful because of the high fresh gas flows necessary to prevent rebreathing. Depending on the type of system, fresh gas flows from 200 to 500 mL/kg are required to prevent rebreathing. This increases the amount of inhalant vaporized and increases oxygen used by the system. Nonrebreathing circuits typically are not effective for larger patients. Patients weighing less than 5 kg are best anesthetized using a nonrebreathing system rather than a rebreathing system.




Endotracheal Tubes


Delivery of gas from the anesthetic circuit to the patient is best accomplished via an endotracheal tube. Maintenance of anesthesia with a mask may be suitable for short procedures; however, masks can leak, which is wasteful and exposes personnel to anesthetic gases. Masks do not maintain a patent airway, cannot be used to provide effective intermittent positive-pressure ventilation (IPPV), and may instill gas into the esophagus or stomach. Endotracheal intubation with an endotracheal tube allows maintenance of a patent airway, and ensures delivery of anesthetic gases to the lungs while preventing aspiration of material from the oropharynx. Anesthetic maintenance using an endotracheal tube also minimizes leakage of waste gases and allows for orofacial surgery. However, endotracheal intubation requires a sufficiently anesthetized patient for placement and may increase airway resistance, work of breathing, and dead space. Endotracheal intubation requires technical expertise and can result in damage to the larynx, trachea, and/or lungs.5,45,147,234 Endotracheal tubes may become kinked or obstructed, causing airway obstruction. Laryngeal mask airways may be used in animals and are designed to fit over the larynx.49,335 A seal is maintained by inflating a cuff around the laryngeal mask airway. The laryngeal mask airway is less traumatic than an endotracheal tube but may be more easily displaced, and provision of IPPV may be difficult.335


Endotracheal tubes may be uncuffed or cuffed. Cuffed tubes have an inflatable balloon near the tip, which occludes the entire lumen of the trachea, preventing aspiration of materials instilled into the trachea from the oropharynx. The cuff is inflated by slowly adding air via a pilot balloon while a positive-pressure breath is administered. When air cannot be heard escaping around the cuff, the cuff is considered adequately inflated. It is recommended to reconfirm adequate cuff inflation before starting procedures that may result in an excess of fluid instilled into the oropharynx (e.g., dental procedures). Overinflation of the cuff, especially in cats, may lead to tracheal tears, whereas underinflation may not prevent aspiration.


Most endotracheal tubes have a Murphy eye near the tip, which allows for continued airflow should the tip of the tube become obstructed (Figure 23-4). Endotracheal tubes are commonly composed of rubber, silicone, or polyvinyl chloride (PVC). Rubber tubes are opaque and stiff, silicone tubes are soft but may require the use of a stylet because of their lack of stiffness, and PVC tubes are clear and stiff. Guarded endotracheal tubes have an embedded metal spiral, which helps prevent kinking, and are particularly useful in orofacial surgery, but they are incompatible for use with magnetic resonance imaging.




Intubation and Preoxygenation


Failure to establish and maintain a patent airway may lead to hypoxemia. In a patient breathing room air, hypoxemia will develop within 30 seconds of apnea or airway obstruction, whereas a patient previously breathing 100% oxygen may not become hypoxemic for upward of 5 minutes after onset of apnea.187 Patients with facial trauma, myositis, laryngeal masses, and similar conditions may be challenging to intubate orotracheally. Because induction of anesthesia is frequently accompanied by apnea or hypoventilation, such patients should be preoxygenated by placing the muzzle or head in a mask and allowing them to breathe100% oxygen for 5 minutes. An oxygen flow rate into the mask of 4 to 5 L/min is adequate for preoxygenation of most small-animal patients. This saturates the alveoli with oxygen, providing a reservoir of oxygen in the alveoli, and particularly within the functional residual capacity, in the event of apnea, upper airway obstruction, or delayed intubation.


Use of a stylet within the endotracheal tube may facilitate placement. Careful passage of a long catheter (e.g., a flexible red-rubber catheter) into the trachea to serve as a guide may be necessary. The catheter may be placed through the Murphy eye to allow the endotracheal tube to be subsequently threaded into the trachea. The anesthetist may also use this catheter to insufflate oxygen during the intubation process. Commercially available airway exchange catheters are available for this use or for simplifying reintubation.342



Difficult Intubation: Difficult tracheal intubation may occur for a variety of reasons. Patients with myositis may be unable to open their mouths to allow direct laryngoscopy or intubation. In these patients, progressively opening the mouth by stacking tongue depressors horizontally between the top and bottom incisors can increase the size of the oral opening, allowing direct laryngoscopy. The use of small stylets to facilitate intubation as described above is another method to achieve a secure airway in patients unable to open their mouths. If nasopharyngeal swelling or a mass prevents direct visualization, fiberoptic or semi-rigid endoscopes may be used to identify and intubate the trachea, providing a pathway for endotracheal tube placement. A through-the-needle jugular catheter may be introduced through a proximal tracheal ring and passed orad into the oral cavity, where the endotracheal tube can be passed over it and into the trachea. If the airway cannot be secured via orotracheal methods, a tracheostomy may be performed to secure and maintain the airway.



Fresh Gas



Carrier Gas


Oxygen is the most common carrier gas for inhalant anesthetics. Use of oxygen as the carrier gas ensures maximal oxygen delivery to the patient and is the standard for veterinary anesthesia machines. Use of 100% oxygen may result in relatively more alveolar collapse than 40% oxygen because the nitrogen contained in air/oxygen mixtures is not readily absorbed from the alveoli. The nitrogen thus provides structural support (the nitrogen “scaffold”) to the alveolus. Because oxygen is rapidly absorbed from the alveoli, an alveolus filled with 100% oxygen will gradually collapse as that oxygen is removed by the pulmonary blood flow (absorption atelectasis).305


Oxygen is supplied to the anesthesia machine via portable tanks or piping from a central source. Larger tanks, liquid oxygen, and oxygen concentrators are more cost-effective than smaller tanks. These tanks are all pressurized, and great care should be taken when handling them. Connections should not be forced when pressurized gases are handled, and tanks should be transported attached to a stable surface, and secured whenever upright. The volume of oxygen in a tank is directly related to the pressure and size of the tank, so that a tank with half the starting pressure will also have half the amount of oxygen remaining in the tank. This is not the case for all medical gases; supply information for commonly used medical gases is shown in Table 23-1.



Oxygen increases the flammability and volatility of existing chemicals. For example, ether in room air causes cool flames, but in 100% oxygen causes explosions. Accumulation of high concentrations of oxygen due to inappropriate connections can result in catastrophe in the face of a spark or use of some surgical lasers or electrocautery. High oxygen concentrations are contraindicated when some surgical lasers are used for surgery adjacent to the endotracheal tube, and shielding tape is available to protect against inadvertent laceration of the tube and oxygen leakage, which would result in fire.103 Helium may be combined in a 70 : 30 ratio with oxygen to protect against fires caused by laser ignition of anesthetic carrier gas, although this requires a specially calibrated additional flowmeter.259


Errors in the connection of gases are minimized by use of the pin index safety system, the diameter index safety system, and color coding of the tanks of gas. The pin index safety system (Figure 23-5) uses protrusions on the machine, which must line up with depressions on the tank or connector. The diameter index safety system uses different diameter adaptors on hoses for each type of gas. Incorrect or forced connections can result in elevated levels of N2O in the circuit (a hypoxic gas mixture), leaks, unknown oxygen concentration and delivery, and the unknown presence of other gases.




Pressure Regulation: An internal pressure regulator on the anesthesia machine reduces the carrier gas pressure from that in the tank or wall outlet to 50 pounds per square inch (PSI). The flowmeter further reduces the oxygen flow to clinically appropriate levels. Most oxygen flowmeters on small-animal anesthetic machines deliver up to 4 L/min. The flow rate is read from the top of a bobbin or the middle of a ball that floats alongside the flowmeter scale. From the flowmeter, oxygen travels to the vaporizer. The oxygen flush valve delivers oxygen, which bypasses the flowmeter and vaporizer and is consequently delivered at high pressure. The oxygen flush valve should not be used with nonrebreathing systems because of the risk of significant barotrauma from the high flow rate (30 to 50 liters per minute [LPM]).


Because the oxygen flush valve delivers pure oxygen, it will decrease the concentration of inhalant anesthetic in a rebreathing system. Therefore, the oxygen flush valve should be used only to pressure-test the anesthesia machine before use, and when the anesthetist wishes to rapidly decrease the concentration of anesthetic in the system.



Gas Scavenging: Inhalant anesthetics are potential toxins and pollutants; therefore personnel and environmental exposure should be minimized.61,196 No causal relationship between exposure to the newer volatile inhalant anesthetics and occurrence of disease among health care providers has been proven, yet caution should be exercised in the handling of waste gases.181 Some evidence suggests that chronic exposure to N2O may result in decreased reproductive health among operating room personnel.160


Once the gas passes through the pop-off valve, it can be scavenged via active or passive mechanisms. Active scavenging involves active suction of waste gases (by vacuum or fan), which typically are vented to the outside air away from personnel. Passive scavenging uses activated charcoal absorbents, through which the waste gas passes. As it does so, the charcoal absorbs any volatile inhalant anesthetics. The charcoal containers must be replaced regularly and are not capable of scavenging N2O. Use of a simple tube acting as a passive conduit to divert waste gases away from the work area (e.g., through a window to vent to the outside) is not as effective as the previously mentioned methods but can be done in certain circumstances. These types of passive scavenging systems may be affected by ambient weather conditions such as wind if they are vented to the outside environment. It is recommended that an active scavenge system is used; if this is not possible, passive scavenging with activated charcoal canisters should be done.



Anesthesia Ventilators


Intermittent positive-pressure ventilation (IPPV) can be supplied by manual compression of the rebreathing bag or the use of a mechanical ventilator. Intermittent positive-pressure ventilation may be necessary to support ventilation in patients suffering from hypoventilation, to manipulate alveolar ventilation in hypoxemic patients, and to limit the development of atelectasis over the duration of an anesthetic episode.


Ventilators used for anesthesia purposes are considerably simpler in design and function compared with those used in critical care settings. The ventilator is typically electrically powered and pneumatically driven, with ascending bellows. The ventilator bellows is depressed by compressed gas, typically air or oxygen, which enters the bellows housing under pressure. This prevents electrical parts from coming into contact with the high-oxygen gas within the bellows, which may be a fire hazard. The bellows typically ascend during expiration. Ascending bellows are superior to descending bellows (which descend during expiration) because a leak is more easily noticed with the ascending bellows, and they will not entrain atmospheric air if a leak is present.


Anesthesia ventilators may be time-, volume-, or pressure-cycled. Time-cycled, volume-cycled, and pressure-cycled ventilators deliver a tidal volume up to a set inspiratory time, volume, or pressure, respectively. Most anesthesia ventilators are volume- or pressure-cycled. With a volume-cycled ventilator, the user sets the tidal volume or minute volume and the respiratory rate in breaths per minute. The inspiratory-to-expiratory (I : E) ratio may also be set. This can range from 1 : 1 to 1 : 4 in most machines, with a default setting of 1 : 2. The inspiratory time is determined by the set volume and respiratory rate. With a pressure-cycled ventilator, the user sets the desired peak inspiratory pressure and the expiratory length in seconds. The inspiratory time, and therefore the I : E ratio, is set by the user or determined by the rate of flow of gas into the bellows housing, with faster gas flows resulting in a shorter inspiratory time.


The initial volume can be calculated at 10 to 15 mL/kg for each breath, although the appropriate tidal volume should be titrated on the basis of the end-tidal CO2 level and avoidance of an excessive peak inspiratory pressure. Volume-cycled ventilators may result in excessive peak inspiratory pressures, however, particularly if lung compliance changes, as may occur with bronchospasm or pneumothorax. Pressure-cycled ventilators are better for patients who may have changing degrees of compliance, such as those with severe respiratory disease who are ventilated for prolonged periods of time, or very small patients with a small tidal volume. Initial pressure settings depend on the size of the animal, with cats starting at a peak inspiratory pressure of 12 mm Hg and dogs starting at a peak inspiratory pressure of 15 mm Hg.


The ventilator is essentially an extra pair of hands; it eliminates the need for the anesthetist to continually provide manual IPPV. Ventilator failures due to a disconnect of the power or driving gas source, a leak in the bellows housing, or an incorrect internal setup after cleaning require rapid diagnosis and management, or the patient should be transferred back to manual IPPV. Insufficient volumes and excessive peak inspiratory pressures can result in alarms and may be dangerous for the patient.



Anesthetic Monitoring Equipment



Physiologic Considerations


During anesthesia, many of the patient’s intrinsic homeostatic mechanisms are blunted or obliterated. Furthermore, many of the drugs used to produce anesthesia result in marked depression of critical body systems such as the cardiovascular and respiratory systems. As a result, the anesthetist must monitor these systems for adverse changes to prevent a poor patient outcome. Fundamentally, during anesthesia, oxygen delivery to tissues, acid-base balance, and the anesthetic triad of narcosis, analgesia, and muscle relaxation must be maintained. In addition, the anesthetist must maintain normal body temperature and cellular energy delivery.


Oxygen delivery to tissues is dependent on both blood flow through the body and the oxygen content of the arterial blood being carried. Blood flow is generated by cardiac output, the amount of blood pumped by the heart per minute (a product of heart rate and stroke volume). In the awake patient, oxygen delivery to tissues is maintained by alterations in cardiac output, in addition to local tissue factors. In the anesthetized patient, sympathetic responses are blunted, necessitating monitoring and possibly intervention by the anesthetist. The oxygen content of arterial blood is largely dictated by the hemoglobin concentration. Decreases in hemoglobin lead to a linear decrease in the oxygen content of arterial blood. Profound hypoxemia can also decrease the oxygen content of arterial blood, but this effect is far less dramatic than the effect of anemia (Figure 23-6). In the face of suboptimal oxygen delivery to tissues, anaerobic metabolism occurs, which produces dramatically less adenosine triphosphate (ATP) for a given amount of glucose than aerobic metabolism, and results in the production of lactate and acid (H+) as byproducts. Prolonged tissue hypoxia results in an insufficient supply of ATP to power important enzymes such as the sodium-potassium ATPase, which produces energy to maintain the cell electrochemical gradient. Without the electrochemical gradient, cell swelling occurs and can lead to cell death. Cellular dysfunction due to prolonged hypoxia may lead to organ failure. In practical terms, the brain, kidney, and heart are most susceptible to tissue hypoxia, and decreased oxygen delivery to tissues most commonly manifests as dysfunction in any or all of these organs. Decreased blood flow to the liver may alter the metabolism of anesthetic drugs and result in prolonged clearance of some drugs.



Enzyme systems are designed to function within a narrow pH range. A normal acid-base balance is thus necessary for maintenance of enzymatic function. Anesthesia commonly alters respiration, which is an intrinsic component of the acid-base balance because of the impact of CO2 on acid-base status. Hypoventilation results in an increase in CO2, causing a respiratory acidosis, which then hampers important enzymatic systems. Clinically, poor cardiac contractility, reduced wound healing, decreased immune defense, and altered drug metabolism may all result from prolonged acid-base disturbances.


Enzyme systems are also designed to function within a narrow temperature range. Both hypothermia and hyperthermia can have profound consequences for the patient, including abnormalities in coagulation, prolonged recovery from anesthesia, decreased immune defense, reduced wound healing, altered drug metabolism, and arrhythmias.


Cells require not only oxygen but an energy source to produce ATP. Energy can arise from carbohydrate, protein, and fat sources. Clinically, alterations in glucose concentrations, particularly hypoglycemia, produce the most significant effects in anesthetized patients. Young animals, patients receiving exogenous insulin or with insulin-secreting tumors, septic animals, and patients in liver failure all may have significant abnormalities in glucose regulation, and close monitoring during anesthesia is warranted. Typically, a baseline glucose level is checked before anesthesia induction, with follow-up glucose levels checked every 30 to 60 minutes, depending on the severity of disease and baseline glucose concentration. Supplementation of dextrose may be warranted in hypoglycemic patients.


In addition to monitoring the above body systems, the patient must be rendered unconscious, be free from pain, and have muscle relaxation for general anesthesia to commence successfully. A poorly anesthetized patient may have sympathetic stimulation or movement in response to a procedure, which may impair the ability of the surgeon to complete the procedure. Excessive sympathetic stimulation may lead to cardiac arrhythmias. Pain causes myriad adverse consequences, and analgesia must be provided during any procedure that induces pain (Table 23-2).





Blood Pressure Monitoring


Although cardiac output is a major determinant of oxygen delivery to tissues, monitoring of cardiac output in the clinical patient can be challenging. Blood pressure is therefore used as a surrogate, because blood pressure is the product of cardiac output and systemic vascular resistance (SVR). Systemic vascular resistance reflects the systemic vascular tone (i.e., vasodilation or vasoconstriction) in the body. If systemic vascular resistance does not change, changes in blood pressure reflect changes in cardiac output. Blood pressure provides three variables of interest. Systolic arterial pressure (SAP) is the blood pressure at the peak of cardiac systole. Diastolic arterial pressure (DAP) is the pressure at the end of diastole. Mean arterial pressure (MAP) is the average blood pressure during a cardiac cycle. The heart spends more time in diastole than in systole, and thus it has a greater impact on mean arterial pressure. Mean arterial pressure is calculated from the systolic arterial pressure and diastolic arterial pressure:


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Normal values for systolic arterial pressure, mean arterial pressure, and diastolic arterial pressure vary with species. In awake dogs and cats, the normal values are typically written as SAP/DAP (MAP) in mm Hg and are 125/85 (105).311 Values in anesthetized animals are typically lower. When the mean arterial pressure is less than 60 mm Hg, perfusion and oxygen delivery to the kidney and brain are likely insufficient to meet the requirements for aerobic metabolism.86 Prolonged hypotension can result in cerebral and renal ischemia, which manifests as alterations in brain function (often blindness, prolonged recovery, and mental dullness) or acute renal failure. Because the heart fills in diastole, a diastolic arterial pressure less than 40 mm Hg indicates poor coronary artery perfusion, and possible cardiac ischemia.99


Blood pressure should be monitored no less frequently than every 5 minutes in anesthetized patients. This interval is chosen because if profound changes to blood pressure are noticed within 5 minutes of development, action can be taken to prevent long-term damage. A multitude of technologies have been developed to measure blood pressure. Although the gold standard is direct arterial pressure measurement, this procedure can be relatively difficult. As a consequence, indirect measurement methods have become popular. Indirect methods will always be less accurate and precise than direct arterial blood pressure.



Indirect Blood Pressure Measurement: The Doppler method of indirect blood pressure measurement relies on the recognition of a change in sound produced as blood passes under the Doppler crystal. The crystal emits a sound wave and, as that wave is reflected by the blood, receives the change in signal created by the velocity of the flowing blood. Although a Doppler crystal may be placed over any artery for assessment of blood flow, to measure blood pressure, it must be placed over a peripheral artery. The most commonly used arteries are the radial artery in the forelimb, the plantar metatarsal artery in the hindlimb, and the coccygeal artery in the tail.


The Doppler crystal is placed over the artery and is secured with tape. Proximal to the crystal, an occlusive inflatable cuff is secured around the limb or tail. The width of the cuff should be equal to 40% to 60% of the circumference of the limb or tail. A cuff that exceeds this recommendation will result in an artificially low value, and the use of a cuff that is too small will result in an artificially high value. A cuff that is loose and is not snugly secured on the patient will also result in an artificially high value. The inflatable bladder of the cuff should be placed over the artery to be occluded. The cuff is then connected to a sphygmomanometer and is inflated until no sound can be heard from the Doppler. Air is slowly released from the cuff until sound can be heard again—this is the systolic arterial pressure. A change in the tone of the signal may be appreciated as the cuff continues to be deflated and may be interpreted as the diastolic arterial pressure. The diastolic arterial pressure can be challenging to define with the Doppler method, and therefore the systolic arterial pressure is the only reliable value obtained with the Doppler.311 In cats, the Doppler tends to underestimate the systolic arterial pressure by up to 25 mm Hg.51 In dogs, the Doppler reading correlates well with the direct arterial pressure reading in most anesthetized patients.53 The Doppler sound pulse is not affected by tachyarrhythmias, bradyarrhythmias, or irregular heartbeats.


A variation on the Doppler method uses the waveform produced by a pulse oximeter device to interpret the point of occlusion of the artery. This is known as photoplethysmography. The cuff is inflated by the sphygmomanometer until the waveform of the pulse oximeter disappears. Air is then released from the cuff until a waveform appears—this is the systolic arterial pressure. This technique relies on a good pulse oximeter waveform with placement of the pulse oximeter probe distal to the cuff. Typically, only the webbing of the front and hind feet is suitable for this purpose. Photoplethysmography seems to offer no advantage over traditional Doppler methods for indirect blood pressure measurement.51


The oscillometric method of indirect pressure measurement is an automated system. A cuff is attached in a similar fashion to the Doppler method (using the same sizing guidelines). The machine then inflates the cuff to a sufficient pressure to occlude the artery. The cuff is then either slowly deflated or deflated in steps by the machine. As oscillations in the cuff occur as a result of the return of pulsatile blood flow through an artery partially collapsed by the cuff pressure, those oscillations are sensed by the machine and interpreted as the systolic arterial pressure. Oscillations with the greatest amplitude are interpreted as the mean arterial pressure, and the point of absence of oscillations is interpreted as the diastolic arterial pressure. Some machines only measure the mean arterial pressure and systolic arterial pressure and calculate the diastolic arterial pressure, and all have proprietary algorithms for determining pressure. In cats, oscillometric devices tend to underestimate systolic arterial pressure but are relatively precise for mean and diastolic arterial pressure.51 In dogs, oscillometric devices tend to underestimate systolic, diastolic, and mean arterial pressures, although this bias is changed in the face of hypotension and other disease states.311 In both species, the trend of readings is consistently correlated with the direct arterial pressure. Because the machine requires regular oscillations to determine pressure, oscillometric machines may fail to generate a reading in the presence of irregular heart rhythms, or tachyarrhythmias or bradyarrhythmias.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Anesthesia Principles and Monitoring
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