Anesthetic Induction and Maintenance

Injectable Anesthesia
Anesthetic induction using injectable anesthetics is fairly simple. It involves admininsistration of the drug and monitoring the depth of anesthesia. Supportive care may be needed. Maintenance of injectable anesthesia can be through repeated bolus doses of the drug or through a constant infusion. Infusion rates are calculated based on the clearance time of the drug. Bolus dosing is simpler. Typically, 1/2 of the original dose is given for repeat doses.

Injectable anesthetics can be administered by various routes depending upon the specific compound. The most frequently used routes of administration in laboratory animals are intraperitoneal, intramuscular and intravenous. Less frequently used routes, among others, are intrathoracic, oral and rectal. Techniques are described below. Contact RAR at 624-9100 for training materials on handling animals and administering injections.

Intravenous(IV)

Method- An appropriate vein must be selected. For large animals, the saphenous, cephalic or jugular veins are best. For rodents, the tail veins are best. For rabbits and swine, ear veins may be used. The vein is held off proximal to the venipuncture site. The vessel may be stroked with a finger to stimulate blood flow into it. The needle is inserted at a 30-45° angle to the vessel. Then the needle is lowered to align with the longitudinal axis of the vessel and advanced slightly. Draw back. If blood appears in the hub of the needle, the drug may be injected. If not, try redirecting the needle (before you pull it out of the skin) and repeat. You may need to try several times while learning. Using a new, sharp needle for each stick, even if it is the same animal, will improve your chances for success. Once the needle is withdrawn, it is necessary to put pressure on the vessel to prevent bleeding.

Advantages- rapid delivery of drug, ability to titrate dose, irritating substances may be given IV

Disadvantages- small veins are hard to access (i.e. small animals), restraint is critical, developing skill in venipuncture takes experience

Intramuscular (IM)

Method- Insert the needle into a large muscle mass. Draw back slightly. If blood is aspirated, you are in a blood vessel. Redirect the needle. When the needle is placed correctly, inject the drug. The best muscle masses to use are for small animals, the caudal thigh muscles. For larger animals, the lateral dorsal spinal muscles or the cranial or caudal thigh muscles may be used. When administering into thigh muscles, inject from the lateral aspect, or if from the caudal aspect, direct the needle slightly lateral. This will help avoid injecting into the sciatic nerve.

Advantages-- Fairly rapid absorption, technique is simple

Disadvantages- IM injections are painful, small volumes are necessary, the animal may try to bite or escape

Intraperitoneal (IP)

Method- The animal is usually restrained in dorsal recumbency. The drug may be injected anywhere in the caudal 2/3 of the abdomen. However, it is best to try to avoid the left side in rodents and rabbits because of the presence of the cecum. After the needle is inserted, draw back. If anything is aspirated, you have likely hit the viscera. Withdraw and get a new needle before trying again. If the needle is placed correctly the drug may be injected.

Advantages- relatively large volumes may be injected (0.5 ml in mice, 2 ml in rats, etc.)

Disadvantages- technique is more difficult than IM injections, drug may be administered into the viscera resulting in no effect or in a complication.

Subcutaneous (SQ)

Method- Pinch an area of loose skin. Inject into the center of the "tent" created by pinching.

Advantages- Technique is the simplest of any, large volumes may be given (basically as much as the tent of skin will hold that doesn't cause discomfort to the animal)

Disadvantages- Irritating substances cannot be given this way, absorption is slow

Inhalant Anesthesia
Induction of inhalation anesthesia can be difficult. Anesthetic gases are irritating to eyes and nasal passages. Animals may resist as they begin to lose consciousness or they may stop breathing temporarily. For this reason induction using a mask or nose cone held over the animal's nose can only be performed on smaller or non-fractious animals. In smaller animals gas can be delivered into an induction chamber large enough to contain the entire animal. Induction via a nose cone or chamber requires delivery of the anesthetic gas at 2-3x MAC. Frequently an injectable anesthetic is used to induce anesthesia and the inhalation agent is used for maintenance.

Maintenance of inhalation anesthesia is normally accomplished by delivering approximately 1.2 MAC to an animal via a mask or nose cone, or directly into the lungs via an endotracheal tube. Intubation is recommended whevever possible, particularly when a procedure will be prolonged. Endotracheal access is essential to provide ventilation support.

Gas Delivery SystemsThe most complicated aspect of using inhalant anesthesia is the delivery system. A delivery system must provide the anesthetic gas to the animal at a known and constant rate. It must also ensure that animals receive adequate oxygen. There are several types of delivery systems typically used in laboratory animals.

Anesthetic MachineThe best method of delivering an inhalant anesthetic is with an anesthetic machine. These machines precisely mix the gas with air or oxygen and can be easily adjusted. Machines can vary in construction and design. Anesthetic machines typically require more training to learn to operate.

Anesthetic concentration is accomplished by sets of mixing valves or a precision vaporizer. Vaporizers are easier to use but are very expensive. Vaporizers are calibrated for the specific anesthetic gas to be used.

Anesthesia circuits can be re-breathing or non-rebreathing.

Re-breathing circuits include typical circle systems used in large animals. The gas/oxygen mixture is delivered to the animal via a one-way valve. When the animal breathes out the gas passes out another valve attached to a y-piece. This is passed over a carbon dioxide absorbent and then back into the system. Additional gas and oxygen are continuously delivered to replace that lost.

Re-breathing circuits conserve anesthetic gas and the animal's body heat. The CO₂ absorbent must be replaced regularly.

Non-rebreathing circuits are primarily used for smaller animals that cannot cycle the valves in a re-breathing system. With newer machines non-rebreathing circuits are normally only necessary for rodents and birds. In older machines with metal valves a non-rebreathing circuit may be necessary for rabbits and cats as well. A Bain system is the most common non-rebreathing circuit available.

The non-rebreathing circuit is attached to the same anesthetic supply as used for a re-breathing system. However, the exhaust line is connected directly to the waste gas scavenging system.

Non-rebreathing circuits depend on gas and oxygen being delivered at a higher pressure than is present in the exhaust line. This tends to increase anesthetic usage and can increase body heat loss in the patient.

Anesthesia machines must have a waste gas scavenging system. Normally the exhaust line on a non-rebreating system or the pop-off valve on a re-breathing system is connected to a vacuum line or to the building exhaust. Other scavenging systems can be used, contact RAR at 624-9100 or Environmental Health & Safety at 626-5804 for further information on anesthetic delivery systems.

Anesthesia breathing circuits.

Apparatus for Rodent Anesthesia
Left: a non-re-breathing nose cone that can be used with a large animal anesthetic machine; Middle: a typical drop system closed anesthetic chamber; Right: a gas scavenging system that can be used with a drop system.

Preparation, Monitoring and Maintenance of Normal Physiology

A variety of things must be done to prepare for anesthesia. Once animals are under anesthesia they must be monitored closely while they are anesthetized to ensure that they do not become too deep and die, and to ensure that they do not become too light and experience pain from the surgical procedure. Normal physiologic functions such as body temperature, respiration and cardiovascular function must also be monitored and supported while the animal is anesthetized. For all major surgical procedures on non-rodent mammals, an intra-operative anesthesia record must be kept and included with the surgeon's reports as part of the animal's record. The anesthetist must be prepared to handle emergencies if they occur.

Preparation

Withhold food and water from large animals for 12 h prior to anesthesia and from small animals for 2 h to prevent regurgitation and aspiration. It is not necessary to withhold food and water from rodents prior to anesthesia. Prolonged food or water deprivation are distressful to animals and are rarely necessary.

Have all drugs and equipment ready before the animal is anesthetized. You may not have time to look for things once the animal is under.

Have an assistant. Anesthesia takes time to perform and monitor. A person should be available to assist so the surgeon does not have to break sterility to monitor the animal or administer medications.

Premedication with atropine or glycopyrrolate (anticholinergics) may reduce the respiratory tract secretions in some animals

Protect the eyes from drying out using an ophthalmic ointment and protect them from being contaminated with surgical scrub solutions. Also protect pressure points, such as bony protrusions, from pressure necrosis or peripheral nerve damage by providing padding between the animal and the table.

RespirationMost anesthetics cause direct depression of the respiratory center in the brain and reduce ventilation. This is complicated by other factors that may interfere with respiration. When an animal is in lateral recumbency the lung that is down is being compressed by the rest of the body. Likewise, animals in dorsal recumbency may experience compression of the diaphragm by abdominal viscera. The airway may be compromised by regurgitated food or pharyngeal and tracheal secretions that normally would be removed by reflex swallowing or coughing. These reflexes are lost during anesthesia. There are several ways to monitor and support the ventilation of an anesthetized animal.

Intubate the trachea whenever possible, even if injectable anesthetics are being used. Intubation can be achieved on animals as small as a rat. This will prevent aspiration pneumonia and allow you to assist respiration if the animal stops breathing. Contact RAR at 624-9100 for training materials.

Assist respiration during the procedure. This can be done with a mechanical ventilator. However, mechanical ventilation is rarely needed (unless a thoracotomy or diaphragmectomy is being performed) and can be detrimental to the animal if over-done. Attaching an AMBU bag to the endotracheal tube or using an anesthetic machine's rebreathing bag will allow you to administer a deep breath every 2-5 min during the procedure. This will inflate all areas of the lungs and improve gas exchange. If the animal is not intubated, ventilation can be performed using a nose cone or face mask.

Monitor respiratory function throughout the procedure and recovery.

Monitor respiratory rate and depth (compare to normal for your species. You can expect them to be slightly decreased). Observe chest movement, or use a stethoscope or esophageal stethoscope.

Monitor the color of the mucous membranes (gums, conjunctiva, vulvar mucosa). A bluish color means the animal is not getting enough oxygen- ventilate!

Red-tinged foam present in the airway along with dyspnea (difficulty breathing) may indicate pulmonary edema. This can result from overventilation or overhydration. A diuretic like furosemide can be administered, but prognosis is poor.

Sophisticated respiratory monitoring can be achieved by measuring blood gasses, or expired oxygen and carbon dioxide concentration or by use of a pulse oximeter.

Fluid Therapy/Cardiovascular SupportMany anesthetics have direct effects on the heart or vasculature, decreasing cardiac output and blood pressure. This is further complicated by increased fluid requirements during anesthesia and surgery that may result in hypovolemia. Fluid requirements are increased because: breathing dry, cold oxygen (if inhalant anesthesia is used) increases respiratory fluid loss; the animal has not received its normal fluid intake since it was fasted; fluid may be lost through hemorrhage or exposure of moist viscera to room air; many anesthetics are metabolized in the kidney (creating a slight diuresis minimizes renal toxicity).

To minimize the effects of surgery and anesthesia on hydration:

Place an intravenous catheter whenever possible to provide access for fluids and medications

Supplement fluids, intravenously if possible; otherwise intraperitoneally or subcutaneously

Fluid should be supplemented at the rate of 5-10 ml/kg/hour during anesthesia

Monitor hydration status- Overhydration results in frequent urination and pulmonary edema, underhydration results in sticky mucous membranes, loss of skin elasticity, the eyes sinking into the orbit, decrease in blood pressure and increase in heart rate

To replace blood loss with saline or lactated ringers, administer 3X the volume of blood lost by slow IV drip. Monitor the hematocrit. If it drops below 20%, whole blood replacement may be necessary.

Monitor cardiovascular function by monitoring one or more of the following:

Mucous membrane color and capillary refill time (the time it takes for the mucous membranes to regain their normal color after pressure is applied)

Heart rate and rhythm- stethescope or esophageal stethoscope

Pulse rate and pressure- using your fingers

Blood pressure- arterial catheter or Doppler cuff required

ECG

If the animal has pale mucous membranes, the capillary refill time is greater than 2 seconds, or if the other cardiovascular parameters are out of normal range (determine normal for the species you are using!) you may have a cardiovascular emergency. Increasing the rate of intravenous fluid administration will improve cardiac output temporarily. However the depth of anesthesia will need to be reduced and if there is a primary cardiac problem it will require specific treatment. Consult with an RAR veterinarian for more information on anesthetic emergencies. Thermoregulation

Animals frequently become hypothermic during anesthesia because of inhalation of cold gases, exposure of body cavities to the room air, and loss of normal thermoregulatory mechanisms and behaviors. Hypothermia depresses all physiologic functions, including respiration and cardiac function, slows the metabolism of anesthetics and results in prolonged recoveries. All of these can contribute to anesthetic death. Hyperthermia is less common, but may occur because of excessive application of heat, hot surgery lights or malignant hyperthermia in genetically pre-disposed animals. To thermoregulate your patient:

Monitor the body temperature frequently using a thermometer during the procedure and during anesthetic recovery. While animal normals vary from species- to-species, in general, when body temperature drops below 99° F, an animal is considered hypothermic. Below 95-96° F an animal cannot regain normal body temperature without supplementation.

Prevent heat loss by insulating cold surfaces with a blanket

Prevent heat loss during gas anesthesia by utilizing low flow techniques that conserve heat

Supplement heat with a thermal blanket (keep blanket temperature below 40 C to prevent burns!) or with pre-warmed fluids

Treat hyperthermia by administering intravenous fluids or applying water to foot pads or exposed skin. Only use an ice bath as a last resort, as it may cause cardiovascular shock.

Water blanket and heater
Monitoring Anesthesia
The depth of anesthesia must be monitored carefully. Animals that are too light will experience pain and may move during the procedure. Animals that are too deep run the risk of experiencing cardiopulmonary arrest. If an animal is too light the anesthesia should be supplemented, if too deep, animals on gas anesthesia can be turned down. Animals given injectable anesthetics can not be lightened directly. Instead respiratory and cardiovascular support must be administered until the anesthetic is metabolized and the animal begins to lighten on its own.
To monitor the depth of anesthesia, perform the following:

Reflexes- these reflexes disappear as the animal becomes deeper in the following order:

Palpebral reflex- touching the eyelids causes blinking. The animal is light if it is blinking.

Toe pinch reflex- pinching the toe or foot web will cause a pain response. If the animal withdraws the toe it is not deep enough. If it doesn't, it is not sensing pain.

Corneal reflex- touching the cornea of the eye with a tuft of cotton results in a

blink. Once the animal has lost its corneal reflex, it is too deep.

Muscle tone increases as the depth of anesthesia decreases, unless the animal is receiving a cataleptic drug like ketamine in the absence of a sedative. Test muscle tone by pulling on the lower jaw or a limb. Rigid tone indicates inadequate depth of anesthesia.

Monitor cardiopulmonary function and body temperature- As an animal becomes too deeply anesthetized, respiration and cardiac output decrease, resulting in poor blood oxygenation and tissue perfusion and decreased blood pressure and temperature. Likewise, elevations in heart rate and blood pressure may be indications that an animal may be feeling pain and is anesthetized too lightly. Monitor as previously described.

Anesthetic Emergency Drugs	Dose (mg/kg)	Indications
Doxopram (Dopram)	1-5 IV (10x in farm animals)	Respiratory stimulant, for complete respiratory arrest only, use with CPR
Furosemide (Lasix)	2- IV, IM	For pulmonary edema. Administer as needed
Naloxone (Narcan)	0.04 IV	For reversal of narcotic sedation or respiratory depression
Yohimbine	0.1-0.15 IV	Reversal of xylazine or detomidine sedation
Atropine	0.02-0.04 IV	For bradycardia
Epinephrine (1:1000)	0.1 ml/kg IV, IT, IC, IM	For cardiac arrest only. Administer IV, intratracheal or intracardiac and perform cardiac massage
Lidocaine	2, IV (0.5 mg/kg in cats)	For diagnosed ventricular tachycardia only. Administer to effect and monitor

Recovery
Monotoring and support must continue until the animal is completely recovered from anesthesia. Complete recovery means the animal is able to hold itself in a normal upright position, has returned to normal body temperature and all physiological indices are within normal limits. Anesthetic recovery can be rapid for gas agents and short anesthetic episodes. Recovery time can be prolonged if animals were under for a long time or if injectable agents were used.