GuidelinesfortheUseofAnalgesicsandTranquilizersinLaboratoryAnimal6

 Anesthetic Machine

The 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. 

• Low-flow anesthetic techniques in large animals.

• Anesthesia breathing circuits.

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 cardiovascularfunction must also be monitored and supported while the animal is anesthetized. For all majorsurgical 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.  Please consult RAR's policy for fasting requests.

• 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. 

Respiration

Most 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 furosemidecan 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 Support

Many 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.

• Click here for clinical case studies in fluid management.

• 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.