The Five Phases of Anesthesia
Surgical Anesthesia is a common practice in nearly every veterinary hospital; however, anesthesia should never be considered routine. Each time it is performed, there is the potential for adverse consequences.
Anesthesia is a procedure that intentionally disrupts a patient’s normal homeostatic functions and renders it unconscious. Therefore, every anesthetic protocol should be tailored to the patient’s needs using best in practice technology and procedures and be executed by trained personnel.
Jurox, one of the world’s leading manufacturers of anesthesia-related products, believes each practice’s anesthesia protocols should be in a state of continuous improvement…with each procedure providing a learning experience for the next. To that end, this section provides a review of a modern anesthetic protocol and highlights best practices in each phase of anesthesia.
Although terminology may differ, every anesthetic protocol follows a basic sequence:
- Patient evaluation and planning
Each phase of anesthesia is comprised of a detailed set of critical activities. Veterinary practices focused on consistency and attention to detail should expect positive outcomes across the entire range of their anesthetic procedures.
Patient history review
Patient evaluation begins with a thorough review of the patient’s medical history. The patient history review begins by updating the patient’s medical record during an interview with its owner. Questions concerning changes in patient health status and behavior should be reviewed. The owner should also be asked about the pet’s previous anesthetic episodes and past and current medication history, including heartworm prophylaxis and any supplements the pet is currently taking. It should also be determined when the patient last had a meal. The patient history review concludes with an explanation to the pet owner of the surgical and anesthetic procedure to be performed, including a risk assessment. Finally, obtain the client’s signature on an Anesthesia Consent form.
Once the pet has been admitted, a physical examination is conducted, including measurement of physiological vital signs, a baseline blood pressure and, if indicated, an ECG.
Pre-anesthetic diagnostic tests
Results of all relevant pre-anesthetic diagnostic tests, including a biochemistry panel, complete blood count, and urinalysis and others, as indicated, should be reviewed and recorded in the patient history. Based upon the history and physical examination, diagnostic imaging may also be indicated. Considering the factors found in the patient history review, physical examination and pre-anesthetic diagnostics, the veterinarian should note the patient’s ASA health status as a factor in the overall risk assessment for the procedure.
Documentation of the anesthetic event should begin simultaneously with the patient history review and chronicle the entire procedure through to recovery. Vital signs, peri-anesthetic events and interventions including medications and dosages, routes and time of administration are recorded. Trained staff should record patient parameters at 5 to 10 minute intervals, more frequently if abnormal trends or unexpected events occur.
Equipment and medications: pre-check
To complete patient evaluation and planning activities, ensure all equipment required for the procedure is accessible and in working order. All required drugs and medications should be available in sufficient quantities and in date. These activities can be performed prior to admission of the patient.
Emergency medications should be available and their dose rates known and or displayed on a visible emergency chart. It is important to pre-emptively calculate emergency drug doses for the patient being anesthetized. Make sure all participating staff are current with procedures for cardiopulmonary cerebral resuscitation.
Here is a list of equipment checks to follow when preparing for an anesthetic procedure:
The CO2 absorbent should be checked and replaced according to its manufacturer’s specifications. Note that patient size and varying metabolic rates affect the frequency with which the material needs to be replaced.
Ensure supply lines are not blocked, not kinked and securely connected to their respective pieces of equipment.
Oxygen quantity and flow
Confirm sufficient medical grade oxygen is available and that the flow meter is functioning properly. Leak check the system prior to every procedure by closing the pop-off valve and blocking the patient connector. Fill the system with oxygen to a pressure of 30-40 cm of water. After turning off the oxygen flow, the system should remain pressurized for at least 10 seconds.
Waste gas scavenger
Verify an appropriate waste gas scavenging system is available and fully functional.
Ensure all monitoring devices and leads are present and operational. Know when monitors were last calibrated.
Supplemental heat sources and positioning devices
Check the adequacy of the supplemental heat sources and the availability and functionality of required patient positioning devices.
The choice of drugs administered during premedication depends on species, temperament, physical status, the procedure to be performed and the clinician’s experience with the medication.
Objectives of premedication - Premedication drugs are administered to minimize patient stress and anxiety and to facilitate restraint and enhance the safety of the patient and staff. When administered properly, premedication drugs decrease the amount of injectable and inhalant anesthetics required and facilitate a smooth transition between anesthetic phases and improve cardiopulmonary stability.
Premedication drugs also provide essential, preemptive and perioperative pain management and reduce overall analgesic requirements. Finally, drugs used to decrease autonomic reflex activity provide a more stable plane of anesthesia and reduce potential cardiac arrhythmias.
Catheterization – As the premedication drugs take full effect, an intravenous catheter is inserted into the patient to provide rapid administration of drugs while avoiding perivascular administration. Additionally, an intravenous catheter facilitates administration of intravenous fluids to maintain vascular volume, which decreases as a result of anesthetic drugs, blood loss, and insensible fluid loss. Finally, the intravenous port provides immediate access to the circulation should emergency drugs be required.
Pre-oxygenation - Administration of supplemental oxygen reduces the risk of hemoglobin desaturation and hypoxemia during anesthetic induction. It is especially beneficial if a difficult or prolonged endotracheal intubation is expected or if the patient is already dependent on supplemental oxygen.
It is best practice to continue pre-oxygenation until it is determined the patient is ready to be intubated. During pre-oxygenation, connect and activate the ECG and blood pressure monitors to observe how the patient's cardiovascular system has been affected by the pre-medication drugs.
The anesthetic induction phase begins when the premedications are deemed to have taken full effect.
The goals of the induction phase include:
- A calm and anxiety-free patient
- Rapid unconsciousness with muscle relaxation
- A secure, protected airway
- Maintenance of acceptable frequency and depth of breathing
- Maintenance of good tissue perfusion
- A smooth transition to inhalant or injectable maintenance anesthesia
Induction with Alfaxan® Multidose injectable anesthetic
Alfaxan Multidose is being used as the principal induction agent in more and more U. S. veterinary anesthetic procedures because of its versatility, its effectiveness and its wide safety profile. Alfaxan Multidose is FDA-registered for use as both an induction and a maintenance agent in both cats and dogs. It can be used with all common premedications.
The recommended dose rates for Alfaxan Multiodose in unpremedicated cats and dogs are 5 milligrams per kilogram and 2 milligrams per kilogram, respectively. Alfaxan Multidose is known to have minimal dose dependent effects on key cardiopulmonary functions in anesthetized dogs and cats. Blood pressure is generally well maintained and provides acceptable tissue perfusion, which is important in sustaining normal tissue and organ function.
Rapid onset of unconsciousness allows for airway evaluation and endotracheal intubation.
Choose the largest diameter endotracheal tube that will fit easily through the arytenoid cartilages without damaging them. The tube itself should be premeasured for the patient such that the distal tip lies midway between the larynx and the thoracic inlet.
Once the patient’s airway has been secured and the transition phase to inhalant anesthesia has begun, check all cardiopulmonary values and compare them against the pre-anesthetic baseline values.
Inflate the cuff of the endotracheal tube sufficiently to create a seal for adequate positive pressure ventilation. It is a best practice to conduct a leak test to ensure a good seal of the endotracheal cuff.
Take care not to over-inflate the cuff due to the risk of tracheal damage. Lubricating the cuff will decrease the amount of pressure required to create an effective seal. To avoid the risk of tracheal tears, disconnect the endotracheal tube from the anesthetic machine when re-positioning the patient.
Transition to the maintenance phase of anesthesia begins with the completion of the intubation process. With the patient transitioned and positioned for surgery apply corneal lubrication to protect the eyes from corneal ulceration.
Administration of Intravenous Fluids – Upon introduction of supplemental fluids into the IV catheter, double check the flow rate and the fluid level. For healthy patients, intravenous fluids should be administered as follows, adjusting for comorbidities:
- Initial rate for cats - 3 milliliters per kilogram per hour
- Initial rate for dogs - 5 milliliters per kilogram per hour
- Procedures > 1 hour, reduce fluid administration by 25% every hour until maintenance rates are reached.
Finally, provide thermal support and monitor the patient's body temperature throughout the peri-anesthetic period.
There are four basic goals of the maintenance phase of anesthesia:
- Maintain required levels of unconsciousness, analgesia and muscle relaxation throughout the procedure,
- Provide effective cardiopulmonary stability and tissue perfusion,
- Allow adjustments to the depth of anesthesia based upon the needs of the patient
- Facilitate a smooth transition of the patient to the recovery phase
Anesthesia is typically maintained using inhalant anesthetics. However, it can also be maintained with a continuous infusion or intermittent administration of an injectable agent called Total Intravenous Anesthesia (TIVA). Also, a combination of inhaled and injectable drugs can be administered simultaneously to maintain anesthesia called Partial Intravenous Anesthesia (PIVA).
Alfaxan Multidose injectable anesthetic may be effectively used in combination with inhaled gases for PIVA or by itself for TIVA. The depth of anesthesia must be continuously monitored and adjusted based upon the patient’s individual requirements. Remember, only utilizing monitoring equipment to measure the patient's vital physiological parameters is not sufficient. Individual data points must be interpreted in light of the entire clinical picture. This leads to informed decision-making throughout the procedure.
Complications during the maintenance phase of anesthesia - Despite the best planning, anesthetic complications are not uncommon. To prevent a minor complication from escalating, it is essential to recognize it and respond to it quickly and correctly.
Common complications of anesthesia are hypoventilation, hypotension and cardiac arrhythmia.
Hypoventilation is an expected effect of inhalant anesthesia. It can be estimated by observing respiratory rate and depth, and quantified using capnography. Normal end tidal carbon dioxide ranges from 35 to 40 mm of mercury in non-anesthetized patients and 40 to 50 mm in patients in a surgical plane of anesthesia. Increases beyond these values may indicate the need for anesthetic adjustments (e.g. positive pressure ventilation).
Hypotension may be diagnosed by measuring blood pressure. Responses to hypotension include lowering the depth of anesthesia, administering a crystalloid fluid bolus and/or anticholinergics, vasopressors and inotropes.
Gas anesthesia is known to cause significant cardiac depression and vasodilation. In a situation where the patient is insufficiently anesthetized but still exhibits symptoms of hypotension, it is possible to reduce the inhaled gas delivery and give an injection of Alfaxan Multidose at approximately 1.0 mg/kg to increase depth of anesthesia while treating the underlying cause of hypotension.
Common anesthesia-related arrhythmias include bradycardia and ventricular arrhythmia. These may be detected via auscultation of the heart or ECG monitoring.
The decision to treat an arrhythmia should be based upon its severity and its effects on other hemodynamic parameters.
Anesthesia best practice: When the procedure has concluded and the inhalant gas is discontinued, supplemental oxygen should be provided for at least 3-5 minutes. During this time, observations may be made to ensure the patient can maintain normal oxygenation without supplementation.
The goals of the recovery phase are
- A pain and stress-free emergence from anesthesia
- Normalization of the patient’s physiological vital signs.
Recovery begins when maintenance anesthesia is discontinued and ends, not with extubation, but when the patient’s mentation status, physiological vital signs, and comfort are deemed clinically acceptable for discharge from the hospital. Monitoring of mentation, vitals, and comfort should be performed periodically based on the individual patient needs.
Thermal support - Recovery may be prolonged in hypothermic patients resulting in increased morbidity. Thermal support should be provided until the patient can regulate and maintain an adequate body temperature.
Providing adequate analgesia and a quiet environment improve the quality of patient recovery. A final evaluation should be performed by caregivers to assess the patient’s mental status and pain management before discharge from the hospital.
Normal mentation and body functions are restored rapidly after an Alfaxan Multiodose-induced anesthesia allowing the patient to be discharged from the hospital in a timely fashion.
- Bednarski R, Grimm K, Harvey R, Lukasik VM, Penn WS, Sargent B, Spelts K. AAHA anesthesia guidelines for dogs and cats. J Am Anim Hosp Assoc 2001; 47:377-385.
- Blunt MC, Young PJ, Patil A, et al. Gel lubrication of the tracheal tube cuff reduces pulmonary aspiration. Anesthesiology 2001; 95(2):377-381.
- Brodbelt DC, Blissitt KJ, Hammond RA, et al. The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg 2008; 35(5):365-73.
- Davis H, Jensen T, Johnson A, Knowles P, Meyer R, Rucinsky R, Shafford H. 2013 AAHA/AAFP Fluid therapy guidelines for dogs and cats. J Am Anim Hosp Assoc 2013; 49:149-159.
- Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment, 5th ed. New York: Williams and Wilkins, 2008.
- McNally EM, Robertson SA, Pablo LS. Comparison of time to desaturation between preoxygenated and nonpreoxgenated dogs following sedation with acepromazine maleate and morphine and induction of anesthesia with propofol. Am J of Vet Res 2009; 70(11):1333-8.