During the perioperative period, patients should be monitored. This review will focus on concepts of anesthesia monitoring and management during anesthesia.
Anesthetic machines should be inspected daily before use (i.e., all connections, kinks, cracks, anesthetic agents, O2, etc.). Low pressure leak tests should be done by turning off all gases and pop-off valves that occlude the patient end of breathing hoses to activate O2 flush valve until pressure reaches 30 cm H2O; the pressure should hold > 10 sec.
Anesthetic plane: During general anesthesia, the surgical plane of anesthesia should be reached before surgery can start. Examples of surgical plane, but not limited to, are loose jaw tone, ventromedial rotation of eyeballs, no paw withdrawal reflex, no palpebral reflex, and no response to surgical stimulation. These signs may not be valid in some species. Vital signs such as heart rate, respiratory rate and depth should also be monitored.
Cardiovascular system: During anesthesia, maintaining normal blood pressure and tissue/organ perfusion are important in maintaining oxygenation and nutrient delivery. Blood pressure is a product of cardiac output and vascular resistance. Cardiac output is a product of heart rate and stroke volume. Mean blood pressure should be kept ~ 70 mm Hg during anesthesia to ensure tissue perfusion. Balanced fluid administration at 10–20 ml/kg/h should be provided. Generally, animals are maintained with inhalational anesthesia (e.g., isoflurane) which can cause hypotension in dose dependent manner. The use of inhalation should be as low but safe as possible. To achieve that, balanced anesthesia should be performed by using other classes of anesthetic or analgesic drugs in combination with inhalation anesthesia (e.g., the use of isoflurane with fentanyl CRI and/or local blocks with local anesthetics).
Heart rhythm: Auscultation and digital pulse can be used to monitor heart rhythm and deficit. In addition, electrocardiogram (ECG) can be used to monitor electrical activity of the heart providing a continuous reading of heart rate and rhythm. Please note that ECG does not provide mechanical activity of the heart. If arrhythmia occurs and if it has impact on hemodynamic status of patients, corrections should be made based on the causes. For example, VPCs can be treated with lidocaine 2–4 mg/kg IV.
Blood pressure: Hypotension is common during anesthesia, so monitoring blood pressure is crucial. Because anesthetics may suppress contractility and cardiac output, it is important to monitor blood pressure to ensure tissue perfusion to vital organs. Systolic blood pressure is 100–180 mm Hg; diastolic blood pressure is 40–100 mm Hg; mean blood pressure is 70–120 mm Hg. Blood pressure can be monitored using either non-invasive or invasive methods. If hypotensive occurs, actions should be taken, (i.e., lighten anesthetic plane [reduce amount of gas anesthesia], bolus fluid administration [5–20 ml/kg], blood administration if due to blood loss, anticholinergic administration if due to bradycardia).
Respiratory system: respiration should be monitored either visually or mechanically by capnographs or arterial blood gas analysis. Capnographs give end-tidal CO2 which should be ~ 35–45 mm Hg. Hypoventilation (PaCO2 > 40 mm Hg) is common during anesthesia due to suppression from anesthetics, in which case manual or mechanical ventilation should be provided.
Pulse oximeter (%SpO2) can be used to monitor oxygen saturation in arterial blood (> 95%). If the reading is low, it can be due to multiple causes (vasoconstriction, location, pigmentation, fur, movement, etc.). To confirm %SpO2, arterial blood gas analysis should be performed.
Maintenance of inhalation: In general, with premedication on board, inhalation agents should be maintained ~ 1–1.5 MAC. Therefore, in healthy animals, isoflurane, MAC 1.3 (dog)/1.6 (cat), should be maintained ~ 1.3–1.95% (dog) and 1.6–2.4% (cat). Inhalation, of isoflurane for example, causes vasodilation and suppresses cardiac contractility in dose dependent manner. Therefore, the use of inhalation should be as low but safe as possible.
Body temperature: Hypothermia is a common complication during anesthesia. Body temperature should be monitored. During severe hypothermia, anesthetic requirement is reduced; therefore, the amount of anesthetics should be decreased accordingly. Hypothermia may cause prolonged recovery.
Hypoglycemia: Hypoglycemia can be seen during anesthesia due to food/water fasting during anesthetic preparation. Therefore, the glucose level should be monitored.
Pain control should be provided throughout the procedure from preoperative, perioperative and postoperative periods. Opioids (i.e., full or partial μ and κ receptor agonists) are the mainstay in controlling pain based on severity of pain and health status of animals. Other classes of drugs that are commonly used during perioperative periods are ketamine and α-2 agonists (premedication). Several routes (IV, IM, SC, epidural, local infiltration, CRI, patches, etc.) of administration can be performed. Understanding pharmacological effects of analgesics, anesthetists' familiarity with analgesics and availability of drugs are important in determining appropriate procedures on a case by case basis.
1. Greene SA. Veterinary Anesthesia and Pain Management Secrets. Philadelphia, PA: Hanley & Belfus; 2002:1–43.
2. Thurmon JC, Tranquilli WJ, Benson GJ. Essentials of Small Animal Anesthesia & Analgesia. Baltimore, MD: Lippicott Williams & Wilkins; 1999.
3. Seymour C, Duke-Novakovski T. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 2nd ed. Gloucester, UK: British Small Animal Veterinary Association; 2007.