Goals of monitoring are to prevent morbidity and mortality related to anesthesia. Monitoring equipment will alert veterinary anesthetists regarding complications so anesthetists can take appropriate actions and make good decisions to correct complications before they become irreversible. Patients under anesthesia should be monitored from the time anesthetic drugs are administered until full recovery is achieved. Good clinical judgment of anesthetists and other basic monitoring methods (i.e., auscultation, palpation of pulse quality, observation of mucous membrane color, and capillary refill time) should always be applied. Multiple body systems should be monitored under anesthesia. One should not rely on only single monitoring equipment to judge the condition of patients.
Electrocardiogram (ECG) displays electrical activity of the heart providing a continuous reading of heart rate and rhythm. ECG shows electrical activity but not mechanical activity. Monitoring ECG during anesthesia for patients who have cardiac problems, blunt force trauma, metabolic or electrolyte disorders is essentially good practice.
Hypotension is common during anesthesia, therefore monitoring blood pressure is crucial. Because anesthetics may suppress cardiac contractility and cause vasodilation, 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. For noninvasive blood pressure measurement, there are two ways to monitor blood pressure noninvasively, oscillometric method and Doppler ultrasound method. For both methods, the cuff width size should be approximately 40% of limb circumference. Oscillometric method provides systolic, diastolic and mean blood pressure, and heart rate. The machine can be set to read as an interval reading (i.e., 5 min.) and will automatically run itself. This method may not be accurate with small patients. For smaller patients (i.e., cats or small breed dogs) Doppler ultrasound method provides a more accurate measurement; Doppler provides an auditory signal of heart rate and rhythm, and measures only systolic blood pressure (should be > 80 mm Hg), both of which can be measured manually. Another method is an invasive blood pressure measurement. This method requires the placing of an arterial catheterization, which can be connected to either a pressure transducer (systolic, diastolic and mean blood pressure, and heart rate) or an aneroid manometer (mean blood pressure). Invasive blood pressure method requires some skill for placing and maintaining the arterial catheterization, but the advantage is that it provides accurate and real time blood pressure readings.
Evaluating and assessing oxygenation and ventilation during anesthesia is important because anesthetics tend to suppress respiration in a dose dependent manner. Examples of commonly used monitoring equipment are below (but not limited to).
This method is a useful tool to measure ventilatory status of the patients. Capnogram displays waveforms; capnometer displays numbers; capnograph displays both waveforms and numbers. Capnograph measures the amount of CO2 at the end of expiration (end tidal CO2 ~ 35–45 mm Hg). It reveals normoventilation (ETCO2 = 40 mm Hg), hypoventilation (ETCO2 > 40 mm Hg), or hyperventilation (ETCO2 < 40 mm Hg). Waveforms from this machine also provide other information, for example, partial or complete obstruction of the airway, endotracheal tube displacement, CO2 rebreathing.
Pulse oximeter measures oxygen saturation in arterial blood shown as %SpO2. During anesthesia, with 100% O2, %SpO2 should read > 95%. This method is noninvasive, easy to use, and gives a real time reading with auditory signal; the machine can be portable. Hypotension, hypothermia, vasoconstriction, ambient light, fur, pigment, and movement may interfere with its reading.
Hypothermia is one of common problems during anesthesia. Hypothermia may lead to other complications, for example, in prolonged recovery, delayed wound healing, and suppressed immune function. Therefore, body temperature should be monitored and water circulating heating pad/Bair Hugger should be provided.
What is the monitoring machine I should have in my practice? Because, ideally, multiple body systems should be monitored during anesthesia, what you should have depends on what procedure is performing on patients, length of procedure, and patients' health status.
1. Seymour C, Duke-Novakovski T. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 2nd ed. Gloucester, UK: British Small Animal Veterinary Association; 2007.
2. Tranquilli WJ, Thurmon, JC, Grimm, KA. Lumb and Jones' Veterinary Anesthesia and Analgesia. 4th ed. Oxford, UK: Blackwell Publishing; 2007:440–533.
3. Thurmon JC, Tranquilli WJ, Benson GJ. Essentials of Small Animal Anesthesia & Analgesia. Baltimore, MD: Lippicott Williams & Wilkins; 1999:1–27.