Hello there! In this video lesson from the “Anaesthesia Unravelled” web series we will be talking about "Anaesthetic Monitoring". The success of anaesthesia is closely related to an adequate monitoring.
It should be based on the assessment of the depth of anaesthesia, cardiovascular, pulmonary and thermoregulation parameters. There are several important parameters in monitoring, but we are going to be addressing the fundamental, and essential, so that we can perform anaesthesia safely.
The assessment of the anaesthetic depth can be done whenever the patient is under general anaesthesia. The pattern was established by Guedel, at the beginning of the 20th century, and it is divided into 4 stages. Stage 3 is subdivided into 4 planes, of which 2 and 3 are considered surgical planes. We currently follow a slightly more flexible nomenclature, dividing stage 3 into superficial, adequate and deep planes. For this, we assess the protective reflexes and cardiovascular changes. In this link you can watch these planes in details.
Basic monitoring includes HR, electrocardiographic tracing and BP. HR can be obtained through several methods, but the best is by ECG, which will provide HR and heart rhythm at the same time. However, it is essential that BP be always monitored, because this way we will get important information on electrical activity and also blood output.
The gold standard for BP monitoring is the invasive method, in which we need to perform arterial puncture. This method will give us continuous and accurate BP values. We can also monitor BP non-invasively, using an oscillometric system or arterial Doppler. In this case, the method is easier and faster, but the values are not continuous and we may have reading errors. Other parameters evaluated are CD, CVP and DeltaPP.
Basic monitoring includes RF, SpO2 and ETCO2. SpO2 is an easy and practical method of monitoring, which provides the % of red blood cells with O2. Values below 94% already indicate hypoxia, which may be associated with hypoxemia. ETCO2 is another very important parameter, as it is closely related to PaCO2 but, unlike the latter, ETCO2 is obtained easily and continuously. We can still assess capnography, which, depending on the shape of the curve, can indicate changes in the breathing pattern. Other parameters assessed are ventilometry and blood gas analysis.
The temperature is sometimes overlooked, but it is very important as well. Hypothermia results in cardiovascular depression, decreased cerebral metabolism and delayed recovery from anaesthesia. On the other hand, hyperthermia can promote an increase in metabolism and an increase in O2 consumption. The ideal, obviously, is to keep the patient in normothermia.
There are other types of monitoring, as aforementioned, as urine output, lactate, bispectral index, among others. However, the most important is to know the patient's pathophysiology, the mechanism of action of the drugs used in the anaesthetic protocol and the physiological data obtained in the monitoring, so that we can maintain the patient in adequate anaesthesia, with minimum cardiopulmonary depression.
– Quadro de Estágios e Planos Anestésicos em Animais Domésticos
– Carregaro AB, Silva ANE. Monitoração Anestésica. In: Luna SPL, Carregaro, AB. Anestesia e Analgesia de Equideos, Ruminantes e Suínos. 247-280, 2019.
– Cerejo et al. Effects of cuff size and position on the agreement between arterial blood pressure measured by Doppler ultrasound and through a dorsal pedal artery catheter in anesthetized cats. Vet Anaesth Analg, 47:191-199, 2020.
– Flaherty D, Musk G. Anaesthetic monitoring equipment for small animals. In Practice, 27:512-521, 2005.
– Garofalo et al. Agreement between direct, oscillometric and Doppler ultrasound blood pressures using three different cuff positions in anesthetized dogs. Vet Anaesth Analg, 39:324-334, 2012.
– Heliczer et al. Accuracy and precision of noninvasive blood pressure in normo-, hyper-, and hypotensive standing and anesthetized adult horses. J Vet Intern Med, 30:866-872, 2016.
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