Imagine yourself, flying a plane through the night. Visibility is extremely poor, and you can only guide yourself through the instruments. Now, imagine one of the parameters shuts down, then two, then three, until they're all gone.
This is the feeling we get in a poorly monitored anaesthesia. But we will learn how to avoid that in this video lesson, OK? Bring it on!
Anaesthesia monitoring What's up, NAVE crew! In today's Anaesthesia Unravelled video lesson we'll be talking about anaesthesia monitoring. In the old days, perianesthetic mortality rates, both in human and veterinary medicine were very high, mainly due to the quality of the drugs and monitoring, that was quite poor.
Today, however, we are living a different reality. The drugs are much safer and monitoring has developed a lot. Some other factors also contributed in decreasing death rates, such as the adoption of anaesthesia check-lists and an adequate preoperative evaluation.
In the veterinary, the mortality rates in healthy patients are of 0. 17% for dogs, 0. 24% for cats, 1% for horses and more than 2% for rabbits.
It seems low, right? But it isn't, especially if we compare it with human medicine, where the death rate in healthy patients is of 0. 005%.
Those rates can increase up to 10x when the patient has any comorbidities. So, even though mortality has decreased a lot in the last decades, we can still improve a lot. And anaesthetic monitoring is a fundamental part of this process.
The monitoring should cover as many parameters as possible, focusing on circulation, ventilation and temperature of the patient. It must be done continuously and recorded in spreadsheets. We have a very detailed video on how to develop and fill out an anaesthetic record, if you haven't seen it, go check it out, OK?
This video lesson will focus on basic transanaesthetic monitoring. So, we will be covering anesthetic depth, pulmonary and cardiovascular parameters, and thermoregulation. We will NOT be covering BIS, CVP, cardiac output, DeltaPP, doppler ultrassound.
. . Our focus will be on quality basic monitoring.
We'll also not be discussing the interpretation of curves and waves right now, this will be done in other video lessons, OK? Promise. Anaesthetic depth When we perform general anaesthesia, we can tell what is the anaesthetic depth of our patient.
Around the middle of the 19th century, Jon Snow No, not this Jon Snow, this other one. This John Snow was the first doctor that tried to establish patterns of anaesthetic depth, but only in the early 20th century did people really begin to understand a little more about anaesthetic depth. An army lieutenant doctor, Albert Guedel, described each stage of general anaesthesia.
He divided anaesthetic depth in four stages, the third of which, the stage of surgical anaesthesia, was further divided into four additional planes. We can see in this original chart that Guedel based his theory mainly in muscle activity, breathing pattern, in the positioning of the eyeball and also in pupillary dilation. So, for a long time, the assessment of anaesthetic depth was based on these anaesthetic stages, and we generally tried to keep the patient between stage II and stage III, which are considered surgical stages.
Today, we follow a slightly more flexible classification, which is based on superficial stage, adequate stage and deep stage. These stages are based on the intensity of the protective reflexes, such as the laryngotracheal, interdigital, the eyelids and corneal reflexes, and also the cardiopulmonary parameters. I suggest you watch this video later, which explains in detail each anaesthetic stage on the dog, but there's also a PDF in the video's description that characterizes each stage.
I don't know if you noticed, but I repeatedly mentioned the anaesthetic depth in general anaesthesia. Those characteristics are not applicable for dissociative anaesthesia, because there won't be generalized depression of the brain. Cardiovascular monitoring Heart rate assessments only tell us that there's electrical activity in the heart, but by itself, does not represent a lot, so it always has to be associated with other cardiovascular parameters, so that we can understand how the cardiovascular system of the patient is faring.
We can evaluate the heart rate by stethoscopy, electrocardiography, oximetry, arterial Doppler and even palpation, in some cases. In fact, the heart rate by itself doesn't have much relevance, because it can give us the false impression that the animal is very deep or very superficial in anaesthesia. Usually, we relate tachycardia with superficialization of the anaesthetic depth.
However, in hypovolemic and hypotensive patients, the tachycardia will be present even in slightly deeper stages, in a compensatory way. Likewise, a patient with bradycardia can also not be deep in anaesthesia, such as in the presence of an intense vagal stimulus. Then there's no point in assessing cardiac rate.
No, that's not what I'm saying. It is important. But we need to understand that it is only important if associated to other parameters, mainly blood pressure.
Electrocardiography, or ECG, on the other hand, is more interesting, as we're going to have, in addition to heart rate, the heart rhythm. This cardiac activity is obtained via electrodes that form leads. It's possible to see in these pictures that in small and medium sized animals, the main leads are DII and DIII.
In large animals, we use the base-apex lead. ECG tracings consist, basically, of three waves: the P wave indicates atrial depolarisation. The QRS complex indicates atrial repolarization and ventricular depolarization, while the T wave indicates ventricular repolarization.
With that, leads DII and DIII will show this wave pattern, while the base-apex lead will give us the complex in an inverted form, due to the electrode layout. Even though the ECG is much better for monitoring than a simple stethoscope, we have to understand that we're not going to have information on the blood flow, so we may find a normal wave pattern in a hypovolemic patient or even in a patient in cardiac arrest. I beg your pardon?
It's true, this is called electromechanical dissociation. So, in the same way than the heart rate, the ECG should also be associated with blood pressure measurements. Arterial blood pressure is certainly one of the most important parameters in anaesthetic monitoring.
It provides us with reliable information, assuring that the heart is ejecting blood properly and that this blood is reaching peripheral tissues. Indirectly, we can also acquire the heart rate, depending on the equipment we are using. AP monitoring is very important, because it consists one of the most reliable parameters to show that, if the animal has hypertension, it is superficial in anaesthesia, and if it has hypotension, it’s deep in anaesthesia.
Obviously, we have special situations, such as patients which are bleeding and shocked or even that have received drugs that alter the blood pressure. But in general, it is very reliable in regards to the anaesthetic depth. Roughly speaking, we should keep these patients with a mean blood pressure between 60 and 100 mmHg.
Below that, we're going to have renal vasoconstriction and muscle and brain hypoperfusion. In horses, we have to keep mean blood pressure at a minimum of 70 mmHg to ensure muscle perfusion. Kept lower than that, it can result in post-anesthetic myopathy.
AP can be invasive or non-invasive. Let's check out the differences between them. Invasive monitoring is known as the gold standard for AP monitoring.
In this case, we'll need to do arterial puncture. Usually, in small animals, we access the dorsal pedal and femoral arteries. In horses, we access the facial, transverse facial, and metatarsal arteries.
In swine, ruminants and also rabbits, we use the auricular artery. So, after arterial puncture, we connect this catheter to a rigid PVC extension tube. We can also use a 3-way cannula to collect samples during anesthesia and this rigid extension is connected to a pressure transducer.
This pressure transducer must be zeroed at the heart level of the patient, in order to give us accurate values. I suggest you watch this video here, because it explains, step by step, how to obtain an arterial line in dogs, and assessment of AP. We can also connect this rigid extension to an aneroid manometer.
This is old, huh? I used it a lot. It works the same, but in this case, we’ll have mean arterial pressure, and not the mean, systolic and diastolic pressures.
So, the transducer will give us this wave pattern, in which we have the peak pressure, that represents the systolic arterial pressure, the bottom depression, that represents the diastolic arterial pressure, and with these two values the equipment calculates the mean arterial pressure. Obviously, it’s always better to evaluate invasive blood pressure, but it has some drawbacks like, for example, the person has to have the ability for arterial puncture, it can promote embolism and even infections. So, usually, in ASA I or ASA II patients, we do not evaluate invasive blood pressure, we usually reserve it for the complex cases, but it's your choice.
Non-invasive blood pressure, on the other hand, is easy to obtain, even you can do it, because it does not require arterial access. It is not as accurate as the invasive method, but we usually agree on a variation of 10 mmHg. To acquire NIBP, we generally use two methods.
The first is the oscillometric method, in which we place a cuff around the limbs or even on the tail of the animal. The monitor inflates the cuff to stop the blood flow, and then it deflates it until the flow returns. In this case, it'll give us the mean, systolic and diastolic blood pressures.
We should remember that this cuff's width needs to be 40% of the circumference, either that of a limb or the tail. With that, you'll get adequate values for the mean, diastolic and systolic blood pressures. Well, as I said, this method is very interesting because it is not invasive and it will usually give values that are very close to the invasive method.
The disadvantage is that it works in cycles, so this cuff inflates and deflates in intervals of, for example, 2, 3 or 5 minutes, which means we don't have the constant monitoring as as with the invasive method. Another important detail is that patients with very intense vasoconstriction, as under the influence of some alpha-2 adrenergic agonists, can have this reading impaired. We can also evaluate NIBP by arterial Doppler.
in this case, we place an ultrasonic probe over a peripheral artery and listen to the sound of the blood flow. Afterwards, we place a cuff above that location, similar to the oscillometric method, andl inflate this cuff until the sound gets interrupted. Then we deflate the cuff until the sound returns.
In this case, we will have systolic blood pressure. Systolic blood pressure should be kept between 90 and 120 mmHg. Pulmonar monitoring Look, I'm going to talk about respiratory rate because, otherwise, someone will say Jesus, Adriano doesn't monitor respiratory rate!
We do so, only it doesn't really bring any useful information. What matters most is the patient's ventilation. If it's well ventilated, it doesn't really matter if the respiratory rate is a little high or very low.
Respiratory rate can be assessed by observation of the chest expansion or even a reservoir bag if the patient is connected to an anaesthetic machine. Pulse oximetry is one of the most popular monitoring because it is continuous and easy to execute. It reflects the proportion of hemoglobin that is saturated with oxygen and, in this case, we can predict and prevent the occurence of hypoxemia before the patient has any clinical signs.
And for that, we place a pair of sensors on a region that is not pigmented and without hair, like the tongue, the lips, nipples or even the vulva, and according to the absorbance obtained, the device determines the percentage of oxygen-saturated hemoglobin. We consider oximetry values acceptable when above 94%. We can see in this picture here that, when these values fall below 93%, 92%, the patient quickly enters an hypoxemic state, where the arterial oxygen tension greatly decreases to limits around 60mmHg.
Ok, there are a lot of people who favours this monitoring, everyone has an oximeter, it's easy to place the sensor on the patient's tongue or lip, and you'll have the heart rate and saturation, but I have serious reservations concerning this technique, first, if the patient is receiving pure oxygen, either in general or dissociative anaesthesia, inhalation or intravenous, the values of oximetry will usually be of 98% or higher, while the PaO2 must be above 400 mmHg. So, we’re hardly going to notice a decrease in this parameter if the patient is receiving supplementation of oxygen. Another thing is that there are a myriad of situations in which there are measurement issues.
In case of hypotension, severe anemia, vasoconstriction, hypothermia, pigmentation and exaggerated compression of the sensor site, all of which will give us inaccurate values, obviously much lower than the reality. Then, what we do is to never trust the oximeter, we take it from a place and put it in another one, moisten the region, do some hocus-pocus, until the oximetry reaches our desired value. The problem is that the values may actually be accurate, the patient can be in a state of hypoxemia, and we are waisting time trying yo check if the values are true.
As it ends up, we don't really trust this equipment too much. I find it interesting to have an oximeter as monitoring in very specific cases, such as deep sedation, outpatient procedures, or even in anaesthetic recoveries in which the patient isn't doing so well, because we'll then have oximetry and heart rate and, generally, this patient may not be under oxygen supplementation, but other than that, for me, it doesn't help much. I can't trust it.
In fact, you shouldn't trust absolutely any choice of monitoring. The capnometry will offer us the CO2 concentration in one respiratory cycle, while the capnography gives up the wave pattern of this cycle. This equipment is very interesting, because it shows the concentration of exhaled CO2, that reflects very well the arterial pressure of CO2.
This value of exhaled CO2 must be between 35 and 45 mmHg. Above that, we may have hypotension due to hypercapnia and, below that, we may have vasoconstriction due to hypocapnia. In small animals, the difference between the exhaled CO2 and the PaCO2 is between 5 and 10 mmHg, whilst in large animals this difference can reach up to 20 mmHg.
This difference, while important, is usually overlooked in healthy animals, but several situations can make this difference increase considerably, such as hypovolemic patients, with reduced cardiac output, severe pulmonary shunt or even on mechanical ventilation or thoracotomy surgeries. In this case, in addition to evaluating ETCO2, it's interesting that we also have PaCO2. Wait a minute, does this equipment assess PaCO2 or not?
No, obviously not, but it's very similar to the PACO2, and we need to understand that it's done continuously. In order to evaluate PACO2 we need to keep taking arterial blood samples and analysing them to get results, so it's a slow and expensive process, while a capnographer will give us these values very accurately and continuously. Well, there are two capnograph models, a mainstream model, where the sensor is attached to the anaesthetic breathing system, between the tracheal tube of the patient and the Y-piece, or even to an open system, and then the inhaled and exhaled gas passes through this sensor, which measures the CO2 concentration on the gas.
Due to the diameter of the sensor, it is only used in small and medium-sized animals. The sidestream works by collecting a sample of the gas from the anaesthetic system. This sample can range from 5 up to 200 ml/min.
In this case, a collection tube is attached to the breathing system, for example, in the Y-piece, and it can be used in any situation. Although there are some differences between the mainstream and the sidestream, they give us very reliable values. The standard capnography curve follows what we can see here.
Here we have phase 0, or A, which reflects the moment immediately before expiration, phase B, which is the moment in which expiration occurs, phase C, representing the alveolar plateau, and phase D, which represents inspiration. Keep in mind the standard curves that we talked about in each type of monitoring, because then we'll be discussing possible changes on these waves and curves, OK? Temperature monitoring Temperature is a simple and quite neglected method.
We can evaluate the temperature with a rectal thermometer or with an esophageal thermometer, which is what we usually use in an operating room. This environment usually brings the patient to a situation of hypothermia, because of air conditioning and room temperature of about 21°C, so the patient tends to have hypothermia, mainly in surgeries involving laparotomy or thoracotomy. In this case, if we are facing considerable hypothermia as, for example, in domestic animals, around 35°C, we'll already have decreased cellular metabolism, cardiopulmonary depression and a delayed anaesthetic recovery, so that is why it's interesting to monitor the patient's temperature while in the operating room and use means to try and keep that patient in normothermia.
Hyperthermia is not very common, but it can happen, mainly in field surgeries or in capture of wild animals, during the warmer hours of the day. So, in addition to monitoring the temperature in these patients, we have to avoid high temperatures for field surgeries and captures, and also favor, when the patient is anaesthetized, places sheltered from the sun. As I promised, we'll talk about the main parameters of monitoring during the perianesthetic period.
What we have at this moment is heart rate, the ECG tracing, the mean, systolic and diastolic blood pressures, with its respective curve, the mean, systolic and diastolic blood pressures from the oscillometric method, the oximetry values with its curve, the inhaled and exhaled CO2 values with its respective curve, and temperature. So, in this case, I can assure you that this animal is well monitored. There are other parameters that are also assessed during anaesthesia, such as arterial blood gas and electrolytes analysis, which we'll be covering in another lesson, also lactate, urinary output, anaesthetic gases, in the case of inhalation anaesthesia, among others.
What really matters is that we have the knowledge of what is happening in the anaesthesia, assessing the anaesthetic depth, the cardiopulmonary parameters and temperature and trying to associate all this with the drugs that we use and with the physiology of the animal. If we follow this, anaesthetic success is practically guaranteed. As the conclusion to this video lesson we have that an adequate anaesthetic monitoring has a fundamental relationship to the success of anaesthesia.
It must be done from premedication to the patient's recovery. Perianesthetic monitoring should provide parameters that allow the assessment of the anaesthetic depth, the cardiovascular and pulmonary systems and the thermoregulation of the patient. Monitoring can be relaxed in situations of sedation and local nerve blocks, but should always be as complete as possible in procedures that involve high complexity.
Also, we cannot forget that it must be continuous and recorded in the anaesthetic record. Well guys, I hope this video has helped you better understand anaesthetic monitoring. If you enjoyed it, leave us a like, share and also subscribe to the channel, so you can be on par with new material, OK?
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