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Understanding Anesthesia-Induced Hypotension with Dr. Vaidehi Paranjape - Ep. 12
874 Plays1 year ago
Imagine you are anesthetizing your last patient of the day, and the alarm for the anesthesia machine is blaring, warning that your patient’s blood pressure is far too low. The machine reads 80/40 (55). It probably doesn’t seem too hard to imagine, because anesthesia-induced hypotension is one of the most common cardiovascular complications of general anesthesia.
In this episode, host Dr. Bonnie Gatson welcomes Dr. Vaidehi Paranjape. Not only is Dr. Paranjape a board-certified veterinary anesthesiologist and assistant professor at Virgina-Maryland College of Veterinary Medicine, but she has also focused her research career on understanding how to appropriately identify, measure, and manage cardiovascular problems under general anesthesia. We will answer your burning questions about blood pressure monitoring, including: what are we measuring when we take an animal’s blood pressure, how do we know if we are measuring blood pressure correctly, which is the most accurate indirect method of measuring blood pressure: oscillometric or Doppler, and what are the most appropriate steps to take when dealing with a hypotensive patient under anesthesia?
We invite our listeners to check out articles mentioned in today’s episode:
- 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats
- ACVAA Recommendations for Anesthesia Monitoring in horses and small animals
- ACVIM consensus statement on the identification, evaluation, and management of systemic hypertension in dogs and cats
- Veterinary Blood Pressure Society
If you like what you hear, we have a couple of favors to ask of you:
Become a member at North American Veterinary Anesthesia Society (NAVAS) for access to more anesthesia and analgesia educational and RACE-approved CE content.
Spread the word. Share our podcast and FB/IG posts, re-tweet, post something on a network or a discussion forum, or tell a friend over lunch. That would really help us achieve our mission: Reduce mortality and morbidity in veterinary patients undergoing sedation, anesthesia, and analgesia through high-quality, peer-reviewed education.
We also ask our listeners to save the date for the NAVAS Virtual Spring Symposium on April 27th and 28th, 2024. For more information about the program, visit the NAVAS Spring Symposium website. Dr. Paranjape will be one of our featured speakers during the symposium, and several speakers will discuss blood pressure management under anesthesia.
Thank you to our sponsor, Dechra - learn more about the pharmaceutical products Dechra has to offer veterinary professionals, such as Zenalpha.
If you have questions about this episode or want to suggest topics for future episodes, reach out to the producers at education@mynavas.org.
All opinions stated by the host and their guests are theirs alone and do not represent the thoughts or opinions of any corporation, university, or other business or governmental entity.
The NAVAS Podcast is published monthly on or near the 15th of the month.
Special thanks to Chris Webster for editing, producer Maria Bridges, and Saul Jimenez for IT support in making this podcast a reality.
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Transcript
Introduction and Listener Engagement
00:00:06
Speaker
Hello, gas pastors everywhere. Welcome to another episode of the official podcast of the North American Veterinary Anesthesia Society, where our goal is to help veterinary professionals and caregivers advance and improve the safe administration of anesthesia and analgesia to all animals by bringing you the latest insights and expertise from the field. I'm your host, Dr. Bonnie Gatson.
00:00:32
Speaker
Before we get started today, I want to thank all the listeners out there that have been listening and providing great feedback on the show. I personally look through and read your reviews and they just really brighten up my day. If you like what you're hearing today so far, please spread the word and help to make this podcast a great resource for you and your veterinary team for all things anesthesia and pain management.
00:01:00
Speaker
As a reminder, you can email myself and the producers of this podcast at education at mynavas.org if you have any questions about a past episode or if you want to suggest a guest or topics for discussion. And for just one last bit of housekeeping, we here at Navas are asking our listeners
00:01:23
Speaker
to save the date for the virtual spring symposium on April 27th and 28th, 2024. It's an excellent way for both veterinarians and technicians to gain high quality anesthesia knowledge from leading experts in the field. More information about the symposium will be available soon on the NAVAS website, www.mynavas.org.
00:01:49
Speaker
I can't wait to see all of you there.
Cardiovascular Complications in Anesthesia
00:01:51
Speaker
On today's episode, we are going to take a deep dive into troubleshooting cardiovascular complications under anesthesia, and we are going to examine one of the more common anesthetic complications we see when anesthetizing many of our domestic species, hypotension.
00:02:07
Speaker
hypotension or low blood pressure can pose significant challenges during anesthesia and surgery. And it's essential for veterinary professionals to understand how to identify and manage this condition effectively.
00:02:23
Speaker
Our expert guest today to help us navigate how to best manage this commonly seen complication is Dr. Vaidahi Pranjapai. She is a boarded veterinary anesthesiologist, an assistant professor of anesthesia and pain management at Virginia Maryland College of Veterinary Medicine, and a leading researcher in the field of recognizing and managing anesthesia-induced low blood pressure in a variety of different species.
00:02:52
Speaker
Together with Dr. Paranjapay leading the discussion, we will shed light on the causes, consequences, and best practices for addressing hypotension in our anesthetized animal patients. We'll talk about some common questions when it comes to understanding blood pressure monitoring, like how do you know what cuff size to use? Does the location of the cuff matter? And does the value you get from the Doppler correlate to the systolic or mean blood pressure?
00:03:21
Speaker
And if you listen all the way to the end, you will hear our stepwise approach to treating hypotension using a few different clinical case examples. Also, just a slight apology in advance, but Dr. Pranjapai was a past resident of mine when I was in academia and I might gush about her just a little bit on the episode. Actually, I take back my apology.
00:03:49
Speaker
because I am very proud of everything this amazingly intelligent trailblazer has done in her short career. So, that being said, this episode promises to provide valuable insights into ensuring the safety and well-being of animals under anesthesia. So, without further ado, let's jump right into this vital discussion with our esteemed guest, Dr. Vaidahi Pranjapai.
00:04:16
Speaker
and learn how to navigate the challenges of hypotension in anesthetized animals. Welcome Dr. Pranjapai to the NavAss Podcast. We're so happy to have you here today to talk to us. Thank you so much for having me. I'm excited to be here. So why don't you start by introducing yourself to our listeners?
00:04:44
Speaker
My name is Vaidihi Paranjape. I'm an assistant professor of anesthesiology and pain management at Virginia Maryland College of Veterinary Medicine. Something you left out from your introduction is that Dr. Paranjape was actually my first resident that I trained when I was working in academia.
00:05:06
Speaker
And so I'm just really excited to kind of touch base with you again because it's been a while since you finished your training. And I'm just so proud of what you've accomplished because the reason I'm having you on the podcast is to speak today about managing and understanding anesthesia induced hypotension.
00:05:33
Speaker
And you have just published a flurry of research on managing and understanding cardiovascular changes that happen in our anesthetized patients. And I just can't gush enough about how proud I am. And I'm just like so happy that I was a part of your journey, your anesthesia journey.
00:05:55
Speaker
Yeah. Thank you so much for those kind words. I have been blessed to have great mentors throughout my journey here in my training as a board certified specialist in anesthesia. So I have a lot of people to thank for where I am today. So I ask all of my guests this question and I think I have an inkling of what you're going to say, but
00:06:20
Speaker
What fascinates you about anesthesia? And as a second part of that question, why have you decided to dedicate your career to understanding the cardiovascular changes that occur in anesthetized patients?
00:06:37
Speaker
Yeah, so I truly believe that anesthesia is the backbone of multi-specialty hospitals. You get to work with a lot of services and all species, any age and size. So our patient population is extremely diverse and presented with a broad range of medical conditions.
00:06:59
Speaker
What I really love about anesthesia is that it's a very hands-on, fast-paced specialty that requires a very strong knowledge base in anatomy, physiology, pharmacology, and medicine, which I enjoy all of those subjects. And as anesthesiologists, we get the opportunity to deal with stressful situations and have to make smart decisions quickly.
00:07:24
Speaker
And as the patient response towards the interventions is in real time, there is also immediate gratification from our clinical decisions. And throughout the perioperative period, we do play a very crucial role in optimizing the quality of patient care by using highly advanced medical devices and technologies to monitor the patient's vital signs and critical life functions.
00:07:53
Speaker
as well as administer the perfect blend of medications that can keep our patients unconscious and comfortable during the entire process. And the specialty allows us to recognize and treat pain in our patients. And not only is that greatly rewarding, but it is also a key contributor to successful patient outcomes.
Understanding Blood Pressure
00:08:17
Speaker
So let's dive into our conversation about blood pressure.
00:08:22
Speaker
Blood pressure is emphasized heavily as an important parameter to monitor in anesthetized patients. And it's really highlighted in the most recent AHA guidelines on anesthesia and monitoring, which to our listeners, I am going to make sure that those guidelines, there's a link to those in our show notes so you can review those as well. Cardiovascular complications like hypotension occur very commonly in anesthetized patients.
00:08:50
Speaker
So let's start with some definitions to guide our listeners through this conversation. With that, can you tell me what is blood pressure and what is
00:09:02
Speaker
cardiac output, systemic vascular resistance. Let's start with defining some of those terms. Yeah. So what you're asking me is basically the core concept underlying the parameters we try to target with various interventions commonly used for hypotension.
00:09:20
Speaker
And I'll start with the definitions. So blood pressure or systemic arterial pressure refers to the pressure within large arteries in the systemic circulation. And it is the force that the flow of blood exerts on the vessel walls, which drives tissue perfusion.
00:09:40
Speaker
This number will split into systolic blood pressure and diastolic blood pressure. So systolic pressure refers to the maximum pressure within the large arteries when the heart muscle contracts to propel the blood throughout the body.
00:09:56
Speaker
And the diastolic pressure on the other hand describes the lowest pressure within the large arteries during heart muscle relaxation between the beats. Since these blood pressure values are going to oscillate between a maximum and a minimum in one heartbeat, the mean arterial pressure is obtained by integrating the pressure curve and averaging the values in the course of a cardiac cycle.
00:10:20
Speaker
On the other hand, cardiac output is the amount of blood that is pumped by the heart each minute and is the mechanism whereas the blood flows around the body, especially providing blood flow to the brain and the other vital organs.
00:10:37
Speaker
Whereas the systemic vascular resistance is the resistance to the blood flow offered by the arteries and arterioles involved in the systemic circulation except those present in the lungs because that's going to be pulmonary circulation. And as the arteries constrict, the resistance will increase and as they dilate, the resistance will decrease.
00:11:01
Speaker
Let's integrate all of these together. So we have blood pressure, which is essentially the force that blood gives against the walls of the artery, our cardiac output, which is how much blood is circulating around the body over a certain amount of time, and our systemic vascular resistance, which essentially tells us how much resistance we have of the vessel wall against blood going through it. I hope I summarized that well. Yes.
00:11:31
Speaker
Good. Along with heart rates, how do all of these parameters relate to one another, especially when we're talking about what we're classically measuring under anesthesia, which is usually only blood pressure and heart rate?
00:11:48
Speaker
Correct. So blood pressure is primarily affected by the cardiac output and systemic vascular resistance. And by the mathematical equation, blood pressure equals cardiac output times systemic vascular resistance. So these three variables which are primary variables are very interdependent and have the ability to change one another.
00:12:11
Speaker
Now, cardiac output is the product of heart rate and stroke volume, where stroke volume is the volume of blood pumped out of the heart during each contraction. And stroke volume is influenced by three determinants, preload, which is the volume of the blood that is received back by the heart, the afterload, which is the other term for systemic vascular resistance, and myocardial contractility.
00:12:40
Speaker
So to summarize, all of these variables together are dependent on each other in some way. And the blood pressure, when we try to target it in order to improve it, it can be changed by variations in heart rate, preload, afterload or systemic vascular resistance and myocardial contractility. Yeah. So I think that is a really good point because a lot of times
00:13:07
Speaker
when I am working with technical staff or other veterinarians, they might say like, oh, you know, what heart rate under anesthesia is too low? What heart rate are you going to treat
00:13:19
Speaker
with an anticholinergic with something like atropine. And a lot of times my response to that is like, well, it depends. It depends on the heart rate and it depends on what medications you've given because all of those values are so tightly integrated to one another. I just find you can't, at least when you're making a decision about whether or not something requires intervention.
00:13:42
Speaker
when you're talking about the cardiovascular system, I never like to interpret any one of those values alone and I don't know if you agree with that or not.
00:13:51
Speaker
No, you're perfectly correct. I think it's important to understand that not every patient will go by the textbook. So not every patient, you know, the heart rate needs to be treated. It's a very individual specific scenario. So it does depend on their resting heart rates. It does depend on the drugs that are given to them.
00:14:13
Speaker
And whenever you decide or you are thinking about treating heart rate, you have to look also at the blood pressure. So I totally agree with you that just focusing on heart rate is definitely not the way to go. Why are we even monitoring blood pressure in the first place?
00:14:30
Speaker
So we are monitoring blood pressure because that is one variable out of the three variables that we discussed that we are able to closely monitor. And it will give you kind of like a rough picture of what's happening for the sex circulatory function because you are anesthetizing the patients. So they are under influence of a lot of drugs which can have a significant hemodynamic impact.
00:14:57
Speaker
So that's your best variable along with heart rate, obviously, that can give you sort of an idea of your hemodynamic picture in the patient. I always think of it as like an indirect marker of organ perfusion. And it's not a perfect marker because as we discussed earlier, it's influenced by many other things that we are not monitoring under anesthesia, for example, like
00:15:23
Speaker
stroke volume. We're not usually standardly monitoring what our patient stroke volume is, but it's probably, in my opinion, the best parameter we kind of have towards getting some kind of understanding of how well organs in the body are being perfused. I don't know if you think of it that way or you have a different way of thinking about it.
00:15:44
Speaker
No, that's absolutely correct. I mean, in the ideal world, it would be great if we could monitor cardiac output, right? That's the main variable that everybody's interested in. But under normal circumstances and in clinical settings,
00:16:00
Speaker
Right now, what we have available is the blood pressure and the heart rate. And the blood pressure is what drives perfusion to the organs and the tissues. So it's one of the main determinants of organ blood flow. So that's exactly why we have different devices which are very commonly available in order for us to do that. So I completely agree with you.
00:16:24
Speaker
Now that we kind of have an understanding of why we're monitoring blood pressure, let's talk about what's normal. So in a dog or a cat, what would you consider to be a normal systolic or mean pressure? And maybe also we could talk about what would be considered normal in another species like horses.
00:16:43
Speaker
Hypertension is a decrease in systemic blood pressure, which is below accepted values. And the ones that are published in small animals are quoted as less than 80 to 90 millimeters of mercury for systolic pressures, less than 60 to 70 millimeters of mercury for mean, and less than 45 millimeters of mercury for diastolic pressure.
00:17:08
Speaker
So measurements obtained above these values are going to be considered as normal and hence if you have a patient that has below normal systolic and mean blood pressures that warrants immediate action.
00:17:23
Speaker
Now, there are going to be species variations with respect to what's normal versus low between, for example, large and small animals. And that's mainly due to their large size and muscle mass.
00:17:38
Speaker
which increases the requirement for the mean blood pressure to ensure adequate perfusion to not just the organs, but to also the vastly distributed skeletal muscles. So that's why in horses, we want to maintain the mean arterial pressure above 70 to 80 millimeters of mercury in horses and large ruminants, because that's going to prevent the occurrence of a muscle compartment syndrome and post anesthetic myopathy and neuropathy.
00:18:07
Speaker
Yeah, and just to emphasize that in horses, something that we ask horses to do, which we don't ask a lot of other species to do after anesthesia is to stand up. And so if they have developed some kind of myopathy or neuropathy from a lack of appropriate perfusion to their muscles under anesthesia, then that can affect their quality of recovery. Yes, absolutely.
00:18:35
Speaker
Okay, so in your opinion, I mentioned earlier that hypotension is a very common cardiovascular complication that we see under anesthesia. So in your opinion, why is this happening? So the way I like to break down the causes of intraoperative hypotension is by classifying them under patient related, anesthesia related, and then procedure or surgery related.
Causes of Intraoperative Hypotension
00:19:04
Speaker
And examples of patient-related causes would be age. So neonatal, pediatric, and geriatric patients are more predisposed to hypotension as compared to adults than the ASA status. So patients with systemic comorbidities such as cardiovascular disease, endocrine disease, sepsis, neoplasia,
00:19:25
Speaker
Patients with pre-existing arrhythmias, hypotension, hypovolemia, dehydration, or any medication the patient is being administered such as anti-hypertensive drugs, beta adrenergic blockers that can also exacerbate the hypotension.
00:19:44
Speaker
Now, as far as anesthesia-related causes, hypotension observed in the first 20 minutes of anesthetic induction can occur with induction drugs such as propofol, alphaxolone, because they can cause systemic vasodilation. And opioids may cause bradycardia, which can precipitate hypotension.
00:20:06
Speaker
Inhalant anesthetics such as isoflurane and cevoflurane, which are commonly used for anesthetic maintenance across veterinary species, are notorious at causing significant impact on the cardiovascular system and that is in a dose-dependent manner.
00:20:22
Speaker
So just to mention a few mechanisms by which inhalants affect the cardiovascular system so significantly would be that they depress the myocardial contractility by dysregulation of calcium channels which are L-type or contractile proteins.
00:20:43
Speaker
They can also decrease systemic blood pressure by reduction in stroke volume, cardiac output, systemic vascular resistance, or combination of all of these. They also can attenuate the baroreceptor reflex, which every constituent of it. So it could be the afferent, efferent, the central processing. So any part of the reflex can get affected and be inhibited.
00:21:10
Speaker
And they can also slow down the rate of the sinusoidal node discharge, which can have a direct action on the cardiac pacemaker cells and the conduction pathways. So these are just examples of anesthesia-related causes. And lastly, procedure or surgery-related causes include any high-risk surgeries, bleeding, longer duration, which can influence the occurrence of intraoperative hypotension.
00:21:38
Speaker
Let's talk about measuring blood pressure.
Measuring Blood Pressure Accurately
00:21:42
Speaker
So we know that there are two distinct ways of monitoring blood pressure in an S size patients. We have our indirect method and we have our direct method. So just briefly, what's the difference between these two types of blood pressure monitoring?
00:21:57
Speaker
Yeah, so direct blood pressure is considered the gold standard invasive technique where we obtain measurements by placing a catheter into an accessible artery which is then displayed on the monitor as a continuous pressure waveform as well as numeric reading of systolic, mean and diastolic blood pressures.
00:22:18
Speaker
Like most clinical skills, the technique for placing arterial catheters is not difficult, but does require sufficient practice. And also there is expense of the equipment and frequent maintenance of the catheter and tubing can be challenging.
00:22:34
Speaker
And also complications can arise from catheterization, right, because it's an invasive procedure at the end of the day. So you may have bleeding from the catheter insertion site. There could be a hematoma formation. There could be significant hemorrhage if the system becomes disconnected and no one notices that infection and arterial thrombosis.
00:22:56
Speaker
On the other hand, the indirect blood pressure techniques like ossiometry and Doppler ultrasonography are affordable, are readily available. They require less technical skill. They pose less risk to the patient because they are non-invasive and are more routinely practical in clinical settings.
00:23:16
Speaker
And these common methods require a superficial artery of sufficient size that can be compressed easily. And they detect the return of the pulsatile blood flow after occlusion of the artery with an inflatable cuff. So these methods are very straightforward, but obviously you have to keep in mind that are going to be less accurate because of their intrinsic differences in methodologies.
00:23:44
Speaker
and they do not have the capacity of giving continuous real-time readings. So instead, they need to be taken as you need them. And depending upon the patient, you may need to restrain, which can falsely elevate the readings due to stress and anxiety. And also, it may be difficult for you to take a reading in a patient which is moving or shivering as far as the indirect methods go. Yeah, I'm going to focus our next part about indirect
00:24:13
Speaker
methods of measuring blood pressure, because those are obviously more clinically relevant. But I also get a lot of questions about those two different methods, Doppler versus Oscillometric. And before we get into that, both of those methods require the use of a blood pressure cuff. So let's focus on blood pressure cuffs first.
00:24:33
Speaker
I find that the literature on this can be somewhat confusing, but question being, what size cuff should I use and where should I put the cuff? At least to get the most accurate blood pressure reading. So in your opinion, where should we put the cuff? What's the best? Is it above a hawk, below the hawk, on the front, on the back of the patient? What is best?
00:25:02
Speaker
So firstly, in most species, what is followed as the recommended guideline is that the cuff width should be 30 to 40% of the circumference of the appendage. So if you're choosing the fore limb or the hind limb or the tail for which the arterial blood pressure is being measured. Now, the inflatable portion of the cuff should be positioned directly over the artery that you're trying to compress.
00:25:30
Speaker
And the appendage from which the blood pressure is being measured should be at the same level as the heart. When you're using a cuff, the appendage where the cuff is applied, like I said, must be mid-heart level. So measuring blood pressure in a location above the heart level will provide you with a falsely low blood pressure, whereas falsely high readings will be obtained whenever the location is below the heart level.
00:25:55
Speaker
So this concept is extremely important when it comes to cuff size and cuff placement, because that will significantly affect the accuracy of your reading. So in an ideal scenario, we want the patients to be laterally recumbent, lying immobile, relaxed, is usually the best position. And if I was to review the literature and try and find cuff positions that have given
00:26:25
Speaker
the best accuracy or the best results in comparison with direct arterial blood pressures. And there are many studies, let me tell you that. The locations that have come up a lot are the anti-brachium, which is above the median artery. And so that's the artery you're trying to target.
00:26:45
Speaker
or the coccygeal artery which is the tail. So either a tail cuff or the cuff placed on the antebrachium. I don't know if you've looked at the literature to see if we've looked at like above or below the hawk because that's another question I get asked.
00:27:01
Speaker
One of the studies that really pops, it's in conscious dogs, they place the cuff above the tibiotarsal joint over the cranial tibial artery and that they did it during hypotensive states and they compared the pressures from that location to direct from the dorsal-pedal artery, direct measurements.
00:27:22
Speaker
They did it with a device which is very readily available. It's called the Cardel, a wet blood pressure monitor. And it was shown that the performance of the device was pretty good using that location. Is there any differences in species? So dogs versus cats as far as a cuff location or cuff size is concerned?
00:27:44
Speaker
So interestingly in cats, there's a study with the Cardel monitor, and they also placed the cuff on the anti-brachium over the median artery. And they showed that the monitor came closest to correctly estimating the mean arterial blood pressure. While the other study, which used an ossometric technique in cats, they placed the cuff on the base of the tail. And it was shown to meet the recommendation criteria, which again, I think they're the similar sites.
00:28:13
Speaker
So, I feel like the locations that have come up a lot are the tail and the anti-brachium, and like I said, the t-butarsal joint. I don't know if you're familiar with literature and horses about using indirect methods, but I'm just curious if, as an aside to our listeners who may or may not be familiar with this,
00:28:33
Speaker
Very commonly in horses, we use direct arterial monitoring for blood pressure because it's very easy to catheterize. Equine arteries are quite large and they're very accessible. We oftentimes do that as opposed to dogs and cats where it takes a little bit more skill to catheterize an artery in those species.
00:28:54
Speaker
In horses, sometimes either maybe we can't get an artery or we just don't have the technology available. So are you familiar with whether or not indirect methods like ossometric techniques actually are accurate at measuring blood pressure in horses?
00:29:12
Speaker
I have gone through a few studies in horses. I will say this, that there's always going to be contradicting results, right? Like you know this, like even in dogs and cats, there are some studies which will say good agreement, there will be some that will say poor agreement. And you're right, like because it's so easy to catheterize an artery in a horse, the only time you would need a non-invasive blood pressure device would be if you're working in field conditions or it's going to be a short
00:29:41
Speaker
induction stall procedure. So a couple of studies that I have gone through, they have put the cuff on the tail, so over the coccygeal artery in horses. And the other place that I have noticed is the cannon bone. So the one on the tail has shown two decent measurements. So that's one place they propose to use in horses. What about bats?
00:30:08
Speaker
I'm just joking. Dr. Pranjapai and I published a study together, which took a long time to get that study published, but we looked at various different indirect blood pressure measurements in flying foxes. So I'm just... Yes. Yes. So I think it's a good segue into talking a little bit more about ossometric versus
00:30:32
Speaker
Doppler, the study that comes up for me is I think there was a study looking at the use of Doppler blood pressure in cats that was quite old and discussing whether or not that correlates better to the mean or the systolic pressure.
00:30:49
Speaker
So I'm wondering if you can comment a little bit about A, the use of Doppler in general as a means of measuring blood pressure and whether or not the number you get correlates to the mean or to the systolecular pressure and its accuracy during different size animals.
00:31:09
Speaker
In conscious and anesthetized normotensive dogs, Doppler has shown consistent underestimation of direct systolic arterial blood pressure taken from the femoral artery, the dorsal fetal artery, radial and coccygeal artery.
00:31:27
Speaker
And it has been very unreliable in detecting hypotension in anesthetized dogs, correctly identifying hypotension only about 66% of the times, which means low precision. While using Doppler in dogs which are less than 5 kilograms, interestingly, it has shown to provide an estimate of direct systolic blood pressure, especially during hypotensive states.
00:31:56
Speaker
Now, in CATs, there are two older studies that observed underestimation of direct systolic arterial blood pressure and better prediction of direct mean arterial blood pressure, which was obtained from the femoral artery. And the authors of that, one of the studies suggested we use a calibration factor of 14 millimeters of mercury to be added to
00:32:23
Speaker
the Doppler systolic blood pressure in order to estimate the femoral systolic arterial blood pressure. Now, this changed in 2014 when there was a study that came out in anesthetized cats that found Doppler measurements to be a poor predictor of both direct systolic and mean arterial blood pressures from the femoral and the dorsal pedal artery.
00:32:47
Speaker
And then there is a study from 2021 in Conscious Cats that says that the Doppler blood pressure does not correlate with ossometric mean arterial pressure. So the data is clearly contradicting and confusion is inevitable. So what I take away from these studies, because they can confuse any reader, and this is my way of approaching this or my recommendation, and this is for both Doppler and ossometric.
00:33:16
Speaker
is that ossometric and Doppler in small animals, the way they should be used is that individual patient should be assessed using the same machine. You use the same cuff size for that patient and position and document whether it is in a conscious state or sedated or anesthetized each time you're taking a reading.
00:33:37
Speaker
so that the changes over time can be assessed and this way they become their own control and we should focus not on the accuracy of absolute numbers but rather the trend of the numbers to help us guide therapeutic decisions during the treatment of hypotension or hypertension. I really like that idea is just using the animal as its own control
00:34:05
Speaker
I get asked a lot like, oh, should we focus on using a Doppler or also metric? I've had my own opinions, but I really like the idea of just picking one thing, documenting what you did, and then using repeated measurements, which I really like that. Yes, because what is tricky is that any reader who reads this literature is going to be confused because the results are very contradicting.
00:34:30
Speaker
And firstly, it's important to acknowledge that in veterinary medicine, consensus validation standards have not been established. But there are recommendations from the American College of Veterinary Internal Medicine. There is a hypertension consensus panel and there is a veterinary blood pressure society.
00:34:52
Speaker
So with all these validation studies are using the passing criteria that has been proposed by these institutions to test these indirect blood pressure devices. And the results are going to significantly vary between the studies because the type of device used is different.
00:35:11
Speaker
The cuff location is different. The animal population studied is different. There's a different methodology used to evaluate the wide ranges of blood pressure. So some studies are using phenylephrine and nitroprusside for hypotensive and, you know, hypotensive states. Some studies are using blood loss to create that same scenario. Some studies are using just playing with anesthetic depth. So the methodology is extremely different.
00:35:39
Speaker
And at the end of the day, the statistical analysis used is also when to affect the results of the study. So confusion is inevitable, like I said. How would you recommend treating hypotension in an otherwise healthy anesthetized dog or cat?
Treating Hypotension in Clinical Cases
00:35:59
Speaker
Yeah, so my first step is always to try and identify the underlying potential cause of hypertension. So I tend to rerun in my head the findings from the physical examination, the blood work, along with reassessment of
00:36:16
Speaker
physiologic parameters such as mucous membrane color, capillary refill time, pulse quality to diagnose if the perfusion is impaired. And at that time, I will also ensure that there are no technical errors which are causing low readings.
00:36:32
Speaker
So effect of cuff size, cuff position, transducer calibration in terms of direct blood pressure, leveling at the location of the heart, and inaccuracy of the monitor. So I check all of these things before I jump to treating hypotension aggressively. The next question is going to be, can the anesthetic depth be reduced?
00:36:55
Speaker
And this should be done extremely carefully after confirming an adequate anesthetic depth by monitoring ocular reflexes, muscle tone, and vital signs. And that is enough to support the degree of stimulation that the patient is exposed to at that point.
00:37:13
Speaker
So, since we are all aware that inhalant anesthetics are notorious by now in impacting the cardiovascular system, we are going to try and see if we can reduce their administration, which can significantly improve hemodynamics.
00:37:28
Speaker
So designing an anesthetic protocol that is balanced and multimodal, for example, using local regional anesthesia and administering additional analgesics and sedatives can tremendously help reduce the requirement of the inhalant that you're administering.
00:37:45
Speaker
Now, the next thing I do is if the patient is on the mechanical ventilator, which is controlling the ventilation, the ventilator settings may be adjusted to reduce the peak inspiratory pressure because that could be a source of hypotension due to reduced venous return and cardiac output. And however, this should be done without causing any significant effects on the alveolar gas exchange.
00:38:13
Speaker
The next thing then is to evaluate heart rate and blood pressure together that we were speaking about earlier. So hypotension is commonly associated with bradycardia or bradyarrhythmias. So treatment with an anticholinergic like atropine or glycopyrrolate is recommended, especially in vaguely induced bradycardia. Hypothermia can be another cause of bradycardia. Now remember, in this case, the anticholinergics probably are not going to be effective.
00:38:42
Speaker
And, of course, if you have used dexmedetomidine, so dexmedetomidine-induced bradycardia associated with high blood pressure is not something you would want to treat with an anticholinergic. The next in line for me would be fluid therapy, and that is in scenarios of suspected dehydration and hypovolemia, which will help correct ongoing losses
00:39:06
Speaker
fluid deficits and improve preload, which will eventually hopefully improve cardiac output. And we use balanced isotonic crystalloids to increase the intravascular volume. We give about 10 to 15 mls per kg intravenously over 15 minutes, which is commonly referred to as fluid challenge.
00:39:28
Speaker
And sometimes we also give colloids and hypertonic saline, which can be administered as plasma volume expanders following a crystalloid bolus. Now, remember that if the sole cause of hypotension in your normovolemic patient is vasodilation, then a fluid challenge may not make the blood pressure better.
00:39:52
Speaker
And now you have to move to the next step, which is going to be interventions that can enhance cardiac inotropy or increase systemic vascular resistance or do both. So common examples are positive inotropes like debutamine, which is a beta 1 adrenergic agonist.
00:40:11
Speaker
that can increase cardiac output in a dose-dependent fashion. There is norepinephrine, which is a potent agonist on the alpha-1 and alpha-2 adrenergic receptors, which results in peripheral vasoconstriction and venoconstriction, but it also acts on beta-1 adrenergic receptor, which causes cardiac stimulation.
00:40:33
Speaker
The other common one is dopamine, which is a hybrid drug with effects on dopamine, alpha, and beta adrenergic receptors, but that's in a dose-dependent manner. And then ephedrine is also administered sometimes as a bolus, which is a mixed adrenergic agonist, which stimulates alpha and beta adrenergic receptors. So if I was to summarize, it sounds like
00:41:00
Speaker
First, we are going to think about our patient and the surgery that's being performed as we spoke about earlier. Next step is going to be checking anesthetic depth and altering that or lowering our inhalant concentrations if we can do so safely for our patients.
00:41:17
Speaker
next step is going to be heart rates. So if our patient is bradycardic and hypotensive, we're going to treat that with something like atropine. Next step would be fluid boluses. And then the last step would be like pharmacologic drugs. Just to give our listeners a little bit more, we could probably do a whole podcast, honestly, on inotropes and vasopressors. There's probably a lot of stuff that we can say about that. But these are all drugs that are going
00:41:42
Speaker
to manipulate the sympathetic nervous system, which is kind of like our flight or flight type of response. So with the goal of either increasing the amount of blood that's being injected from the heart for each time the heart beats or potentially vasoconstricting our patients. I do have a few follow-up questions really quickly about those things that you just mentioned.
00:42:07
Speaker
The first question actually has to do with diastolic blood pressure because I feel like a lot of people understand that we need to intervene when our mi arterial pressure is below 60 or our systolic is below 80 millimeters of mercury. So I'm curious about what you do with low diastolic pressures and what that represents in our patients.
00:42:31
Speaker
So diastolic blood pressures are recorded during diastole, which is basically when the heart is filling with blood. And it represents coronary perfusion, it represents the volume, it represents the vasomotor tone. So typically anything less than 45 millimeters of mercury for diastolic pressures is considered low and needs to be treated.
00:42:58
Speaker
And the good question is how do you know if it's a volume issue or if it's a vasomotor tone issue, correct? So I think that's when you will have to evaluate the patient and reassess perfusion parameters and things like that to ensure volume is not an issue.
00:43:14
Speaker
But most of the times in an healthy, non-movalemic patient, the culprit is going to be the vasomotor tone because the patient is under influence of inhalant anesthetics, if you're using them, and that's what's going to cause vasodilation. So that's probably going to cause a reduction in the diastolic blood pressures as well.
00:43:33
Speaker
I don't think that there is a lot of data on this, but I have some questions sometimes about if we should adjust our treatment targets based off of our patient's pre-anesthetic or normal blood pressure.
00:43:52
Speaker
I have heard some anesthesiologists who will measure a patient's blood pressure before the procedure and then adjust their targets based off of pre-anesthetic measurements. Mostly this is for patients who have underlying hypertension. So I was wondering if you have any thoughts, any insights, or you know any studies about doing something like this. I would love to hear your thoughts about it.
00:44:21
Speaker
Yeah, so it's interesting you asked me this because I have been intrigued by the potential benefits of taking a set of pre-anesthetic blood pressure measurements on small animal patients. And if we review the literature in human patients from multiple cohort studies, it has been shown that the average pre-induction blood pressure is higher than the pre-operative blood pressures which are taken during the initial appointment.
00:44:51
Speaker
So this difference between the measurements is thought to be because of stress induced effects. I have been collecting data recently to evaluate whether the pre-operative blood pressure readings in dogs can be useful to predict the incidence of hypotension intraoperatively.
00:45:10
Speaker
And initial quick analysis that I ran on the available data that I have, these values are significantly higher than post premedication, post induction, and intraoperative. And this makes sense, right? Because there is influence of drug-related effects during the later time points.
00:45:31
Speaker
But even then, the pre-operative values are visibly significantly impacted by the temperament of the animal, stress and anxiety levels being in the hospital settings. So this is just something to be aware of when you're considering taking these measurements.
00:45:51
Speaker
Now, from my overall experience and the literature I have reviewed, I do see some advantages that I think it's a good practice to still take pre-anesthetic blood pressure measurements so that the patients with pre-existing hypotension and hypertension can be singled out. And based on these initial readings, one can define the peri-operative blood pressure targets that are specific for them.
00:46:21
Speaker
and also be prepared with a treatment plan sketched beforehand in patients with pre-existing hypotension as they can have a higher risk for chemodynamic instability under anesthesia. That's so interesting. Well, I can't wait to see the results of that study. Me too.
00:46:42
Speaker
I hope they're exciting and not disappointing, but yeah. I'm going to ask you something that I think would be a little fun to do because, well, at least fun for anesthesiologists. So I'm going to give you a clinical scenario, maybe a few clinical scenarios, and I am just curious how you would approach treating hypotension in these patients.
00:47:07
Speaker
in these different clinical scenarios. So let's start with an older dog. Maybe it's 10 or 11 years old and let's make it a chihuahua. So it's less than five kilograms. It's getting a dental cleaning, maybe some extractions.
00:47:23
Speaker
And the dog presents with a heart murmur, and we've taken some chest rheographs. The heart is enlarged. And maybe the dog also has some coughing, so it's clinical. And it's on some cardiac medications. Let's say it's on Pemobendan.
00:47:40
Speaker
And our anesthesia plan is going to be appropriate dosages of buprenorphine, alphaxolone for induction, and we're maintaining with isoflurine. So let's say this dog does become hypotensive during the procedure and its heart rate is about 70 beats per minute. And this happens maybe 30 minutes into the procedure.
00:48:03
Speaker
So, I am curious what your approach would be to treating hypotension in this particular patient. Yeah. So, firstly, we have to acknowledge that this is a geriatric patient. So, this is not classified as your normal ASA1 and 2, also because the dog has an evidence of cardiac disease as a non-cadiac medication.
00:48:25
Speaker
So firstly, for this dog, what would be important is to see whether we can decrease our inhalant requirement, which usually this is some more like a pre-anesthetic consideration, but a multimodal approach would have been selected where you could do partial intravenous anesthesia combined with a locoregional anesthetic technique.
00:48:48
Speaker
which they can do dental blocks in this case, right? So that should have helped with the pain aspect and helped you to keep your inhalant requirement in check. Now, in the current scenario, clearly the dog is on Pemobendin, which is a drug that is given for improving systolic function. So I'm assuming this dog has systolic dysfunction and with the cardiac disease. So it's very, very important that under influence of an inhalant,
00:49:16
Speaker
we maintain the myocardial contractility in this case. So one of the things which can be done is starting a positive inotrope such as debutamine which is a beta 1 adrenergic agonist and which improves myocardial contractility but also increases cardiac output in a dose dependent manner. So that's something which I would have incorporated in this case. Now you have to be careful a little bit here because
00:49:46
Speaker
Fluid therapy is not going to fit in this scenario because the dog has an evidence of cardiac disease. So it's probably best for you to be very cautious with the amount of fluids that you're giving. And that's why here, you know, we had one of our steps was to administer fluids. So that's not going to be a step here, which is going to be applicable.
00:50:07
Speaker
The other thing to consider is that the heart rate of this dog is 70, which it's a chihuahua. So in my opinion, it is getting very close to being bradycardic. So you could try a dose of glycopyrrolate to see if the heart rate increases, which can help you to increase the blood pressure.
00:50:30
Speaker
Now the other thing which can happen here is the heart rate may just go up by 40 beats so it would be about 110 which is tolerable for this dog but in an event with this glycopyrilate the heart rate increases too much that's also not good so that can affect your diastolic filling and reduce coronary perfusion
00:50:49
Speaker
So that's not going to be good for this dog. So as long as you maintain a higher normal heart rate for this dog, that would be beneficial, which in this case can be about 110 to 120 beats per minute. I think something interesting that sometimes gets overlooked when you're treating hypotension is sometimes just trying to get the patient
00:51:10
Speaker
off of the isoflurine. So you try to drop the isoflurine as much as you can, but sometimes you need some assistance. So I do get asked questions sometimes like, oh, is it okay to like, let's say instead of people referring to the dog was on like fentanyl. So should I increase my fentanyl to try to get the dog's isoflurine decrease? And in that clinical scenario, I might say I think it might be appropriate to try that. And if there is any bradycardia associated with increasing the fentanyl, we could just try to reverse that with some
00:51:40
Speaker
like with an anticholinergic. I have also sometimes tried, and this sometimes is successful, but I think it's like an option I oftentimes don't think about, which is I get them off of isoflurin. I try like Tiva, which is total intravenous anesthesia. Sometimes I've tried just switching them off isoflurin, put them on a propofol CRI. Propofol can cause hypotension, but
00:52:02
Speaker
Sometimes they respond a little bit better to that so I've done that before as well I don't know if you prioritize doing things like that for dogs they're just not being very responsive to some of our more traditional treatments. Or dogs like this one where they really can't get a big fluid bolus because physiologically it's just more challenging to for them to receive a large fluid bolus.
00:52:23
Speaker
Yeah, absolutely. So usually for me in critical ill patients, I always tend to design a multimodal well-balanced anesthetic approach. So it's going to be a partial intravenous anesthesia. So like you said, fentanyl CRI, ketamine CRI, lidocaine CRI. So of course, in this case now, fentanyl CRI would be a good choice because the dog has a cardiac disease, which will also help you to keep your inhalant requirement on the lower side.
00:52:49
Speaker
and then good dental blocks, effective dental blocks will also help keeping the dog in a good plane of anesthesia without having to increase the isoflurine too much. Now, I use a lot of intravenous anesthesia protocols combined with locoregional anesthetic techniques for sicker patients or high-risk surgeries or procedures or patients where I want to completely avoid the hemodynamic impact of inhalant anesthetics.
00:53:17
Speaker
So I feel like these kind of patients do pretty well on the protocol. Okay. I'm going to ask one more clinical scenario. Now we have a very angry German Shepherd and it is trying to eat everybody in the hospital and needs
00:53:34
Speaker
a mass removal, like a small mass removal from, let's say, the ansobrachium. And in order to be able to handle this animal, you have tried giving it some oral medications. So let's say the owner gave
00:53:49
Speaker
trasodone gabapentin at home, maybe also some ace-promazine. The dog comes in, it's still very angry. We've given it like 10 mics per kilo of dexmedetomidine and 0.1 mics per gig of hydromorphone. So it just had like all the dexmedetomidine. And it finally is handleable. We get a catheter in, be induced with propofol and
00:54:14
Speaker
hook it up to all your monitoring equipment and maybe 45 minutes into the procedure, we're noticing that this dog's heart rate has been 40 beats per minute, like pretty much the whole time. But when we hook it up to blood pressure monitoring, initially the dog's blood pressure is like 160 systolic. And now that
00:54:35
Speaker
the procedure has been going on for a little while. It's been like 30 minutes or so. Now we're starting to get systolic into the 80s and our means into like the low 60s, maybe 55. So how are you going to go about combating hypotension in this patient? And I guess where I'm really trying to focus this is how
00:54:58
Speaker
how are we treating dexmenetomidine related hypotension or hypotension and bradycardia when we see that with dexmenetomidine?
00:55:07
Speaker
Yeah, I think that's a great question. What's important to pick up in this scenario is that it has been a while since the dexmedetomidine has been administered. Clearly, whenever you are looking at blood pressure as an entity, you also have to look at heart rate. If you're looking at heart rate as an entity, you also have to look at the blood pressure.
00:55:28
Speaker
So clearly in this scenario, what I get from it is that the heart rate is low and the blood pressure is trending low as well now. And the reason for this is because even though the dog received dexametamidine, the initial hypertensive phase of the dexametamidine seems to be going away. And now you're actually seeing the hypotensive phase because the dog is on an inhalant anesthetic.
00:55:56
Speaker
So what's happened now is that the inhalant is becoming dominant. And that's why it's also contributing to the peripheral vasodilation. So over time, because the peak effect of the dexmenetomidine or the hypertensive phase of dexmenetomidine has now started to go away, the blood pressures are starting to come down. So in this scenario, what I would do is, because both of these entities are trending downwards,
00:56:24
Speaker
I would think it would be safe to give this dog a dose of glycopiolate to increase the heart rate in an event to increase the blood pressure. Now, I will say this as a side note because I think it depends on how anesthesiologists are trained because everybody does things very differently.
00:56:41
Speaker
There are different ways to do the same thing. I will say that sometimes in this situation, dependent upon who was training me, some anesthesiologists will also first try and reverse the dexmedetomidine to have absolutely no impact of dexmed on board and then give the anticholinergic.
00:57:04
Speaker
I guess the thought process is that you don't want to dialogue if the anticholinergic went dexmed, any effect of dexmed is on board. So there are people who still do that. I think for me, considering both the variables are trending downwards, it tells me that the hypertensive phase of dexmeditomidine is going away. So it should be safe now.
00:57:28
Speaker
Because the blood pressures are on the lower side, it should be safe now to administer an anticholinergic and slightly increase the heart rate of this dog. I'm sure we could spend so much more time talking about hypotension because it is such a fascinating topic and something that is
00:57:46
Speaker
So commonly seen as a cardiovascular complication in our anesthetized patients, but I think we've just run out of time to cover everything.
Closing Remarks and Call to Action
00:57:55
Speaker
But I really appreciate your time. I think I feel like we've just kind of covered the tip of the iceberg. Like there's so much more we could talk about, but I really appreciate you coming on the podcast and sharing your knowledge about treating cardiovascular complications in anesthetized patients.
00:58:12
Speaker
I am honored and I'm extremely grateful for this opportunity. It was lovely seeing you and chatting with you after so long. And it revived a lot of my residency memories with you as a mentor that helped me become what I am today. So I really appreciate everything you've done so far for me. Yes. Well, I can't gush enough about how proud I am of you.
00:58:39
Speaker
Okay, good. It's both ways. Amazing impact for the anesthesia community. So thank you for everything that you've done. You're very welcome. Thank you so much for having me.
00:58:58
Speaker
If you like what you heard today, I encourage you to check out NavAss and consider becoming a member. As a member of the North American Veterinary Anesthesia Society, you get tons of benefits, including access to CE events, focusing on anesthesia and pain management, blog posts, fireside chats with boarded anesthesiologists, as well as specialty technicians, and just so much more.
00:59:22
Speaker
visit www.mynavas.org to advance your anesthesia journey today. Also, as a reminder, we ask that you save the date for the Navas Spring Virtual Symposium on April 27th and 28th, 2024. More information, including programs and speakers, will be coming out shortly, so stay tuned for more updates.
00:59:47
Speaker
If you have been listening and enjoying this podcast, I would sincerely appreciate it if you could give us a like or subscribe to our podcast, write a review, or simply just tell a friend about this podcast. We appreciate any and all listener support. If you have any questions about this week's episode or the Navas podcast in general,
01:00:09
Speaker
Or if you want to suggest topics you would like for us to discuss in future episodes, please reach out to us at education at mynavast.org. We would love to hear from all of you. Also a huge thank you to our sponsor, Decra, without whom this podcast would not be possible. Visit their website, www.decra-us.com to learn more about their line of veterinary anesthesia products.
01:00:38
Speaker
This podcast was produced and edited by Chris Webster of Chris Webster Productions. Learn more at propodcastnow.com. I want to thank our guest, Dr. Vaidhehi Pranjape, for this insightful discussion on recognizing and treating hypotension in our anesthetized patients. More information about the studies and manuscripts mentioned in the podcast can be found in the show notes for this episode.
01:01:04
Speaker
And lastly, a huge thank you to all the gas pastors out there who choose to spend their time with me today on the Navas podcast. Becoming a skilled anesthetist is a lifelong journey of learning and self-discovery. So I hope you consider listening in the future. Until next time, I'm your host, Dr. Bonnie Gatson, and thank you for listening. I hope you consider tuning in next month for another episode of the Navas podcast.