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The Veterinary Detective:
The Case of the Low-Pressure Labrador

 

released February 2, 2021

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Dr. Ashley Mitek: Hi, I'm Dr. Ashley Mitek, a member of the faculty at the University of Illinois, College of Veterinary Medicine. This week we present our final installment of, "The Veterinary Detective". In each episode we discuss a case with a veterinary clinician who walks us through the diagnostic process to help us understand how they apply clinical reasoning in their practice.

This week we look at the unique role of veterinary anesthesiologists in the patient care team. These specialists oversee all patients receiving anesthesia in a large hospital. There could be several patients anesthetized at the same time, all having different surgeries in different areas of the hospital.

Just as in human medicine, the anesthesiologist reviews the case and the procedure and helps develop the anesthetic plan before the patient has surgery. During the procedure, a veterinary technician monitors the patient while they are anesthetized. Occasionally, a patient will have some abnormalities under anesthesia, and then the staff managing the case will call the supervising anesthesiologist to help assess the situation.

We’ll take a look at one of these examples in this episode we call “The Case of the Low-pressure Labrador".

Joining me is Dr. Graeme Doodnaugt, a board-certified veterinary anesthesiologist and pain management specialist here at the University of Illinois veterinary teaching hospital.

Let’s hear about the case.

Dr. Graeme Doodnaught: The case we're going to talk about today is a six month old, 20 kilo Labrador. She's currently a female intact, because our plan today is to spay her. And a bit of the story is going to focus a little bit more kind of on the hypotension occurring during the case.

Dr. Mitek: Hypotension means that your patient’s blood pressure is below normal. This is a problem because the body’s cells could be damaged when blood pressure is too low because they aren’t getting the oxygen they need, and waste products are not being removed.

In anesthesia, we typically focus on the Mean Arterial Pressure, also called MAP or M.A.P. for short. This is the average pressure in a patient’s arteries.

It’s different from the systolic and diastolic numbers you see when you take your blood pressure. For our anesthetized patient, MAP is more important to us because that variable helps us determine if the body can appropriately perfuse it’s organs. 

If your MAP is normal, between 60 and 140, the body is able to do something we call autoregulation, meaning the body’s organs regulate their own blood flow. If MAP drops below normal, which is below 60 millimeters of mercury (or mmhg), then these organs may not be able to appropriately perfuse themselves with blood. Perfusion is another way of describing the delivery of blood to an organ or area of the body. Lack of perfusion is very detrimental to the body if sustained for extended periods of time.

Let’s hear how Dr. Doodnaught responds to these situations.

Dr. Doodnaught: I think the first thing I'll say is, I calmly come over, because I think something that's really important is always just staying calm. Okay, so you got to think you know, myself as the anesthesiologist, you're absolutely right. I'm coming to a situation that, you know, maybe I haven't been there the entire time. Or, you know, as the veterinarian, you know, it might be your technician working on this case. If you come over in a panic, they're gonna panic, and then this is when mistakes are made. Right?

So I think focusing on that. But certainly, once I've come over, I'm going to assess, you know, you know, is this hypotension real? Okay, how many readings have we had? Okay, so how do I assess whether it's real? Well, it's actually fairly hard in general practice. I have the benefit in the University, you know, sometimes we have arterial catheters. And that arterial catheter, if I connect to my monitor gives me an invasive blood pressure, which is the closest to the real blood pressure of our patient and is constantly monitored. Non-invasives are nice because we don't have to place a catheter which is invasive. But the problem is they're not always that accurate.

Dr. Mitek: Dr. Doodnaught talks about two different types of blood pressure measurement: invasive arterial catheters which are typically only found in specialty hospitals, and non-invasive blood pressure measurement. That cuff that your doctor puts on your arm is an example of a non-invasive blood pressure device.  

It’s important to recognize what type of blood pressure equipment is being used when you interpret the patient’s data for a hypotension diagnosis.

Let’s hear more about other factors to consider when interpreting patient data under anesthesia.

Dr. Doodnaught: Generally, I'm trying to look at the whole picture as well. Okay. Has this patient been cruising around at the same blood pressure and this is just one sudden drop and it doesn't really make sense? I might think about, is this reading real? Okay, so I might reflect on well, what were we doing at that point? Does it make sense, either physiologically, or based on the drugs we just gave? Or, you know, is this something else to do with surgery? You know, were we moving the dog as the reading was taken, you know, these can lead to artifacts.

Dr. Mitek: Artifacts are erroneous blood pressure readings. This could be a motion artifact, for example, when you are pushing a dog on a gurney. Or choosing the wrong size blood pressure cuff could also result in artifacts.

Now when anesthesiologists are presented with a problem, one of the first steps is to understand what drugs have been given to the patient so far. Often that information provides a clue to treating or understanding the underlying problem.

Understanding pharmacology is a fundamental skill for any anesthesiologist. In this case, the patient received a drug called Acepromazine for sedation and Hydromorphone, an opioid, for pain management. Let’s hear what Dr. Doodnaught has to say about drugs in this case.

Dr. Doodnaught: I'm not just thinking about dose. I'm not just thinking about the drug. I'm thinking, “When did we give it?” Okay, so, you know, if we're at the middle of that procedure, because we've probably removed both ovaries at this point. You know, I'm thinking back to well, we pre-meded We did about 15 minutes, then got a catheter in. Had to do our prep. So you know, we might be 45 minutes, an hour after that pre-medication. So I'm at the peak effects certainly have my Acepromazine. Acepromazine. What does it do? It causes vasodilation. That's probably its principal mechanism for decreasing blood pressure. Hydromorphone. Definitely going to be at its peak level there. It causes a bradycardia. Right.

Dr. Mitek: At this point in the case the patient’s heart rate is 40 beats per minute, which is low. And the mean arterial pressure is 55, which is also a little low.

With any case, when a veterinarian is presented with a problem, we start to think of the potential causes of that problem. And with hypotension, it has three possible root causes.

One is vasodilation, when your blood vessels relax and lose their tone.

Second is decreased cardiac contractility (which is also known as decreased inotropy, meaning the heart is not squeezing hard enough with each beat)

Finally, hypotension can be caused by a low heart rate, also known as bradycardia.

You can always trace hypotension back to at least one of these causes, although it could be a combination of these. And identifying the cause of hypotension is the first step in correctly treating it.

Dr. Doodnaught: What's easy, what's attached to the patient already is going to be fluids. So I might start a fluid bolus. Now things I think about there is, you know, if there's any cardiac disease, pulmonary disease, renal disease, I might be a little bit more cautious in my use of fluids. But certainly, for a healthy six-month-old dog, puppy that's coming in for a spay, fluids are absolutely going to be up there on my list of here's something I'm going to want to try.

So what fluids are going to do for me is potentially fill up my vascular space. Okay, and that's going to help counteract that vasodilation. I'll kind of give you a visual in a second. If you also think back to sort of basic physiology, Starling told us that if you increase preload, you're going to improve contractility. So as those myofibrillar in the heart stretch, as you give a little bit more fluid, they contract with just that little bit more force. Now, it wasn't necessarily a column, I put an X in actively, but as I mentioned, virtually every anesthetic drug is a negative inotrope. So it's decreasing cardiac contractility. So I think it's going to help. In terms of dose, I'm probably thinking about 10 ml per kilo over about 10 minutes. Partly because it sounds good, but you know, it's an effective dose in my experience. Certainly, if I'm thinking about a patient who I want to judiciously use it, I'm probably going to slow down that rate. I'm going to give smaller volumes. But that's just not the case here.

Dr. Mitek: And what did the patient do after you gave the bolus of fluids?

Dr. Doodnaught: I kind of said I was gonna give you like a little visual because I kinda want to explain what I'm expecting to happen in the patient.

So in your head, I just want you to imagine kind of like a water bottle. Now, okay, let's just take like a 500 ml water bottle, get rid of a quarter of it. Okay, and why did we get rid of a quarter of it is because we just starved this patient overnight. Okay. Patients are very good at managing - healthy patients are very good at managing their hydration level. But when we stop eating, you know, even if we say not to withdraw water, you know, this patient might vomit after that Hydromorphone I gave. It might have might be my last case of the day. It's been in the prep cages for a little while and hasn't had access to water. My patient might just be nervous and not want to eat not want to drink. They're not used to the change in it. So but because we're young and fit and healthy, we're not dehydrated. And if you kind of imagine that water bottle, just imagine the top of that label as being normo-tension.

Okay, so how that dog is managing its blood pressure as it's slowly producing more urine, also having some more ongoing fluid losses during the day is it's just squeezing that water bottle. It's increasing its systemic vascular resistance. It's vasoconstricting that little bit more just to maintain its blood pressure. This is the same reason why, during the day, if you haven't eaten anything, if you haven't drank anything, you've been working hard, and then you squat down and you go to stand up, you get a little lightheaded. That's orthostatic hypotension. That’s your body just not, it's already squeezed too much, it just can't adapt to a sudden change on top of it.

Now imagine that water bottle is nice and kind of constricted, but maintaining a good blood pressure. The body's happy. Organs are perfused. And then I premedicate it with Acepromazine. That bottle relaxes a little bit, you know, maybe not enough to be hypotensive. But certainly, maybe blood pressures come down a little bit. Then I gave that bolus of propofol at induction. That bottle relaxes more. And then I had to start isoflurane. Okay, well, now we're getting towards the bottom end of that label. And that label for me is kind of where hypotension starts. So that might be our mean arterial pressure below 60.

So if I have a sick patient, and you know, I do have the benefit of working in a referral hospital. I've got lots of drugs that can just very easily squeeze that bottle. Okay? Unfortunately, in general practice, we don't use a lot of those drugs. Those drugs aren't readily available. So what is the other option? Well, if my patient can tolerate it, I can just take my other bottle of water, so my fluids, and I can just refill my system. And that's going to bring that fluid line back up to normo-tension. And that's really what I'm imagining happening in this patient. So I'm kind of doing a little like, sort of estimate of how much of my how much dehydration I think my patient could have. And I'm kind of going along the way of, you know, did pressures come down incrementally at each step where I vasodilated? If the answer was yes, I'm really thinking we might have that kind of subclinical hypovolemia. Fluids are going to be definitely high up on my options. So I'm expecting this to have a positive impact. So I'm going to wait probably at least five to 10 minutes once I've started that to see. Okay, fluids are not instantaneous in, I would never advise you hammering in 10 ml per kilo in less than a minute. Okay? Your catheter probably would blow. You'd have a whole host of other problems even in a healthy patient. However, if after about five or so minutes, I'd expect to start to see that blood pressure trend up.

What's nice in this case, too, is I'd expect propofol to disappear at that same time. So I'd kind of expect my blood pressure to come back. However, I can understand physiology only so much, because that's what the textbooks tell me is going to happen. But unfortunately, after about five minutes, nothing has changed. I think what's really important is five minutes later, what am I doing and what have I done from the start is I've checked the depth of my patient. Right? So when I got into the room after that propofol bolus there was there was no palpebral there was no jaw tone, but I thought the propofol was gonna fade out and that would get us lighter. Now I have a palpebral. But pressures are still low. So then really, the question is, where do I go next?

Dr. Mitek: And where did you go?

Dr. Doodnaught: Where did I go? It's a great question. So I just told you that fluids are going to refill that circulating volume. So it's kind of increasing that systemic vascular resistance. Kind of making up for that vasodilation we have. So that column, you know, I put a tick there and OK, I've tried to correct that. And I think I've done a pretty good job with the fluids thinking about kind of maybe like a 2% to maybe 5%, subclinical dehydration there.

If I feel I've addressed that, then okay. Well, what is the other column? Well, I told you, inotropy. So cardiac contractility. Well, I've worked on that, theoretically, with my fluid bolus, I've probably helped cardiac output. I just can't see that in my blood pressure, but my blood pressure is still too low to perfuse the organs. It's what's the one column I haven't done yet? Well, that's increasing my heart rate. Okay. So, at that point, I might think, well, my patient starting to get a palpebral reflex back so I maybe don't want to lighten the plane of anesthesia at the moment. So I might think about a dose of an anticholinergic. Now, I don't really have a true preference, which one I think you need to be aware of kind of what the pros and cons of each are.

Dr. Mitek: And can you explain what is an anticholinergic is, Dr. Doodnaught and, and why it might again be beneficial for this case?

Dr. Doodnaught: Anticholinergics, so glycopyrrolate atropine. They're used to treat bradycardia. So when your heart rate is low, what it does is it comes in and blocks the parasympathetic nervous system. So remember, your parasympathetic nervous system that's rest digest. That's being calm, okay? Well, we don't want that. We want the heart rate to pick up. So think about that sympathetic side. Well, we're not going in stimulating that. All we're doing is we're getting, we're blocking the body's kind of calmness. And that should cause the heart rate to increase. So we're kind of abolishing that vagal control. Couple little quirks with the drugs, probably a little bit more with glycopyrrolate rather than atropine is very early on, so if you go to give it like I said, you want to wait three to five minutes. And what can be a little bit disconcerting sometimes is, you know, this dog I said, has a heart rate of 40. I can give it maybe after the first minute or two, I might actually see heart rate drop. I want to encourage you to gain comfort in that and know that your drug is going to work. Okay?

What has happened is we've just blocked a little bit higher up in this system, something that we kind of see like a presynaptic block. And what that's doing is, that's actually causing the heart rate to slow down a little bit more. So there's two things we need to do. We need to wait longer. So if we haven't hit the peak effect of the drug yet. So glyco, that's why I'm saying three to five minutes. Atropine, this is why I say it's a little less common is maybe one to two minutes. I want to wait for that time. If I get to that time point, you know, three, five minutes plus in the heart rate is only starting to slow down now. I'm gonna then think about repeating my dose because that's the other thing is I just haven't given enough, right? So actually a heart rate slowing down, it might be disconcerting, because I have hypotension right now my blood pressure is still 55. Heart rate still 40. I want that heart rate to come up. But suddenly, it drops down to 32. If you have an ECG, sometimes you'll see second degree AV block. So where you see sort of P waves with no corresponding QRS or ventricular contraction, it can be very disconcerting. But it's actually a good sign, the patient is trying to tell you, hey, this drug is going to work for me, you just got to give it time or give me more.

Dr. Mitek: I think there are few things that initiate fear in practitioners and young veterinary students as when they start to see those arrhythmias with anticholinergics, and you wonder if you made the problem worse, or you're gonna make it better. So I think we hear the message loud and clear. Hold on, hold on, give it a little bit of time to work, right?

Dr. Doodnaught: Exactly. The problem is sometimes in that scenario is you're also trying to fix a problem. So you're you do have that fear of making it worse, but by giving too much too quickly, you might actually cause the a tachycardia hypertension, which could create a whole different problem. So I think just being aware of what the drug does there, knowing when that peak effects and just taking that moment. It's the same, like think of it as fluids. Okay, I start fluids. I'm not going to correct blood pressure. Now, it's going to take time for my drug to work. Fluids are a drug, Glycopyrolate, it's a drug. I just don't wait for it to work.

Dr. Mitek: And did the anticholinergic administration, in this case, fix your problem?

Dr. Doodnaught: So now that I've covered all three categories, increasing heart rate, to correct bradycardia, increased contractility, and increased my systemic vascular resistance, again, physiology dictates that it should have. However, my heart rate came up to 110. And my blood pressure didn't change. It's still 55. Okay, you know, not common, but these things can happen. Right? You know, I think it's one thing, you know, I sometimes get questions about, you know, what's the best way to manage blood pressure for this kind of case? And the answer is I don't know. The only one who knows the answer to that question is the patient and it's just, it's a little puzzle to solve.

And, you know, sometimes that fluid will work on that first instance. Sometimes it won't. Sometimes you go down a few different options and finally, find the solution. In sometimes you especially here, the U of I, you have I sometimes I go down multiple different perfusions, different drugs, and I can never do it. What's the best solution there? Minimize the time in hypotension. Getting the procedure done. Unfortunately, I joke sometimes that my colleagues in surgery tend to fix my problems, whether it's surgical stimulation, increasing blood pressure, or finishing the procedure and getting me out of it.

Because really, and that kind of leads to what my next plan was in this case is I'm creating the problem. This was a young, fit, healthy Labrador before I started it. I went and pre-medicated it for the benefit of the patient but creating a potential problem in anesthesia. So what did I do next? We're now 10 minutes into hypotension here. Heart rate did come up. You know, this fluid bolus is done, my pressures are still low. Well, what am I going to do? Well, again, I'm double-checking my depth. You know that that should be my first check all the time. Again, palpebral’s there. There is no jaw tone at the moment. Unfortunately, I can't go take back Acepromazine. You know, I can't I can't stick my needle in and aspirated out. I can't take back my hydromorphone. But what I can change is my isoflurane. So for that isoflurane, I'm going to probably think about decreasing it.

Dr. Mitek: Isoflurane is an inhaled anesthetic that is vaporized to keep patients asleep. One side effect can be hypotension. So by decreasing the percent of isoflurane delivered to the patient, that may help improve the blood pressure.

In anesthesia, it’s important to understand that we may try several treatments before the problem is resolved.

Let’s see if turning down the isoflurane worked.

Dr. Doodnaught: So yes, in this instance, it did. So fortunately, after about that, three to five minutes, I started to gradually see that blood pressure start to creep up. Okay? No, it didn't completely correct. You know, I didn't immediately have a mean arterial pressure of 90. But I got back into the 60s. And, you know, is it ideal? No, but we're now in that zone of autoregulation. At least the organs in the body will be able to get the nutrients, they can get the nutrients they need. They need to get the oxygen they need out of the blood. And also, importantly, kind of get rid of all the products of metabolism that they produced. And get them out, get things like CO2 back to the lungs, so the patient can breathe it off, etc. So that did fix the situation.

But ultimately, a lot of this boils down to kind of so you talked about it. And I got this discussion from my parents both trained as anesthesiologists, human. So I got kind of two tips before I started first one was exactly like you said is as anesthesiologists we know exactly what's going on with the patient when we walk in. So you know how bad it is, so you can't panic. You gotta keep it on the inside. Between that and coffee, I'm sure I've got a few gastric ulcers. But the next thing is, you should always be able to answer three questions. And you know, I kind of argue now four is to safely give any drug you should know what the effects are, what the side effects are, and how to treat the side effects. But what we've kind of talked about in this, this applies for the pre-medications. This applies for my anesthetic drugs. This applies for how I'm treating this, whether it's fluids, how isoflurane changes how glycopyrrolate works is onset and duration of action, because then I can start to predict when will I see this problem? And even better, maybe I can completely prevent it altogether.

Dr. Mitek: All right, well, this was a great case. And something I think our students and other veterinarians are gonna see pretty commonly. I'm gonna ask you one final question, which is, what did you - what did you not understand when you first graduated that you understand now?

Dr. Doodnaught: I'm going to maybe tweak my answer a little bit from just answering it directly. Because I feel like it's one of my flaws that I work on, on a daily basis. And that is time management. So certainly, as a student, whether it was high school, undergrad, vet school, and then even through internship, residency, time management was always a challenge for me. You know, leaving things to the last minute. Always kind of thinking like, okay, you know I always I always get things done. And that's always my excuse to myself of why, why it's fine is, you know, I get things done that need to be done.

But I think there was always this feeling of, I don't want to sound pessimistic here. But, you know, I remember that feeling vividly going, like, you know, my first year of vet school, oh, my goodness, this is the hardest work of my life. And then you get to second year, like, I didn't even know what I was talking about last year. And then you get to your final year in vet school, and there's this kind of just precipice of like, this is it. This is, this is what I've been working for. This can be an incredibly hard year. But then I'm going to be a doctor. And there's that sensation of, you know, the grass is gonna be greener when it gets to the other side. And I don't want to say it's not. There is an incredible accomplishment there. And that's fantastic. And, you know, I think I had a pretty unique route to where I am today. You know, I, obviously, undergrad, vet school. I did an internship straight out of school. But then I worked in mixed practice for about a year and a half. Then I did a Master's sort of at the same time as that. And then I started my anesthesia residency. So, you know, I've kind of worked in a number of countries, a sort of multiple different levels. And it's always a work in progress.

And I think that's the thing is, I haven't perfected it today. You know, I don't think there's a flaw in my character that I had when I was younger, that I've magically whisked away. But certainly, it's, it's something I try and work on on a daily basis. Okay, you know, technology I always assumed would make it easier. You know, having reminders to do's and it helps. But it almost sometimes makes life a little bit harder is that to do list constantly grows. Every email that comes in and, you know, with each step comes more responsibilities. So maybe there's a little less time but there's a little bit more importance on how you answer that. And I think what's helped me a lot is trying to just think about prioritizing what needs to be done now. What needs to be done short term. What needs to be done in the long term. Divide that and try and give myself that sense of accomplishment when I get through. I just no longer get through all my tasks for a day, because even when I come into my office in the morning, it's here are the six things I need to do today. And I walk out feeling like I've accomplished a lot today and think back to the six original things I thought I was going to do. And I may be lucky to have gotten through two of them because another 10 - you know, I had a couple other meetings, I had to come down help with something else, you know, other things get in the way. And that's just the reality of kind of each step. As more responsibility comes you're split into multiple different ways. So I kind of started by saying my time management skills were weak. They're still a work in progress, but just that feeling of accomplishment is there it's just sometimes a little bit more frustrating. And I don't know if that necessarily it's your question but time management.

Dr. Mitek: I think that's a perfect answer and explanation and it's something that I think we all we all struggle with. It's always a constant learning curve to that. So great advice sector do not and a great case. Thank you for joining us.

Dr. Doodnaught: It was my pleasure.

Dr. Mitek: So that’s the case. Our thanks to Dr. Graeme Doodnaught for joining us.  One thing to remember is that while an anesthesiologist may apply several treatments to remedy a problem, we don’t always know which intervention was most helpful.

And thanks to all of you for joining us. Please subscribe and tell your friends about the show.  It's available on iTunes or the podcatcher of your choice.

One last thing. In addition to this podcast we offer a wide range of learning opportunities for veterinary students and veterinarians. You can learn more about those by visiting  online.vetmed.illinois.edu

I’m Dr. Ashley Mitek, your veterinary detective.

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