The Cardiovascular System: A Pump, Some Pipes, and Fluid


One of my favorite mental pictures of the cardiovascular system is that of A Pump, Some Pipes, and Fluid. Broken down to this simple picture, it’s easier for me to interpret the information I gather as I monitor anesthetized patients.

A Pump

The heart is at the top of this minimalist’s view of the cardiovascular system by acting as a pump.  Its job is to pump blood around the body. The left side of the heart pumps oxygenated blood from the lungs to the rest of the body. The right side pumps stale blood from the body back to the lungs for a fresh supply of oxygen.

Some Pipes

The pipes, of course, are those estimated 60,000 miles of veins and arteries distributed throughout the body.  Most blood vessels can alter their size in order to accommodate the necessary flow of blood.  When a vessel’s interior grows larger to allow more blood flow, it’s called vasodilation.  When it shrinks down to decrease blood flow it’s called vasoconstriction.  Under normal circumstances, the vessels automatically vasodilate and vasoconstrict to help regulate blood flow through the body.  However, many anesthesia drugs alter the body’s ability to respond automatically in this manner.


It is generally accepted that most domestic animals have blood volumes of about 7% of their body weight (cats have a little lower percentage).  That equals about 70ml per kilogram or about 35ml per pound of body weight.  That means your 60 pound Labrador has a blood volume of about half a gallon.  When you consider that a half gallon of blood is pumped through 60,000 miles of blood vessels, you realize that it can’t be all places all the time.  The body is constantly making choices to route blood where it is most needed at any given point in time.

Pressure is the driving force for blood flow through capillaries that supply oxygen to organs and tissues. Blood pressure is needed to propel blood through vascular beds, with priority to those of the brain, heart, lungs and kidneys.  When I notice a drop in blood pressure, I immediately run through this simplified picture of the cardiovascular system. Why is the pressure dropping? Is the problem with the pump (ie not pumping hard enough or fast enough)? Is the problem with the pipes (ie vasodilation or positional occlusion of major vessels)? Or is the problem with the fluid (ie blood loss or vascular pooling)? The answers to these questions can help me anticipate a corrective treatment.

Anesthesia guidelines from the American College of Veterinary Anesthesia and Analgesia (ACVAA) and the American Animal Hospital Association (AAHA) urge us to monitor blood pressure during anesthesia, yet specialists say that blood pressure equipment alone is not the main ingredient to a smooth anesthetic event.  It’s the anesthetist’s knowledge that provides the greatest margin of safety for the patient.

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 


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Decision Fatigue – Building a Case For Checklists

How many food-related decisions do you make in a day? Ten? Fifty? A hundred?

When Cornell University staff and students were asked this question in a recent study, the average response was fifteen. However, after they answered specific questions about when, what, how much and where they ate, researchers found they actually made an average of 221 food-related decisions each day.

The more decisions we make throughout the day, the harder each choice is for our brains to process. No matter how rational and clear-minded you try to be, you can’t make the estimated 35,000 daily decisions we all make, without paying a biological price. It’s called decision fatigue. Although you are not consciously aware of being tired, each decision you make takes its toll on your mental energy. Eventually our brains look for shortcuts. One shortcut is to become reckless: to act impulsively instead of expending the additional energy to think things through. The other shortcut is to simply do nothing at all.

“Good decision-making is not a trait of the person, in the sense that it’s always there,” says social psychologist Roy F Baumeister. “It’s a state that fluctuates.”

Baumeister, a researcher and professor of psychology at Florida State University says, “The best decision makers are the ones who know when not to trust themselves.”

In high-pressure environments like a veterinary hospital, we are up against two main decision-making difficulties. The first is our own fallible memory and attention, especially when routine matters are overlooked under the strain of more pressing events. In the swirl of sedating a patient, placing a catheter, inducing anesthesia, clipping for surgery, and transporting the patient to the surgery table, it’s easy to overlook the routine task of checking the amount of oxygen left in the tank.

The second difficulty is that we sometimes lull ourselves into deliberately skipping steps, even when we remember them. Even the most competent people have been known to tell themselves that certain steps don’t matter. For example, it’s standard to check a patient’s pulse and heartrate during a physical examination. But being diligent to do both – listen to the heart and feel the pulse – rarely uncovers a worrisome issue. So sometimes we skip one. “It’s never been a problem before,” we tell ourselves. Until one day it is.

Simple checklists can provide protection against these mental failures. According to Atul Gawande, in his book The Checklist Manifesto, checklists remind us of the bare minimum necessary steps in any procedure by making them explicit. They catch mental flaws inherent in all of us – flaws of memory and attention.

A good checklist doesn’t have to be long to be effective. But good checklists are always precise and efficient. They leave zero room for interpretation. Don’t attempt to spell out every single step, instead provide reminders of only the most critical and important steps. Good checklists are, above all, practical.

Checklist ManifestoOnce a checklist is written, it’s important to be disciplined and stick with it. No matter how smart, talented or experienced we are chances are we’ll still drop the ball sometimes. And we’re not alone. The fact is, we are all plagued by missed subtleties, overlooked knowledge, and out-right errors. No one is immune to screwing-up. Checklists are absolutely essential for handling the high stakes and complex situations that happen to anesthetists. As Gawande points out in The Checklist Manifesto, simple checklists save time and money. But more importantly, in our line of work, checklists save lives.

For additional reading:
“Mindless Eating: The 200 Daily Food Decisions We Overlook,” Environment and Behavior, 2007, 39:1 (January), 106-23, Brian Wansink and Jeffrey Sobal.
“Do You Suffer From Decision Fatigue?” New York Times 08/21/2011, John Tierney
The Checklist Manifesto: How to get things right, by Atul Gawande
Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 


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Unlearn Old Habits to Avoid Compassion Fatigue


Many of us think we’re feeling burn-out with our jobs when we’re actually feeling compassion fatigue. Two terms for the same thing? Here’s how you recognize the difference. Burn-out always arises from dissatisfaction with your work environment. It’s generally because of supervisors, poor working conditions, low pay, and/or the relationships you have with the people at work. Compassion fatigue arises from the work that you do.

Compassion fatigue is a more user friendly term for Secondary Traumatic Stress Disorder, which is nearly identical to Post-Traumatic Stress Disorder (PTSD), except it affects those emotionally affected by the trauma of another. Charles Figley, professor of Disaster Mental Health at Tulane University’s School of Social Work and coauthor of Compassion Fatigue in the Animal Care Community says, “It’s the burden of caring. It’s the psychosocial sadness we take with us. It’s the stress of dispensing compassion.”

The solution to burn-out is pretty straight forward: find another job.  However, the residual emotional effects of intense medical experiences such as euthanasia aren’t so easily solved. Dr. Kristin Neff, associate professor in Human Development and Culture at the University of Texas thinks self-compassion is at the heart of relieving compassion fatigue. She says self-compassionate people tend to be gentle with themselves when confronted with painful experiences. When people try to deny or resist their reactions to painful experiences, emotional suffering escalates into stress, frustration and self-criticism.

People who find it easy to be supportive and understanding to others – including their animal patients – often berate themselves for their own self-perceived shortcomings. Research suggests that giving ourselves a break and accepting our imperfections may be the first step toward better health. People who score high on tests of self-compassion have less depression and anxiety, and tend to be happier and more optimistic.

For those low on the self-compassion scale, Dr. Neff suggests a set of exercises — like writing yourself a letter of support, just as you might to a friend you are concerned about. She says to include in the letter a list of your best traits, and add steps you might take to help you feel better about yourself.

“The problem is that it’s hard to unlearn habits of a lifetime,” she says about our tendency to equate self-compassion with self-indulgence. “People have to actively and consciously develop the habit of self-compassion.”


For more information about Dr Kristin Neff’s work in self-compassion, visit

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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The Dangers of Prolonged Exposure to Waste Anesthetic Gas

WAGIs there a relationship between exposure to trace concentrations of waste anesthetic gasses and the development of health concerns?  After two independent groups analyzed more than seventeen studies, including one well-designed prospective study, the consensus is that there is no risk of adverse health effects to personnel where waste anesthetic gases are scavenged.

Studies published in the late 1960’s and early 1970’s pointed to waste anesthetic gas (WAG) as a direct contributor to everything from fatigue, exhaustion, and headaches to cancer, infertility, spontaneous abortion, and birth defects.  These studies resulted in a 1974 National Institute for Occupational Safety and Health (NIOSH) recommendation that waste anesthetic gas be scavenged in all areas.  Three years later NIOSH recommended that WAG exposure standards be established.

In the 1980’s, researchers examined the conclusions drawn from these earlier studies.  Seventeen studies were examined and all were found to have flaws.  They found that the results of these studies could have been influenced by confounding variables such as occupational stress, and exposure to blood, drugs, aerosols or radiation.  Even the wording of the questionnaires and the inability to verify reported outcomes by the responders may have influenced the conclusions drawn from the earlier studies.  One prospective study using annual questionnaires, surveyed all British female medical school graduates working in hospitals during the years 1977 to 1984.  Analysis showed that female anesthesiologists had no increased risk of infertility.  Another study using Finnish National Health Registry data demonstrated no statistical differences between patients who had been exposed to WAG and those who had not, when medical records were examined.

However, even with the flaws in the early WAG studies, some good came out of them: waste anesthetic gas is now scavenged.  A 1992 study in the New England Journal of Medicine and a later study in the American Journal of Epidemiology reported reduced fertility and increased spontaneous abortion among dental assistants employed in practices where nitrous oxide was not scavenged.  And despite the fact that the modern anesthetic gases of halothane, isoflurane, enflurane, sevoflurane and desflurane are believed to be harmless in trace concentrations, their predecessors were once thought to be safe as well.  Up until 1977 trichloroethylene and fluroxene were used as general anesthetics and thought to be safe.  Chloroform before that.  But none of the three are now in use as anesthetics because they were found to be hepatotoxic, mutagenic and carcinogenic.

The bottom line is that it’s important to scavenge waste anesthetic gases.  It’s also important to stay mindful to otherwise reduce your exposure to waste gases.  And finally, implement an education program for all personnel working in these areas.  Studies have shown that with these procedures in place, trace anesthetic gasses can be maintained below the levels recommended by NIOSH and OSHA.

Here is a good article with more information.

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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Cuff Stuff – Is your way the right way?

In a recent clinical study, four different techniques for sealing an endotracheal tube cuff were evaluated. Eighty client-owned dogs were used in the study. Once intubated, each had its cuff inflated four times, by four different people. After each inflation, the cuff pressure was measured, the cuff deflated, and then the next technique was evaluated.

Spoiler alert: [I always jump ahead to the results.] None of the methods evaluated in this study consistently resulted in cuff pressures within a recommended range.

Each of the four anesthetists attempted one of these four techniques: (A) feeling the tension of the pilot balloon; (B) feeling the tension of the pilot balloon after a week’s practice inflating the cuff to a known pressure; (C) inflating the cuff to occlude at an airway pressure of 20 cmH2O; (D) incrementally deflating the cuff until a leak could be heard at an airway pressure of 25 cmH2O. Although the results showed none of the techniques adequate, it was encouraging to see that technique (B) approached success. It affirms the value of practice.

The article makes me wonder how effective my favorite techniques are at achieving an appropriate cuff pressure. Fortunately, it gives me the method to test them. I was also surprised at the techniques they chose to test. I was sure that by testing four different methods, I would see at least one of my favorites on the list.

The first method I learned was fast and easy. I would over-inflate the cuff, and then take my thumb off the syringe plunger and allow the pressure in the cuff to push the plunger back. When the plunger stopped moving backward, I would then add 1ml of air back into the cuff and disconnect the syringe.

In subsequent years, I learned a more precise method. It’s similar to technique (C) and (D), in that I use airway pressure to determine the cuff seal, rather than pilot balloon pressure. I seal the cuff at 15 cmH2O, but adjust so it leaks at 20 cmH2O pressure.

And now my interest in piqued. What is your favorite method for inflating an endotracheal tube cuff?

The clinical study: Evaluation of the endotracheal tube cuff pressure resulting from four different methods of inflation in dogs. Vet Anaesth Analg. 2012 Sep;39(5):488-94
Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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Now We Can Warm From Within

Peri-Anesthesia Hypothermia –


Warming hypothermic animals is difficult. Yet the majority of anesthesia patients have lost a significant amount of body heat before the procedure even begins. Smaller dogs and cats lose nearly 2ºF while waiting for induction of anesthesia. Then they rapidly lose  another 2º-4ºF during the first fifteen to thirty minutes after induction. This describes the challenges we face with the complex and difficult to manage syndrome: Peri-Anesthesia Hypothermia.

The Pre-Warming Solution

The reasons for up to 80% of anesthetized cats and dogs to become hypothermic include their small body size, vasodilation, inhaling cold, dry anesthetic gases, lack of shiver-response, and open body cavities during surgery. The key to solving this problem is pre-warming the patients.

It seems counter intuitive to provide thermal support before a patient needs it, but recent research shows that warming patients before induction can prevent that initial drop in body temperature, and may slow the rapid heat loss immediately following induction. This rationale sounds similar to pre-oxygenating, doesn’t it? It turns out that pre-warming is highly effective and easy to do. Just place the pre-medicated patient in a warm cage.

Warming a cage may sound simple enough, but all cage heating techniques are not created equal. Heating devices like heat lamps, jugs of warm water or heated rice bags, which are not specifically designed to warm sedated or anesthetized animals, can burn them. The margin of safety for causing significant thermal injury to animals is surprisingly narrow.

Forced warm air blanket systems are ideal to pre-warm cages because they safely deliver a constant flow of warm air at thermostatically controlled temperatures. However, all blanket systems are not created equal either. Forced warm air blanket systems built for humans are not specifically designed to make use of the fur that traps warm air against an animal’s body, and that limits their effectiveness.

Warmed Inspired Anesthetic Gas

There are many ways to reduce patient heat loss during anesthesia. You can minimize surgical prep time, insulate the patient’s feet, use warm IV fluids, and use forced warm air blankets before, during, and after anesthesia. However, none of these methods adequately address the rapid heat loss in the first fifteen minutes after induction. Just imagine how useful it would be if you could warm the inspired anesthetic gases. That way you could capture control of a patient’s body temperature at intubation, rather than playing catch-up as the procedure progresses.

Advanced Anesthesia Specialists ( recently introduced two innovations to the veterinary market. One is the Cocoon® forced warm air blanket system, that is proven to warm patients rather than just slow the rate at which they cool. The other is the world’s first heated breathing circuit. These smooth-wall heated circuits warm the inspired gases, allowing you control of patient hypothermia from the moment of intubation. Using these in combination is shown to adequately manage Peri-Anesthesia Hypothermia. For more information, click here.

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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Age is not a disease…

When it comes to anesthesia, pet owners and their veterinarians sometimes see age as an obstacle to necessary health maintenance for older pets. It reminds Dr Ralph Harvey of the way people maintain older cars. “Owners are reluctant to care for the engine because they assume the body will go bad, and they fail to care for the body because they think the engine will go,” Harvey says. “If nothing is being cared for, it will surely fall apart.”

In a VPN article, Contributing Editor Jessica Tremayne-Farkas spoke with leading veterinary anesthesiologists about meeting the anesthetic challenges we face with geriatric patients.

While customized monitoring and drugs are provided for every patient, those falling into the geriatric category—in the last 25% of their life expectancy—require extra precaution because underlying health conditions are more likely to be present.

“Older patients are more likely to have concurrent disease or mild to moderate organ dysfunction that would require the routine anesthesia protocol be adjusted,” says Dr Khursheed Mama. “A thorough pre-operative physical examination and blood work should help identify most of these and allow the veterinarian to provide appropriate care.”

Some of these conditions lie beyond the reach of routine tests. “In addition to testing,” says Dr. Harvey, “a veterinarian must be in tune with an older patient’s psycho-social issues. Sometimes an older animal doesn’t do as well at the veterinary practice, away from its family and home. Considerations should be made to make the stay less stressful.”

Not all elderly patients require additional or unique care, but identifying this ahead of time and planning for it when necessary is appropriate. Dr Mama explains that “there is a need to educate the general veterinarian on options available to them to manage these patients.”

“Clients will expect their primary care practitioner to be able to meet the needs of their senior pet,” Dr Bednarski says. “Because of this demand, there is a lot of continuing education available on senior pets for veterinarians and their staffs.

The read the complete article, click here.

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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Pick a number between 1 and 5…

ASA Score

You’ve probably seen it on your anesthesia form, called “Physical Status”. It’s the classification system used to assess the health of patients prior to anesthesia. Studies show that the score you assign to your patient can be an important predictor of the outcome of a procedure. It’s simple: patients with a higher score have a higher risk of anesthetic complications.

The American Society of Anesthesiologists (ASA) adopted a physical status scoring system that’s a subjective assessment of overall health based on five classes.

  1. Patient is a completely healthy fit patient (ex: young patient for elective surgery)
  2. Patient has mild systemic disease (ex: obesity or mildly abnormal blood values)
  3. Patient has severe systemic disease that is not incapacitating (ex: compensated renal failure)
  4. Patient has incapacitating disease that is a constant threat to life (ex: congestive heart failure)
  5. A moribund patient not expected to live 24 hour with or without surgery (ex: acute anaphylactic shock)

“E” is placed after the number when the procedure is an emergency. An emergency is when a delay in treatment would significantly increase the threat to the patient’s life or body part.

Being simple and widely understood, the value of the ASA score has been frequently validated in clinical research studies. In one study of 3,438 elective total hip and total knee arthroplasty patients (human), the ASA score correlated significantly to the incidence of postoperative death. The study showed that class III patients had a higher risk of postoperative death than patients with lower ASA scores.

ASA scores also correlate well with operating time, hospital stay, infection rate, and overall morbidity and mortality. The strength of this association has been well documented and can be considered a predictor of postoperative outcome. So, picking a number between 1 and 5, and assigning your patient its ASA physical status score, is a great way to prepare yourself to protect the life of your patient.

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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6 Tips to Help Keep Patients Safe Under Anesthesia


Refining anesthesia protocols has been at the top of our to-do list since the American Animal Hospital Association released its anesthesia guidelines for dogs and cats. Patient safety is a primary goal for all anesthetists, so take a look at these tips from a recent VPN article to create a better experience for patients undergoing anesthesia.

1. Pay particular attention to the induction and recovery phases, as these are the most critical. According to a 2008 study reported in Veterinary Anesthesia & Analgesia, postoperative deaths accounted for 47% of deaths in dogs, 61% in cats and 64% in rabbits. Most other small animal species had higher mortality risks during the recovery phase. Patients need close monitoring while waking up and that special attention should be paid to pediatric and geriatric patients, patients with low body temperatures and those with unique anatomies, such as brachycephalic dogs.

2. Assign a qualified member of the veterinary staff to monitor patients recovering from anesthesia. Even a few minutes of unsupervised recovery may lead to major problems.

3. Designate a quiet area where anesthetic patients can recover peacefully. Recovering a patient in a quiet, low-light area with a way to heat the patient is imperative to a smooth recovery.

4. Tailor anesthetic protocols to each patient, including pre-medication and pain management. One size does not fit all when it comes to anesthetic drug selection.

5. Have the right tools and equipment for the job. A good stethoscope is probably the most obvious, and a means for measuring blood pressure is a must. Other equipment includes those that help maintain a patient’s body temperature. Even very short procedures under anesthesia can lead to significant decreases in core body temperature, which can prolong the recovery period. A proper heating support system will help prevent the unnecessary hypothermia typically encountered during anesthetic procedures.

6. Keep accurate anesthesia records for the medical records. Beyond the legal need of these documents, the staff can use them as case studies during training of new employees or refresher topics for existing staff.

To read Somyr McLean Perry’s entire article, go here

Ken writerKen Crump ( is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 
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The Blood Pressure Can of Worms – Part III

The Blood Pressure Can of Worms – Part III

Tales of surgery before the use of anesthesia are gruesome. Without the ability to block pain, surgery was reserved as a last and most desperate resort. On Friday October 16, 1846, only one surgery was scheduled at Massachusetts General Hospital. The operation was to be the removal of a growth from a man’s neck. But this surgery would be uniquely different: there would be no pulleys, belts and burly attendants to restrain a patient screaming in pain.

After first experimenting on his dog, and then himself, William T.G. Morton made history in that surgery room when he demonstrated the first use of general anesthesia. His use of ether as an anesthetic was so significant to the advancement of medicine that Morton is ranked the 37th most influential person in human history in Michael Hart’s book “The 100 – A Ranking of the Most Influential Persons in History”.

More than a century and half later, it’s hard to imagine a time before general anesthesia.  As anesthetists today, our attention is focused on learning better ways to protect the integrity of a patient’s cardiovascular system during anesthesia.   We devote our third installment of this four-part series to hypotension.  In this article we take a little deeper look at blood pressure itself, and how it might be affected by general anesthesia.

The Pump

The main circulation pump of the cardiovascular system is, of course, the heart.  There are valves in veins which prevent blood from moving backwards, and arteries have their own ability to squeeze and relax which assists the forward movement of blood, but the heart is the true workhorse of the system.  As complex an organ as the heart is, it controls blood flow in only two basic ways.  Blood flow is dependent on how hard the heart squeezes and on how often.  The strength of the heart’s “squeeze” is called its contractility.  How often the heart squeezes is called its rate.  Heart rate is usually expressed in beats per minute.  The amount of blood the heart pumps in a specific amount of time is called cardiac output.  It’s easy to see that a reduction of cardiac output could cause hypotension and could occur when the heart slows its rate or when its contractility is decreased or as a result of both, slow rate and decreased contractility.

The drugs that improve contractility are called inotropes, and those that improve rate are called chronotropes.  Once you’ve isolated the “pump” as the most likely contributor to your patient’s hypotension during anesthesia, the choice between inotrope and chronotrope can be made by simply taking a heart rate.

The Pipes

The pipes are the miles and miles of arteries, veins, and capillaries through which blood travels on its mission to perfuse the body. The impact vessels have on blood pressure comes from their ability to dilate and constrict.  It’s called vascular tone.  By relaxing vascular tone, blood vessels get bigger and expand their interior lumen dimension. That’s called vasodilation.  When they get smaller and reduce their interior lumen dimension, it’s called vasoconstriction.  It’s easy to conclude that vasodilation might cause blood pressure to fall and vasoconstriction might cause blood pressure to rise.  A proof-of-concept challenge might go like this.  Take a dose syringe filled with water and attach it to a red rubber feeding tube.  Feel the amount of pressure required to move the water through the tube.  Now take the same syringe of water and attach it to a red rubber feeding tube that is half the lumen diameter of the first tube.  Feel how much more difficult it is to move the water through.  You’ve experienced that the smaller the internal lumen size of the tubing, the more pressure that is required to move fluid through it.  The concept translates directly to blood vessels. However, things get a little dicey when you correlate blood pressure directly to perfusion.

Let’s take a couple of steps back to refocus on the mission. The function of blood circulation is to deliver oxygen and other substances to tissues, and to carry waste and byproducts of metabolism away.  This perfusion is accomplished by blood flow through tissue capillaries.  Blood pressure, however, is not the sole determinant of perfusion, and adequate blood pressure does not necessarily equate to adequate perfusion.  We’ve just demonstrated that vasoconstriction will increase blood pressure.  But at the same time, the act of decreasing the lumen size of a blood vessel which increases pressure also decreases the amount of blood passing through it.  So it’s possible that tissue perfusion could be inadequate despite adequate blood pressure. Conversely, low blood pressure due to vasodilation may actually allow more blood to flow through vessels, which is likely to yield adequate perfusion. Therefore, we can say that good blood pressure usually means good perfusion, and poor blood pressure usually means poor perfusion, but not always.  Conflicting information like this is what makes anesthetists the special kind of crazy that we all are.

Many drugs used in anesthesia and pain management have specific actions on blood vessels.  Some cause vasoconstriction, some cause vasodilation.  Be cautious when interpreting any single monitored parameter out of its context.  An anesthetist’s study of anesthetic drugs has to be as thorough as his/her study of anesthetized patients.

The Fluid

We’ve talked about the pump, the pipes, and now we get to the fluid: blood.  To recognize a change in blood pressure due to loss of blood seems pretty straight forward. And it is, as long as you know a few things about blood before you start; like how much blood your patient has, how much it can afford to lose, and how you can tell if it’s lost that much.  These are good numbers to calculate before a surgery begins.

Accepted blood volumes for dogs and cats usually have a range, depending on how they were determined.  Because there are so many different ways to determine blood volume, which can yield very different answers, it’s important that we use these numbers as estimates and not as absolute values.  With that said, an accepted blood volume for dogs is 8% of body weight or 80ml/kg.  For cats the estimates are slightly lower at 7% of body weight or 70ml/kg.  That translates to about 6oz per pound for dogs, and about 5oz per pound for cats.  So, a 25 pound dog has just under a gallon of circulating blood.  A cat weighing 8 pounds has 1 ¼ quarts of circulating blood, or about the same as a Super Big Gulp at your local 7-11 store.

Gallons, quarts, and Big Gulps are easily visualized in our US brains, but the beauty of the metric system is how easily we can translate fluid volumes to weights, and back to fluid volumes using simple math.  So even though it may be a little harder to visualize how many milliliters are in a Super Big Gulp, I strongly suggest you use the metric system as your standard for medical calculations.


Most surgical procedures will cause some degree of blood loss.  As anesthetists, it’s important that we monitor blood loss throughout a surgery.  As blood loss approaches 10% to 15% of the patient’s blood volume, steps should be taken to avoid significant anesthesia complications.  For the sake of visualization, 10% of a 25 pound dog’s blood volume is 90ml; much less than half a cup.  Although you may have read that a dog can lose up to 40% of its blood volume and still survive, that’s knowledge you don’t want to have to use very often.

Here are a couple ways to estimate blood loss during a dentistry or surgical procedure.  Of course, if the surgeon is using suction, watching the amount of blood in the suction canister is a good way to monitor blood loss.  But be mindful also of the lap sponges and gauze sponges that are going in the kick bucket or on the floor.  You can get a good estimate of how much blood they’ve soaked up by simply weighing them.  Pretend that the sponges weigh nothing and attribute the weight all to blood.  Here’s where the metric system is useful.  Convert the weight to kilograms.  Since one kilogram equals one liter, it’s very easy to compare the weight of the sponges to the calculations of blood volume and blood loss you made before the start of the surgery.

The Elephant and the Blind Men

There’s a parable that originated in India which tells of six blind men who tried to describe an elephant, based on the one part they each could feel.  The man who felt a leg gave a much different description from the man who felt a tusk.  And the man who felt the tusk gave a different description from the man who felt the trunk, and so on.  As anesthetists we must be cautious not to lose sight of the whole patient.  We must be careful not to isolate our focus on one monitored parameter, but rather recognize the individual cardiovascular components of the pump, the pipes and the fluid, and how they all interact together to perfuse the body.  In that way we can combine all that we learn about anesthesia and use that knowledge for the benefit of our patients.

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