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 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 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.
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.