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.
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.
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 Crump (kencrump.com) 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.