The Blood Pressure Can of Worms – Part I
This is the first of a four-part series intended to untangle the blood pressure can of worms. We begin by looking at what blood pressure is, and examining the pump, pipes, and fluid of the cardiovascular system.
Monitoring blood pressure in anesthetized animals is the Seat Belt Issue of veterinary medicine. According to a James Madison University report, more than 90% of all motorists believe safety belts are a good idea, but less than 14% actually use them. Similarly, veterinarians universally accept that monitoring blood pressure during anesthesia is a good idea, yet estimates are that fewer than half of anesthetized patients have their blood pressure monitored. A 2006 retrospective study of routine surgical procedures performed in private practice indicates that more than a quarter of all patients are hypotensive during anesthesia. However, in another study, only one veterinarian surveyed considered hypotension to be a common problem during anesthesia. Coincidentally, that veterinarian was the only one surveyed who routinely measures blood pressure in all anesthetized patients. Clearly, if blood pressure is not measured, hypotension cannot be recognized and corrected.
Although anesthesia guidelines from the American College of Veterinary Anesthesiologists (ACVA) and the American Animal Hospital Association (AAHA) urge us to monitor blood pressure during anesthesia, specialists say that blood pressure equipment alone is not the main ingredient to a smooth anesthetic event. It’s the technician’s knowledge that provides the greatest margin of safety for the patient.
A Pump, Pipes, and Fluid
Broken down to its simplest form, the cardiovascular system consists of a pump, some pipes, and fluid. The pump is the heart. The pipes are all the arteries and veins. And the fluid is blood. This “naked” view of the parts of the cardiovascular system helps us conceptualize a very complex transportation network. Once we understand the system in its simplest form, we can better target problems caused by anesthesia, and correct them.
The chief function of blood is to carry oxygen and nutrients to tissues in the body, and to carry away carbon dioxide, lactic acid and other waste material. It is generally accepted that most domestic animals have blood volumes of about 7% of their body weight (cats have a lower percentage). That equals about 70ml per kilogram or about 35ml per pound of body weight. Therefore, a 60 pound dog has a blood volume of about 2 liters, or roughly half a gallon. When you consider the estimated 60,000 miles of blood vessels in the body, half a gallon doesn’t go very far. So, the body is constantly making choices to send blood where it is needed at any given point in time. It’s important to realize that only one thing reduces a patient’s blood volume during anesthesia: blood loss.
The pipes, of course, are those 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 drugs used during anesthesia alter the body’s ability to respond automatically in this manner.
The heart completes this minimalist’s view of the cardiovascular system by acting as the 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.
Pressure is the driving force for blood flow through capillaries that supply oxygen to organs and tissues of the body. Blood pressure is needed to propel blood through vascular beds, with priority to those of the brain, heart, lungs and kidneys. Blood pressure values are expressed as three measurements: systolic, diastolic and mean.
The systolic pressure is the pressure generated when the left ventricle of the heart is fully contracted. Diastolic pressure is the pressure measured when the left ventricle is completely relaxed. Mean blood pressure is usually a calculation that represents the average pressure at which blood flows through the systemic vessels. The mean arterial pressure (MAP) value is closer to diastolic than systolic because during each pressure cycle, the systolic pressure peaks for a shorter time. In other words, the heart spends most of its time in diastole. As anesthetists, we focus on the mean arterial pressure. A mean arterial pressure of at least 60 mmHg is needed to properly perfuse the heart, brain and kidneys.
In the next three parts of this four-part series, we’ll discuss different ways to monitor blood pressure during anesthesia, common causes of hypotension (including the hypotension caused by anesthetic drugs), and how and when to treat hypotension. By further expanding our knowledge of the whole blood pressure can of worms, we’ll continue to provide the greatest margin of safety for our anesthesia patients.