Technicians aren't all destined to be cardiologists. But all technicians should understand what a normal cardiac rhythm looks like and how it's generated. When we started working in a referral clinic, our first cardiac lesson was from a cardiologist who asked us to let him know if we saw anything "wide and bizarre" on the electrocardiograph monitor. He was planting the seed to help us understand why a cardiologist would be concerned about the electrocardiogram (ECG) pattern for a patient with gastric dilatation-volvulus (GDV). We wondered what the heart had to do with the stomach. We also wondered what "wide and bizarre" meant.
Our cardiologist questions and consults didn't end there. Concern also arose with other patients including cats with urinary obstructions. How does the inability to urinate cause problems with heart function?
We were fortunate to work with a team that wanted to educate us on the relationship between heart function and many common veterinary emergencies. We hope to do the same for you in this article.
The cardiac cycle
In order to understand what causes the heart to produce an abnormal rhythm, you must first recognize and appreciate a normal sinus rhythm. To do that, you need some background on the circulatory system. This system transports nutrients, water, and oxygen throughout the entire body and carries away wastes such as carbon dioxide. The circulatory system also oxygenates red blood cells and delivers them to all parts of the body. If there is a breakdown of oxygen delivery, cells will die, organs will fail, and the body will eventually shut down.
This cardiac cycle plays the most significant role in the circulatory system. The cycle describes the flow of blood as it enters the heart, is pumped to the lungs, travels back to the heart, and is pumped out to the rest of the body. In order to cycle the blood, the heart has a contractility phase (systole) for pushing blood out and a relaxing phase (diastole) for filling up the heart. Anatomy, electricity, and volume all contribute to the synchronicity of these systolic and diastolic phases.
The heart is divided in half by a partition (the septum), and the halves are further divided into four chambers (two atria and two ventricles), with a series of valves controlling blood flow. Muscle makes up most of the walls of the atria and ventricles, and a fluid-filled sac called the pericardium surrounds the heart. The cells of the myocardium, or cardiac muscle, can conduct electricity. These specialized myocytes, or cardiac "beating" cells, are interconnected by intercalated disks that have low electrical resistance.1 The disks allow the electrical impulse to easily spread through the atria, down the septum, and up the free walls of the ventricles. This coordination of cardiac muscle cells facilitates heart contraction to propel blood from the atria to the ventricles and out to the lungs and blood vessels of the circulatory system (Figure 1).
1. An illustration of the cardiac cycle.