Electrical impulses flow across the heart in a current
The electrocardiogram machine plays a dramatic supporting role in movies and TV shows, its flat line and piercing tone alerting the medical team to race to the bedside of a patient whose heart has stopped beating.
Electrocardiograms, or ECGs, are equally important as an everyday diagnostic tool used by veterinary cardiologists, such as Dr. Ryan Fries, at the University of Illinois Veterinary Teaching Hospital in Urbana. ECGs are also routinely used to monitor patients undergoing anesthesia, because many anesthetic drugs can affect heart function.
The heart is a muscular pump that circulates blood throughout the body. Electrical impulses flow across the heart in a current (similar to the electric current that lights lightbulbs), stimulating the heart muscle to contract in a very precise pattern.
“The electrical current in the heart is created by positively charged ions moving through channels along the membranes of the muscle cells. When one cell reaches its maximum ionic charge, it depolarizes and the positive charge is conducted to the next cell,” says Dr. Fries. This cell conduction is extremely fast, and the cumulative effect of all these cells depolarizing is a muscle contraction.
“An ECG is a graphic representation of the electrical activity of the heart,” explains Dr. Fries. “With an ECG we can get a patient’s heart rate, view the cardiac rhythm, and learn information about the heart muscle. If there is a disease process that has caused dilation or thickening of a chamber, for example, we may be able to detect this change on an ECG.”
To take readings of the heart’s electric current, electrodes are attached to each of the animal’s limbs in a specific order. These do not hurt the animal and do not introduce electricity, but merely make it possible to monitor the heart’s conduction pattern. The ECG reading depicts the electrical activity in the form of a wave.
In dogs and cats (and all other mammals), the heart is made up of four chambers: the left and right atria and the left and right ventricles. One heartbeat, or one wave on the ECG, consists of the two atria depolarizing (which causes a contraction), followed by the ventricles depolarizing, and then the ventricles repolarizing.
“By investigating the time intervals, consistency, heart rate, and shape of these waves, we can locate which portion of the conduction system might be having a problem,” Dr. Fries explains. For example, if the atrial depolarization wave exists without a corresponding ventricular wave, there is probably a problem in the conduction path between the atria and ventricles.
“Some abnormal rhythms depicted on an ECG may resolve on their own, but other problems may indicate serious heart issues that require treatment or additional diagnostics with radiographs and echocardiograms,” Dr. Fries says.
For more information about ECGs and heart function, ask your local veterinarian.
By Melissa Giese
Photo by L. Brian Stauffer