Learning Module 4:
Exploring ECG’s in Nature
Introduction
Electrocardiograms are very useful for demonstrating a number of important
physiological concepts related to cardiac function. For example, the ECG can not only
provide basic information such as heart rate, but can also help diagnose a number of
human cardiac disorders. ECG’s can also be useful for understanding how the heart
functions not only in humans, but also other vertebrates. The following examples are
meant to help you understand how ECG’s are used in some novel situations.
Example One – Human Cardiac Disorders
Background
As previously demonstrated, the normal human ECG consists of 3 basic parts: 1) p-wave,
2) QRS-complex, and 3) the t-wave. The temporal relationship amongst the three parts
can often be useful in detecting abnormal heart conditions. For example the amount of
time between the p-wave and the beginning of the QRS complex is typically 0.16 seconds
and represents the normal delay in electrical conduction between the atria and ventricles
as the electrical signal passes through the AV-node and into the ventricles (Figure 1).
Remember that conduction between the atria and ventricles normally only occurs through
the AV-node.
Disorder 1: Atrioventricular Block
Certain diseases and heart attacks can damage heart tissue. Occasionally the heart tissue
involved in transmitting the electrical impulses can become damaged, preventing the
electrical signal of the pacemaker cells from being transmitted correctly. For instance,
heart attacks and diseases such as diphtheria can damage the AV-node, thereby
preventing the transmission of the electrical signal from the atria to the ventricles, a
condition known as an atrioventricular block. Remember that the SA-node (atria)
depolarizes at the fastest rate amongst the pacemaker cells (65 bpm), with the AV-node
(40 bpm), Bundle of HIS (30 bpm), and Purkinje fibers (20bpm) all firing at much slower
rates. With this in mind, draw what you think the ECG from a person suffering
from complete atrioventricular block might look like.
According to your ECG, what would the heart rate for the atria be? The ventricles?
What kind of symptoms would a person suffering from this malady expect to
experience during heavy exercise?
Disorder 2 – Wolf-Parkinson-White Syndrome
The atrioventricular block is a great example of what can happen when the heart’s
electrical signal can’t pass from the atria to the ventricles. But what happens when the
electrical signal can pass too easily between the atria and ventricles?
The ECG in Figure 3 is taken from a patient with Wolf-Parkinson-White Syndrome.
What do you notice that is different from the ECG in Figure 1?
People suffering from Wolf-Parkinson-White have an accessory electrical pathway
between the atria and ventricles. Because the AV-node is the only pathway in normal
hearts, there is usually a small delay between the depolarization of atrial cells and the
depolarization of ventricular cells as the electrical signal makes its way through the AVnode. This delay is demonstrated by a p-Q interval of around 0.16 seconds. However, in
Wolf-Parkinson-White some depolarization spreads immediately from the atria to the
ventricles through the accessory pathway. This is typically manifest in the shorter p-Q
interval and the presence of an up-sloping delta wave just prior to the QRS-complex.
If conditions are just right, occasionally an electrical signal can be transferred in reverse,
from the ventricles back to the atria in a person with Wolf-Parkinson-White. When this
occurs, heart rate can suddenly jump from normal levels of 65 bpm, to upwards of 250300 bpm.
Example 2 – Cardiac Regulation in Fish
Background
Fish are unique amongst vertebrates in that they have 2-chambered hearts: one atria and
one ventricle. Despite this difference from mammals and birds, the ECG from fish looks
surprisingly similar to the ECG’s from humans (Figure 4). The reason for this is that the
atria and ventricle in fish depolarize in the same fashion that they do in human, we just
have two atria depolarizing together and two ventricles depolarizing together. What is
the heart rate for the fish in Figure 4?
Diving (or not) Bradycardia
We’ve seen in previous labs that the human response to diving is a sudden bradycardia.
This occurs because the body no longer has access to oxygen, and blood distribution is
limited to the vital organs (brain, heart). Because blood flow is greatly reduced and
oxygen is limited, the heart rate slows in an effort to conserve oxygen. The
electrocardiogram above is from a fish that has been taken out of water (Figure 5). What
is the heart rate for this “fish out of water?” Why do you think heart rate has
slowed down?
Fish Exercise
Total cardiac output is dependent on a combination of stroke volume (how hard the heart
contracts) and heart rate. In humans, we have seen that heart rate increases greatly during
exercise, thereby increasing total cardiac output. Consider the ECG from a fish that is
vigorously swimming (Figure 6).
What is the heart rate of this fish? What can you say about the stroke volume of
vigorously swimming fish?