Materials Included In Kit

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Before beginning this activity it is crucial that students have a thorough understanding of human heart anatomy.
• A concept which many students find perplexing is that downward deflection on the ECG corresponds to depolarization of the heart. The key is to remember that an ECG represents many simultaneous action potentials taking place. Therefore the graph does not appear the same as the graph of an action potential.

Teacher Tips

• In most education settings a three-electrode ECG machine is used. Clinically, a 12-electrode machine is most common. The additional nine electrodes are placed on the chest and trunk. They provide more points of data collection therefore yielding more accurate information on the electrical conduction within the heart.

Answers to Prelab Questions

1. What is a typical resting heart rate of a person?

   A typical resting heart rate is 60–100 beats per minute (bpm).

2. What mechanical action is the heart taking during the PR segment?

   The atria are contracting during the PR segment.

Answers to Questions

Case Study 1

1. 1. Each small square on the chart is 1 mm and 25 mm represents one second. Based on the ECG above, what is the heart rate of this patient in beats per minute?

      From the center of one P wave to the next is 20 mm. Since one second is 25 mm the heart beats 1.25 times per second. There are 60 seconds in a minute so this person’s heart rate is 75 bpm.

   2. Is this within a typical resting heart rate?

      Yes

2. Is the heartbeat regular or is there an arrhythmia?

   The heart rate is regular. Identical patterns occur consecutively at equal intervals.

3. Does the QRS complex always appear after the P wave? Is the PR segment the same length after each P wave?

   Yes, the QRS complex always follows the P wave and the PR segment is consistent in length.

4. What does the above ECG indicate about the heart’s function?

   The ECG indicates that the heart is functioning normally.

Case Study 2

1. 1. Each small square on the chart is 1 mm and 25 mm represents one second. Based on the ECG above, what is the heart rate of this patient?

      Since the heart rate is irregular it is best to take a wider reading than of two consecutive peaks. The distance between the peak of the first QRS complex and the last is 32 mm/3 waves. Therefore one beat represents 10.666 mm and there are 2.34 beats per second or 140 bpm.

   2. Is this within the typical resting heart rate range?

      If not, is it a case of tachycardia or bradycardia? The heartbeat is greater than normal and it indicates tachycardia.

2. Does the QRS complex always appear after the P wave? Is the PR segment the same length after each P wave?

   No, the QRS complex does not always appear after the P wave. The P waves are abnormally shaped and also last longer than they traditionally should. The PR segments are difficult to distinguish.

3. Is the rhythm of the heartbeat regular or is there an arrhythmia?

   An arrhythmia is present.

4. What does the above ECG indicate about the heart’s function?

   As mentioned previously, an arrhythmia is present. This most likely indicates the SA node is not serving as an accurate pacemaker.

Case Study 3

1. 1. Each small square on the chart is 1 mm and 25 mm represents one second. Based on the ECG above, what is the heart rate of this patient?

      Since there are multiple P waves and not all of them correspond to a QRS complex it is difficult to measure the heart rate based on them. Based upon the QRS complexes the heart rate is approximately 48 bpm.

   2. Is this within a typical resting heart rate range? If not is it a case of tachycardia or bradycardia?

      No. Since the heart is beating under 60 bpm it would indicate bradycardia.

2. Is the rhythm of the heartbeat regular or is there an arrhythmia?

   An arrhythmia is present.

3. Does the QRS complex always appear after the P wave? Is the PR segment the same length after each P wave?

   No, the QRS complex is only appearing after the P wave approximately half of the time.

4. What does the above ECG indicate about the heart’s function?

   Since a QRS complex does not always follow the P wave, this indicates that the SA node does not reach the ventricles. This condition is known as heart block where the action potential cannot propagate to the proper location.

Case Study 4

1. 1. Each small square on the chart is 1 mm and 25 mm represents one second. Based on the ECG above, what is the heart rate of this patient?

      This patient’s heart is beating at about 41 bpm.

   2. Is this within a typical resting heart rate range? If not is it a case of tachycardia or bradycardia?

      No. Since the heart is beating under 60 bpm it would indicate bradycardia.

2. Is the rhythm of the heartbeat regular or is there an arrhythmia?

   An arrhythmia is present.

3. Does the QRS complex always appear after the P wave? Is the PR segment the same length after each P wave?

   No, the QRS complex is also not appearing as it should. The ECG goes straight from R to S, there is no decrease in the baseline voltage for Q. The PR segment varies in length.

4. What does the above ECG indicate about the heart’s function?

   The decreased heart rate along with consecutive P waves indicates the electrical signals are not reaching the ventricles.

Teacher Handouts

11100_Teacher1.pdf

References

Silverthorn, D. U., Human Physiology: An Integrated Approach; Pearson Benjamin Cummings: San Francisco; 2004; pp 471–474.

Types of Heart Block. National Heart, Lung and Blood Institute. Accessed February 2011. http://www.nhlbi.nih.gov/health/dci.Diseases/hb/hb_types.html

Introduction

Electrocardiograms (ECGs) are used to obtain information regarding the functionality of the heart. Learn how to read an ECG and correlate the steps involved in a normal heart rhythm with electrical signals recorded on an ECG.

Concepts

• Electrocardiograms
• Cardiovascular physiology

Background

An electrocardiogram (ECG) is a graphical recording of the electrical activity of the heart. Electrodes placed on the skin measure electrical signals corresponding to contraction of the heart muscles and the heartbeat. The signals are only about 1 mV by the time they reach the skin but they are strong enough to be recorded and produce an electrocardiogram.

The heart is a muscle, and like other muscles, it contracts and relaxes in response to electrical impulses or signals. An ECG records or traces the path of these electrical signals through the chamber of the heart and provides a picture of the heart rhythm. A normal ECG consists of three major waves—the P wave, the QRS complex (combination of waves), and the T wave (see Figure 1). Each wave is related to a specific electrical event within the heart and is followed by a muscle contraction. The P wave corresponds to the depolarization of the atria, which signals the atria to contract, pushing blood into the ventricles. The QRS complex represents a simultaneous depolarization of both ventricles. The T wave represents the repolarization of the ventricles. Note that atrial repolarization is represented as part of the QRS complex.

ECGs are also described in terms of segments and intervals. Segments are sections of the baseline between two waves. Intervals are combinations of waves and segments. See Figure 2 for a description of the standard segments and intervals of an ECG. The mechanical actions of the cardiac cycle immediately follow the electrical signal. Atrial contraction begins at the second half of the P wave and continues into the PR segment (see Figures 3a and 3b). Ventricular contraction begins just after the Q of the QRS complex and continues through the T wave (see Figures 4a and 4b). Electrocardiogram machines have three electrodes—positive, negative and ground. If the net current read by the ECG monitor is positive the wave deflects upward from the baseline toward a more positive voltage. If the net current moves toward the negative electrode the wave deflects downward from the baseline, toward a more negative voltage.

ECGs provide many pieces of valuable information for studying heart rate and heart rhythm. Interpreting the data requires examining the following questions:

• What is the heart rate? To determine the heart rate, the ECG is measured from the beginning of one P wave to the beginning of the next P wave. A typical resting heart rate is 60–100 beats per minute. Note: Trained athletes usually have slower resting heart rates. A heart rate above this range is called tachycardia, and a rate slower than this range is called bradycardia.
• Is the rhythm of the heartbeat regular or irregular? It is important that the beat occurs at regular intervals. An irregular heart rhythm is known as an arrhythmia. This can appear as a result of a simple extra heartbeat. Other times an arrhythmia occurs because the SA (sinoatrial) node is not serving as an accurate pacemaker, signaling more contractions than necessary.

The final step in analyzing an ECG is to determine the relationship between the various waves. Does the QRS complex always appear after the P wave? Also in this step it is important to examine whether or not the PR complex is consistent in length after each P wave. If this is not the case, it may be indicative of a problem with conduction of signals through the AV (atrioventricular) node. The AV node is part of the electrical control system of the heart that controls the heart rate. If the signal from the SA node does not reach the ventricles, the condition is known as heart block. The action potential cannot propagate to the proper location. This is witnessed on an ECG by having consecutive P waves with no QRS complex to follow. See Table 1 for further information about each degree of heart block.

Experiment Overview

In this activity, four case studies of ECGs will be observed. Each diagram will be reviewed using a series of questions to determine the heart rate as well as analyze other characteristic patterns. The patterns will be used to identify how the electrical signal corresponds to the mechanical action of the heart.

Materials

Prelab Questions

1. What is a typical resting heart rate of a person?
2. What mechanical action is the heart taking during the PR segment?

Student Worksheet PDF

11100_Student1.pdf