I. Introduction

Background Information

The electrocardiogram (EKG) is a record of electrical activity during depolarization and repolarization of the heart muscle. Three important things to consider about an EKG reading are that 1) the EKG recording only measures the electrical impulse generated from the heart that actually reaches the surface of the body, 2) the EKG represents the total summated electrical activity of the entire heart, not just a single cell, and 3) the recording of an EKG is representative of a difference in voltage between two separately placed electrodes. In respect to the last point the electrodes act as eyes and will elicit a different recording of the exact same electrical potential if placed on different parts of the body. By convention there are 12 electrodes or leads placed on the body - six leads on the limbs and six leads on the chest.

Purpose

The purpose of this lab is to generate and analyze an EKG recording.

Hypothesis

The tracings from a subjects EKG can be used to calculate a mean electrical axis as well as reveal any major cardiac events.

II. Methods

Equipment Used

PB-3 Oscilloscope
EKG Lead Selector Switch
EKG Cable
Four electrodes
Electrolyte gel
Straps
Single Channel Recorder

Procedure

All the proper equipment was identified prior to starting the procedure and the oscilloscope and recorder were plugged in. The recorder box was then connected to the oscilloscope and the EKG Lead Selector Switch was connected to the proper terminal in the front of the Oscilloscope. The EKG cable was connected to the lead selector switch and the recorder turned on to the "Standby" position.

The subject was prepared by having him remove shoes, socks, and all metallic devices and lay flat on a table. Next the four metal electrodes were placed (using a small amount of electrode gel) at the wrists of the left and right arms and just above the ankles of the left and right legs using the straps to secure them into places

The EKG selector switch was set to the "Cal" position and the low frequency filter adjusted to 0.05Hz. Calibration were taken by adjusting the vertical deflection of the trace to one unit on the oscilloscope screen. Tracings were taken for leads I, II, III, aVR, aVL, and aVF.

Finally, the mean electrical axis was calculated and the tracings from individual leads were analyzed for their physiological significance.

Discussion

In analyzing the lead tracings from subject A (Data 1-A, 1-B) the presence of an abnormally large T wave was revealed. Reasons for this may include left ventricular hypertrophy due to vigorous cardiac exercise. The tracings on lead II are imprecise due to inadequate space on the recording paper to show the full negative deflection during the QRS complex. The subject also indicated that he was on medication for borderline hypertension but this did not seem to have a significant effect on any of the tracings. The mean electrical axis was calculated using numerical information from leads I and aVF and appeared to fall within normal parameters.

In analyzing the lead tracings for subject B (Data 2-A, 2-B) the presence of an abnormally large T wave was also detected and may be attributed to reasons mentioned previously. Lead III showed evidence of interference and thus cannot be used to obtain precise numerical data. The subject also indicated that he was on an antibiotic medication but its effects should not have interfered with normal cardiac function. The mean electrical axis was calculated as with the previous subject and was found to fall within normal parameters.

In conclusion, this experiment showed that the lead tracings from an EKG could be used to determine both mean electrical axis and physiologically significant cardiac events. In future experiments better attempts to calibrate the machine between lead tracings should be made in order to better interpret data collected. The participation of more subjects would also help to gain a clearer perspective on physiologically significant cardiac events by a comparison of data.

IV. Questions/ answers

1. Mean Electrical Axis (see Results)

2. Definitions

a. P wave - recording of the depolarization of the atria's.
b. PR segment - recording of the isoelectric activity during AV nodal delay.
c. PR interval - interval measured from the start of the P wave to the start of the QRS complex indicating depolarization of the atria, AV node, Bundle of Hiss, and Purkinje fibers.
d. QRS complex - recording of the depolarization of the ventricles.
Q wave - septal depolarization
R wave - apical depolarization
S wave - depolarization of lateral walls (base)
e. QT interval - interval measured from the start of the QRS complex to the end of the end of the T wave; commonly measured as an indicator of ventricular systole.
f. ST segment - recording of the isoelectric activity between the QRS complex and the T wave during which time the ventricular cells are completely depolarized.
g. ST interval - interval measured from the end of the S wave to the end of the T wave indicating restitution of the ventricles.
h. T wave - recording of the repolarization of the ventricles.

3. Einthoven's Triangle

Three bipolar limbs leads constitute Einthoven's triangle. The axis of Lead I is from shoulder to shoulder. The negative electrode is on the right arm, and the positive is on the left arm. The axis of Lead II is from the right shoulder to the left leg. The negative electrode is on the right arm, and the positive electrode is on the left leg. The axis of Lead III is from the left shoulder to the left leg. The negative electrode is on the left arm, and the positive electrode is on the left leg. These electrodes are all about equally distant from the heart; thus the triangle they form is truly equilateral.

4. The Triaxial System

In the triaxial system each side of the Einthoven's triangle has been shifted parallel to itself, to the center of the triangle. The three axes intersect at the center of the triangle, which is the "zero" point for the three axes. The negative and the positive ends for each axis are the same as in the triangle. The figure also shows the portion of the axes in terms of degrees of circumference (360°), in which the lower half goes from 0° to +180° and the upper half goes from 0° to -180°. This allows one to describe the direction of the cardiac axes in degree.

5. The Hexaxial System

In the hexaxial system three axes are added to those present in the triaxial system. The new axes are those of the three unipolar limb leads aVR, aVL, and aVF. the figure above shows a) the position of the positive electrode for each of the six axes, and b) the degrees for each axis.

6. Einthoven's triangle was used to record the electrical activity of both subjects during the laboratory experiment.

Patient Heart Survey Form

Brief Health History for EKG Subject
Name:
Age:
Race:
Height:
Weight:
Activity Level (Circle one)
	High (Athlete)
	Medium (Mod Exercise)
	Low (Couch Potato)
Health Risk
	Smoking:	Yes (How many packs/ week)				No
	Drinking:	Yes (How much?)				No
Health History
	High Blood Pressure:		Yes (Indicate)			No
	Heart Trouble:		Yes (Indicate)			No
Are there any other factors that you think could affect your EKG?