The purpose of these experiments is to understand the function and importance of an electrocardiogram (ECG) and finger pulse reading as well as see how they relate to one another. This lab will demonstrate how changes in the environment and stress levels affect human heart rate. Furthermore, the equipment used in the lab will demonstrate the anatomical function of the ulnar, radial, and brachial artery in the arm as well as the QRS complex and the waves associated with an ECG. Finally, the lab will serve as a way to learn how to use an ECG to record and calculate heart rate.
An ECG can be used to record activity during the cardiac process of pumping and returning blood to the body and heart because of the electric current that spreads through the tissue of the heart and to the surface of the body. By using three electrodes: one on each wrist around the radial artery and one that is grounded on the upper, left foot, we can record these electrical potentials produced by the heart.
The first part of the lab involves reading an ECG and calculating heart rate by analysing the waves associated with an ECG recording. Each portion of an ECG reading is divided into five waves or collectively three sections. The first part of an ECG is a small peak that is called the P wave. The P wave is produced by the depolarization of the atria before the heart contracts1.
The QRS complex, which consists of the Q, R, and S wave, occurs because of the depolarization of the ventricles before contraction. As a result, the P, Q, R and S wave represent the depolarization of the heart. In contrast, the T wave occurs as a result of repolarization of the ventricles (recovery from depolarization). It is important to note that the P wave occurs immediately before the contraction of the atria and the QRS complex occurs immediately before the contraction of the ventricles. The ventricles remain contracted until the end of repolarization or until the end of the T wave. The atrial T wave is not usually distinguishable on the ECG because atria depolarization occurs 0.1- 0.2 seconds after depolarization occurs. As a result, the atrial T wave is often over shadowed by the QRS complex. On the other hand, the ventricular T wave is noticeable on the ECG because repolarization of the ventricles occurs over a longer period of time and is seen after the QRS complex occurs. The voltage recorded on an ECG is usually very small as compared to a normal action potential. The voltage reached by the QRS complex is approximately 1 mV (from the top of the R wave to the bottom of the S wave). In contrast, the P wave is much smaller and ranges between 0.1 and 0.3 mV, whereas the T wave ranges from 0.2 and 0.3 mV 2. The heart rate can be determined from a normal ECG. The time interval between two beats is the reciprocal of the heart rate. The average interval between two QRS complexes is 0.83 seconds. Therefore, to calculate heart rate, the time in one minute (60 seconds) is divided by the time interval between two QRS waves. As a result, the number of beats that can occur in one second is derived. The equation is given below.
Heart rate=60/(Time interval between successive QRS waves)
The beating of the heart is accompanied by electrical activity. The combined electrical activity of the different myocardial cells produces electrical currents that spread through the body fluids. These currents are large enough to be detected by recording electrodes placed on the skin. The regular pattern of peaks produced by each heart beat cycle is called the electrocardiogram (ECG). By analyzing the information contained in the ECG signal, we can diagnose some heart diseases.
Cardiac contractions are elicited by the electrical activity. Firstly, a group of muscle cells that themselves generate little force act as the pacemaker for the heart (Figure 1 ). These cells rhythmically produce action potentials that spread through the fibers of the atria, result in atrial contraction, which pushes blood into the ventricles. The only electrical connection between the atria and the ventricles is via the atrioventricular (AV) node. The action potential spreads slowly through the AV node (thus giving a time delay for ventricular filling) and then rapidly through the AV bundle and Purkinje fibers to excite both ventricles. This causes the ventricles to contract and push blood into the aorta and pulmonary trunk.
Figure 1. Components of the human heart involved in conduction 3
Figure 2. A typical ECG showing the fundamental parts 4
The components of the ECG can be correlated with the electrical activity of the atrial muscle and ventricular muscle (Figure 2). The normal ECG is composed of a P wave, a QRS complex, and a T wave.
The P-wave is produced by depolarization of atrial muscle.
The QRS complex is produced by ventricular depolarization. Its shape, amplitude, and direction depend on the position of the heart in the chest its size relative to body mass, and the time relationship between right and left ventricular activity. An abnormal lengthening of the duration of the QRS complex suggests interference with the spread of excitation through ventricular muscle, as may occur in Purkinje failure or myocardial infarction.
The T-wave is produced by ventricular repolarization.
METHODS & MATERIALS
The protocol of this lab experiment involved a few things. The first thing that had to be done was to gather the machine that will be used to test the heart. Connect the push-button switch to the BNC socket for Input 2 then, plug the Bio Amp cable into the Bio Amp socket. From that point jewellery should be removed and if alcohol swabs are available swab the area of the skin the electrodes will be placed on. Place an electrode on there are two ways which they can be done.
Standard connection: attach the positive electrode to the left wrist, the negative to the right wrist, and the ground to the right leg. If after looking at the signal in the Bio Amplifier dialog window during the first exercise, this does not produce a good signal, the alternative method can be used 5.
Alternative connection: attach the positive electrode to the left upper arm, the negative to the right upper arm and the ground to either wrist. NOTE: Do not place the electrodes over the major muscles of the upper arm because muscle activity interferes with the signal recorded from the heart. Attach the electrodes on the outer side of the arm, midway between the elbow and the shoulder 5.