Since its discovery, over 100 years ago, electrocardiogram (ECG) has become a routine and an important part of any complete medical evaluation and has been used continuously over this time as a diagnostic test. [4] Most of the damages caused to the heart’s tissue can be perceived by the ECG due to the fact that electricity is conducted through the heart muscle, also known as myocardium. The information obtained from the ECG is represented as the ECG waveform. This waveform provides us with information about the electrical activity associated to different aspects of the heartbeat;
making it a very important tool to assess an individual’s heart health and cardiac rhythm.
For a successful ECG measurement system design, it is important to understand the basic foundations of the electrocardiogram measurements. The often referred as a
“sinus” rhythm is the normal cardiac rhythm; it is referred as sinus due to the fact that it originates in the sinoatrial node (S-A node). The heart beat is triggered when cells deemed as “pacemaker cells” found in the S-A node are stimulated. The disturbances of the normal sinus rhythm are commonly known as arrhythmias. The normal behaviour of the “pacemaker cells” is to generate action potentials at 60-100 beats per minute, when resting. The action potential later depolarizes the tissue before disseminating throughout the myocardium in a predefined way, triggering the tissue to contract. Then after leaving the S-A node, the action potential depolarizes both of the atrial chambers of the heart, from there it moves down to the atrioventricular node (A-V node), this being a node that is found between the atrial and ventricular chambers. The A-V node acts as a backup pacemaker, however, it acts at a lower rate of 40-60 beats per minute. This node acts as a delayer for the electrical impulses, slowing them down by 120 ms giving the atria enough time to pump their blood into the ventricular chambers, before they contract. Then the action potential leaves the A-V node and continues down to the area known as “HIS” bundle, from there it goes to the bundle branches to the left and to the right. Finally the ventricles are depolarized by the conductive fibres named Purkinje fibres. Finally after the repolarization and contraction of the ventricles, the cycle starts
again. The ECG waveform is a recording of the superposition of these different action potentials, as can be seen in Figure 2. [5]
Figure 2 ECG waveform in detail, showing different components of the ECG
waveforms generated by different nodes, muscles and fibers in the heart [5]
As mentioned before, ECG is a mere representation of the electrical activity of the myocardium’s cells. It can be better explained as a differential measurement across the surface of the human body, and can be thought as a measurement of the electrical potential with a vector. As can be seen in Figure 2, the typical ECG waveform contains three primary features: the P wave, the QRS complex and the T wave. The working of each part of the heart can be observed to each of these waves. The P wave represents the depolarization of the atria, thus normally being first. The PR interval is representative of the delay that happens in the A-V node giving the atria time to contract before depolarizing the ventricles. The QRS complex is the strongest wave in ECG, since it shows the depolarization of the ventricles. The T wave represents the repolarization of the ventricles. [6]
ECG Leads:
ECG vectors are generated by taking differential measurements of electrical potential on different locations on the body’s surface. The ECG standard for clinical measurements consists of 12 leads; six of them are in the parallel plane to the body, on the chest. These leads are known as precordial ECG leads and they can be seen in the lower right corner of Figure 3, labeled V1, V2, V3, V4, V5 and V6. The remaining six leads are giving a view of the heart from the perpendicular plane. These leads are known as the frontal leads, that can be seen in the upper right corner of Figure 3, labeled as aVR, aVL, aVF, lead I, lead II and lead III. [6]
Figure 3 Vector view of the standard 12 lead ECG. [7]
In the measurement of ECG leads, three different electrodes are placed on the body;
this would be the formation of the frontal ECG leads. The electrodes are traditionally placed on the left arm (LA), right arm (RA) and left leg (LL). The 3 main electrodes form a triangle which is known as the Einthoven’s triangle that can be seen in Figure 4.
[6]
Figure 4 Einthoven's triangle formed by the 3 main electrodes [6]
Lead I ECG System
Biomedical engineers have been interested in finding simplified methods for detecting chronic diseases for a long time. This led them to the research of a simple method for recognizing abnormal heart behavior. The 12-lead ECG has been used to detect heart abnormalities for many years, even though it is a bulky measurement system. As an alternative to the 12-lead system there have been studies comparing the single lead ECG with the 12 lead ECG, with quite positive results regarding the lead I system. [8]
The lead I system or single lead system provides a one dimensional low frequency signal through the application of three electrodes, two of them being active and one being a ground electrode. A possible arrangement to obtain a lead I system ECG can be seen in Figure 5. This arrangement is used by one of the most interesting and novel ECG bracelet, known as Nymi, the bracelet that uses ECG as a biometric recognition technology. [9]
Figure 5 Possible electrode arrangement for 1 lead ECG, where B and W are active
electrodes and R is the reference/ground [9]
There exist two different types of leads in existence, the unipolar and the bipolar.
The bipolar being the one that has one positive and one negative pole, while the unipolar leads also have two poles, however, as a voltage is measured the negative pole is a composite pole. The type of leads that is used in tis electrode arrangement is the bipolar leads. [10]
When the comparison was made between 1 lead ECG and 12 lead ECG, they were compared by studying a group of 2000 patients and having professionals analyze the measurements taken. Although it wasn’t the intent of the study, it went on to compare the relative merits of both diagnostic methods, which can be observed in Table 1 and Table 2. [8]
Table 1. 12 Lead vs. Final clinical impression from 2000 patient records [8]
Final Clinical Impression
Number Percent
Normal Doubtful Abnormal Total Normal Doubtful Abnormal Total No Heart
As can be seen in Table 1 the 12 lead ECG is very reliable in selecting those people free of heart disease but there is a relatively high percentage of not recognizing people with cardiovascular disease.
Table 2. Lead I vs. Final clinical impression from 2000 patient records [8]
Final Clinical Impression
Number Percent
Normal Doubtful Abnormal Total Normal Doubtful Abnormal Total No Heart
It is apparent in Table 2 that doctors lose the ability to categorize as normal, those patients that have no cardiovascular disease. However the ability to pick up some of the more noteworthy cardiovascular diseases with this method is just as good as the 12 lead system, if we were to observe only the number it would appear that lead I is in fact slightly better. Taking this information into consideration, we proceeded to accept the lead 1 system as an acceptable method to obtain ECG measurement.