27-07-2012, 03:05 PM
Embedded Microcontroller based digital Telemetry system for ECG
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Introduction
Need of a portable wireless ECG system:
In recent times cardiac related diseases are on the rise. Worldwide surveys conducted by World Health Organization (WHO) have confirmed this. Many of the cardiac related problems are attributed to the modern lifestyles, food habits, obesity, smoking, tobacco chewing and lack of physical exercises etc. Various kinds of operations may be carried out for patients suffering from heart related diseases. The post-operative patients can develop complications once they are discharged from the hospital. In some patients the cardiac problems may re-occur, when they start doing their routine work. Hence the ECG of such patients needs to be monitored for some time after their treatment.
A brief history of the Electrocardiogram:
The electrocardiogram (ECG) is the recording on the body surface of the electrical activity generated by the heart. It was originally observed by Waller in 1889 using his pet bulldog as the signal source and the capillary electrometer as the recording device. In 1903, Einthoven enhanced the technology by employing the string galvanometer as the recording device and using human subjects with a variety of cardiac abnormalities. Einthoven is chiefly responsible for introducing some concepts still in use today, including the labeling of the various waves, defining some of the standard recording sites using the arms and legs, and developing the first theoretical construct whereby the heart is modeled as a single time varying dipole. We also owe the EKG acronym to Einthoven’s native Dutch language, where the root word cardio is spelled with a k. In order to record an ECG waveform, a differential recording between two points on the body are made. Traditionally, each differential recording is referred to as a lead. Einthoven defined three leads numbered with the Roman numerals I, II, and III. They are defined as where RA = right arm, LA = left arm, and LL = left leg. Because the body is assumed to be purely resistive at ECG frequencies, the four limbs can be thought of as wires attached to the torso. Hence lead I could be recorded from the respective shoulders without a loss of cardiac information. Note that these are not independent, and the following relationship holds: II = I + III.
The evolution of the ECG proceeded for 30 years when F.N. Wilson added concepts of a “unipolar” recording. He created a reference point by tying the three limbs together and averaging their potentials so that individual recording sites on the limbs or chest surface would be differentially recorded with the same reference point. Wilson extended the biophysical models to include the concept of the cardiac source enclosed within the volume conductor of the body. He erroneously thought that the central terminal was a true zero potential. However, from the mid-1930s until today, the 12 leads composed of the 3 limb leads, 3 leads in which the limb potentials are referenced to a modified Wilson terminal (the augmented leads), and 6 leads placed across the front of the chest and referenced to the Wilson terminal form the basis of the standard 12-lead ECG. The voltage difference from any of the two leads of a 12-lead system will record an ECG signal. Einthoven chose the letters of the alphabet from P to U to label the waves and to avoid conflict with other physiologic waves being studied at the turn of the century. The ECG signals are typically in the range of ±2 mV and require a recording bandwidth of 0.05 to 150 Hz. Full technical specification for ECG equipment has been proposed by both the American Heart Association and the Association for the Advancement of Medical Instrumentation.
There have been several attempts to change the approach for recording the ECG. Application of computers to the ECG for machine interpretation was one of the earliest uses of computers in medicine. Of primary interest in the computer-based systems was replacement of the human reader and elucidation of the standard waves and intervals. Originally this was performed by linking the ECG machine to a centralized computer via phone lines.
The ECG waveform
This is a stylized version of a recording showing the P wave, QRS complex, and the T and U waves. The PR interval and the ST segment are significant time windows. The peak amplitude of the QRS is about 1 mV. The vertical scale is usually 1 mV/cm. The time scale is usually based on millimeters per second scales, with 25 mm/s being the standard form. The small boxes of the ECG are 1 × 1 mm.
Literature Survey
Basic anatomy of the heart:
The heart is located in the chest between the lungs close by diencephalons and is surrounded by the pericardial and connected to the pleura. Two thirds of the heart is located on the left side of the median line through the body. The heart has four larger different chambers, divided into atriums (left and right) and ventricles (left and right). Right half of the heart contains venous blood (deoxygenated). The right side of the heart collects blood from the body and pumps it to the lungs while the left side of the heart receives blood from the lungs and pumps it to the body.
Anatomy of the heart 1
Blood flows through the body in the following way:
• Oxygen-rich blood from the lungs enters the left atrium through the pulmonary veins.
• Blood then flows into the left ventricle where it is pumped into the aorta and is distributed to the rest of the body. This blood supplies organs and cells with oxygen and nutrients necessary for metabolism.
• Blood that returns to the heart is depleted of oxygen and carries carbon dioxide, the waste product of metabolism. The blood enters the right atrium though the vena cava, where it is collected and pumped to the right ventricle.
• The right ventricle then pumps blood through the pulmonary artery to the lungs where carbon dioxide is stripped off, oxygen is replaced, and the cycle begins again.
Anatomy of the heart 2
Like any muscle, the heart requires oxygen and nutrients to function. Oxygen and nutrients are supplied by arteries that originate from the aorta. These vessels branch out to supply all the regions of the heart with oxygen rich blood.
Electrically, the heart can be divided into upper and lower chambers. An electrical impulse is generated in the upper chambers of the heart that causes the atria to squeeze and push blood into the ventricles. There is a short delay to allow the ventricles to fill. The ventricles then contract to pump blood to the body and the lungs.
The generation of Bio Potential:
Signals that control the physiological activity in the heart and other organs are a result of ions propagating over the cell membrane which some of the cells has. Walls of the cells such as nerve-, muscle- and gland cells have semi permeable membrane. This means that the membrane lets certain substances pass but stops other.
When the cells are in a resting state, the cell fluid inside of the membrane (cytosol) has excess of potassium ions and on the outside it is excess of chloride- and sodium ions. This difference in concentration is maintained by energy exacting pumps, which give a way for ions through the cell membrane. In resting state is the inside of the cell more negative than the cell’s outside. The explanation to this is that when the cell is in resting state the inside of the cell membrane has a collection of negative ions, and on the membranes outside are a corresponding collection of positive ions.