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Chapter: Medical Electronics - Electro-Physiology and Bio-Potential Recording

Electrocardiography(ECG)

A very widely used medical instrument, which is utilized to diagnose and monitor cardiac beat abnormalities, is the electrocardiograph.

ELECTROCARDIOGRAPHY (ECG)

 

·        A very widely used medical instrument, which is utilized to diagnose and monitor cardiac beat abnormalities, is the electrocardiograph.

 

·        It measures the electrical activity of the heart (more precisely biopotential differences arising from the electrical activity of myocardium). We’ve already talked about the genesis of the ECG signal.

 

·          The ECG machine uses surface electrodes and high input impedance

 

·        Differential amplifiers with good common mode rejection ratio to record the electrocardiogram

 

·        Normal ECG amplitude ranges between 0.5-4 mV. Normal frequency content of ECG (for diagnostic purposes) is 0.05-100 Hz. A typical ECG waveform is shown below:

Significant diagnostic features of the ECG signal are:

 

·        Duration of component parts of the signal

 

·        Polarities and magnitudes

 

·        The details of the ECG signal and the degree of variability in different parts of the ECG signal is shown below:

 

 

·        The QRS amplitude, polarity, time duration, the RR interval (indicator of heartbeat per min.) and the T-wave amplitude are some very important and distinctive features of the ECG signal.

 

·        The heart rate in BPM = Beats Per Minute) is simply = 60 (RR interval in seconds)

 

Some ECG waveform abnormalities that may indicate illness are:

 

·        An extended PR interval may be diagnosed as AV node block

 

·        A widening of the QRS complex may indicate conduction problems in the bundle of His

·        An elevated ST segment may indicate occurrence of myocardial Infarction (MI)

·        A negative polarity in the T wave may be due to coronary insufficiency

 

1. ECG Leads

 

A Normal ECG recording for the standard lead connections leads I, II and III (Lead II provides the strongest signal)


Obviously, all human hearts are not the same and this results into a high degree of variability.

 

Some abnormalities that may indicate illness:

 

·        An extended P-R interval  may be diagnosed as AV node block

 

·        Widening of the QRS complex conduction problems in the bundle of His


·        Elevated ST segment may indicate occurrence of MI

 

·        Negative polarity T wave may be due to coronary insufficiency QRS amplitude, polarity, time domain, PR interval (indicator of heat beat per min. & T-wave amplitude are some very important.

 

Distinctive features.

 

1.Loss


2. Origin of the ECG signal

 

·        We have already covered this concept extensively in the previous lectures (The Dipole filed of the heart, the Eindhoven’s Triangle, the electrical circuit model for the electrocardiographic problem, etc.)

 

Standard Limb Leads (I, II, III)


 

·        The lead wires are color-coded according to some conventions. One example is: White – RA (Right Arm), Black – LA (Left Arm), Green – RL (Right Leg), Red – LL (Left Leg), and Brown – C (Chest)

 

Augmented Limb Leads

 

·        These leads offer a free 50% increase over leads VR, VL, and VF connections (unipolar leads) with respect to Wilson terminal AVR = -I – III/2, AVL = I – II/2, aVF = II – I/2




Each measurement is made from the reflected limb and the average of the other two limbs.

 

2. The ECG Machine

 

Most representative Specs:

 

Zin = 10 MΩ

Frequency response = 0.05 –100 Hz

Strip Chart Recorder Speed = 25 mm/sec.

Fast Speed = 100 mm/sec.

For detailed Specs. Refer to the Table in your text “Summary of performance requirements for electrocardiographs”

 

Location of the Heart

 

         The heart is located between the lungs behind the sternum and above the diaphragm.

 

         It is surrounded by the pericardium.

 

         Its size is about that of a fist, and its weight is about 250-300 g.

 

         Its center is located about 1.5 cm to the left of the midsagittal plane.



Anatomy of the heart

 

         The walls of the heart are composed of cardiac muscle, called myocardium.

 

         It consists of four compa rtments:

 

–  the right and left atria and ventricles

 

The Heart Valves

 

         The tricuspid valve regulates blood flow between the right atrium and right ventricle.

 

         The pulmonary valve c ontrols blood flow from the right ventricle into the pulmonary arteries

 

         The mitral valve lets oxygen-rich blood from your lungs pass from the left atrium into the left ventricle.

 

         The aortic valve lets oxygen-rich blood pass from the left ventricle i nto the aorta, then to the body.


Blood circulation via heart

 

         The blood returns from the systemic circulation to the right atrium and from there goes through the tricuspid valve to the right ventricle.

         It is ejected from the rig ht ventricle through the pulmonary valve to t he lungs.

 

         Oxygenated blood return s from the lungs to the left atrium, and from there through the mitral valve to the left ventricle.

         Finally blood is pump ed through the aortic valve to the aort a and the systemic circulation.

 

Electrical activation of the heart

 

                     In the heart muscle cell, or myocyte, electric activation takes place b y means of the same mechanism as in the nerve cell, i.e., from the inflow of Na ions across the cell membrane.

 

         The amplitude of the action potential is also similar, being 100 mV for both nerve and muscle

 

         The duration of the card iac impulse is, however, two orders of magnitude longer than in either nerve cell or sceletal muscle cell.

 

         As in the nerve cell, repolarization is a consequence of the outflow of K ions.

 

         The duration of the action impulse is about 300 ms

 

Mechanical contraction  of Cardiac Muscle

• Associated with the electric activation of cardiac muscle cell is its mechanical

• contraction, which occurs a little later.

 An important distinction between cardiac muscle tissue and skeleetal muscle is that in cardiac muscle, activatiion can propagate from one cell to another in any direction.

• Electrical signal begins in the sinoatrial (SA) node: "natural pacemaker." causes the atria to contract.

         The signal then passes through the atrioventricular (AV) node.

–  sends the signal t o the ventricles via the “bundle of His”

 

–  Causes the ventricles to contract.

 

The Conduction System



The Action Potential

 

Recording an AP requires the isolation of a single cell.

 

·   Microelectrodes (with t ips a few μm across) are used to stimulate and record the response. A typical AP i s 2-4ms long with an amplitude of about 100 Mv


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