ELECTROCARDIOGRAM

ecg pdf

 

Introduction

Figure 83 Isochronous lines of ventricular activation of the human heart. The 30 ms activation front into the QRS is almost a closed region.

The ECG is a result of potentials appearing at the surface of the chest as a result of the summation of depolarisation and repolarisation of many myocardial fibres simultaneously.

 

         Figure 84 Heart equivalent circuit. Points A and B are arbitrary observation points on the torso, RAB is the resistance between them, and RT1, RT2 are lumped thoracic medium resistances. The bipolar ECG scalar lead voltage is f A - f B, where these voltages are both measured with respect to an indifferent reference potential.

Depolarisation initiated at the SA node spreads as a wavefront across the two atria, and also at a higher speed along the three inter-nodal tracts to the AV node. There is a delay at the AV node, then the wavefront travels at high speed down the His bundle in the interventricular septum, dividing the two ventricles. The His bundle branches into the Purkinje system which conducts the wavefronts along much of the endocardial surfaces of the two ventricles. The wavefronts then spread more slowly through the normal myocardium. They spread from the inside to the outside of the ventricles (Figure 83). Wavefronts travel with higher velocity in the direction of the fibre orientation. The morphology of the resulting ECG recorded on the chest surface depends on the orientation of the heart, the active recording electrode and the reference electrode (Figure 84).

 

The signal waveform produced for each heart beat consists of the P wave, due to atrial depolarisation, the QRS complex due to ventricular depolarisation, and the T wave caused by ventricular repolarisation. The effects of summing all the electrical activity in the heart can be represented by an electrical dipole whose magnitude and direction is constantly changing (Figure 85). The scalar magnitude of the ECG is then the dot product of the dipole and the electrode orientation.

Figure 85 Approximate dipole field of the heart at the peak of the R wave. The dipole consists of the points of equal positive and negative charge separated from one another and denoted by the dipole moment vector M.

 

Generally 12 lead positions are commonly used to record the ECG (Figure 86). The first three are known as leads I, II and III. They are left arm to right arm for I, ie the active lead is on the left arm (usually the left wrist) and the reference electrode is on the right arm (wrist). Lead II is left leg (ankle) to right arm, and lead III is left leg to left arm. The right ankle is usually grounded. The lead vectors can be represented by an equilateral triangle, known as Eindhoven's triangle. The direction of lead I vector is 0 degrees by convention. The direction of lead II is 60 degrees and lead III is 120 degrees.

   Figure 86 Cardiologists use a standard notation such that the direction of the lead vactor for lead I is 0o, that of lead II is 60o, and that of lead III is 120o. An example of a cardiac vector at 30o with its scalar components seen for each lead is shown.

 

 Figure 87 Connection of electrodes to the body to obtain Wilson's central terminal.

The remaining lead positions use a common reference, known as Wilson's central terminal (Figure 87). This consists of the right and left wrists joined to the left ankle, each through a suitably large resistor, eg 5 M ohms. The first three active electrodes are the right and left wrists and the left ankle. In practice this means that when one limb is an active electrode, it is shunted by the resistance that is part of the Wilson's central terminal circuit. To avoid this shunting, the active limb is connected by a resistor of half the value of the others, to the non-inverting input of the amplifier. The limb is not connected to the Wilson's central terminal. This is known as the augmented lead system (Figure 88). The leads are called aVL, aVR and aVF for active lead connections to the left wrist, right wrist and left ankle respectively. The other six leads also use the Wilson's central terminal as reference and the active lead is placed in different positions over the front of the chest overlying the base of the heart and the apex.

 Figure 88 Augmented limb lead connections, (a), (b), (c). Vector diagram in (d) shows standard and augmented lead-vector directions in the frontal plane.

 

Electrocardiograph recorders require a number of features to make them usable in clinical practice. These are:

 

1. Protection circuitry against defibrillation shocks that may be given to the patient. These shocks may be up to 3,000 volts.

 

2. Lead selector. The default mode is to automatically record all 12 leads simultaneously. Otherwise one or more leads are selected for recording. In cheaper machines, three or four leads are recorded simultaneously for five seconds at a time, with automatic switching to each group of three or four leads.

 

3. Calibration signal of 1 mV is automatically applied to each channel for a brief period.

 

4. Preamplifiers have a very high input impedance and high common mode rejection ratio (reject signals appearing on both the active and reference leads simultaneously).

 

5. Isolation circuit separates the patient from the power supply.

 

6. Driven right leg circuit. In older instruments the right leg was grounded. Now, as part of the isolation, the right leg is not connected to ground, but is instead driven by an amplifier to remain at a virtual ground.

 

7. Driver amplifier follows the pre-amplifier and drives the chart recorder. It also filters the signal to remove any dc offset and high frequency noise.

 

8. Microprocessor system contains circuitry for digitising the signal, and storing and analysing it. Most systems can automatically calculate the rate, analyse most of the common arrhythmias, report the axes of some features, and detect old and recent myocardial infarcts (heart attacks, coronary occlusions).

 

9. Recorder printer is used to provide a hard copy of the ECG, together with the patient information and the analysis and diagnosis.

 

Digitising and filtering is done so as to prevent aliasing, that is the false representation of a high frequency signal as a low frequency one (Figure 89). This happens if the sampling and digitising rate of the signal is less than twice the highest frequency in the signal, as determined by the filter. In that case, signals of higher frequency are digitised as low frequency signals.

Figure 89 Aliasing during analogue to digital conversion. The voltage level of a signal is captured at regular intervals, t1, t2, t3.... and each captured value converted to digital form and stored in memory. If the analogue signal contains higher frequencies than half of the sampling frequency, as shown above, the interpolated digitised signal (bold line) will have lower frequencies than in the original analogue signal.

 

 Figure 90 Effects of frequency distortion on ECG: (a) true ECG, (b) low pass filtered (high frequency distortion), (c) high pass filtered (low frequency distortion).

Frequency distortion can occur if the filters are not set so that the pass band ranges from 0.02 Hz to 150 Hz (Figure 90). If there is too much dc offset on the input signal, the output of the amplifiers may saturate, clipping the signal (Figure 91). Ground loops may induce 50 Hz noise on the recording if other equipment is connected to the patient but has a separate earthing system (Figure 92).

 

High voltage shocks or similar transients can cause the amplifiers to saturate temporarily, producing artefacts on the recording. Noise may be capacitively or inductively coupled to the recording leads (Figure 93).

 

Electrode movement can also cause noise. All electrodes can be represented by a battery in series with a parallel resistor and capacitor. The battery represents the polarisation voltage produced by dissimilar active and reference electrode materials being in contact with an electrolyte, the saline solution of body fluid. Alternatively the polarisation voltage can be seen as due to the layer of ions produced at the electrode by the current flowing between the electrode and the body fluid. The resistance is due to the resistance between the electrode and the tissue of interest. The capacitance is formed by the metal electrode plate as one electrode, deeper tissue as the second electrode and body fluid as the electrolyte.

 

The polarisation voltage is an unwanted dc offset that can swamp the signal being recorded. Any op amp dc bias current flowing through through the patient's body will also cause a dc voltage drop across the resistance of the electrode. In addition this dc current can charge the equivalent capacitor, also producing an unwanted dc offset.

 

Figure 91 Saturation and cut-off distortion on an ECG: (a) original signal, (b) clipping of peaks due to positive saturation in te amplifier, (c) clipping of negative peaks due to cut-off in the amplifier.

Electrodes used to record ECG's consist of metal disks with a conductive gel between the disk and the skin to ensure good contact. The metal disk and the skin form two plates of a capacitor and the conductive gel is the electrolyte. If the capacitor is charged and the distance between the disk and skin changes due to movement, the capacitance changes, so for a given charge the voltage must change. This produces movement noise. This can be overcome by making the electrode of an inverted cup made of insulating material with the metal electrode at the top. The inside is filled with conductive gel. The insulated edge of the cup sits on the skin so that the distance between the electrode and the skin remains constant. This prevents the changes in capacitance which cause movement noise.

 

               Figure 92 Ground loop between instruments (Electrocardiograph and Machine X). (a) Each instrument is grounded separately and also connected to the patient, so there is a closed path from ground A on the electrocardiograph to ground B on machine X. Current returns through dashed line representing connection between grounds in the wall. Voltage drop down this line causes each instrument ground to be at a different potential, so current will flow through patient and cause 50 Hz interference, or in extreme cases, electrocution. (b) The ground loop can be eliminated by connecting both instruments to the same ground and having only one connection to the patient.

There are two main consequences of coronary heart disease: arrhythmia and myocardial infarction. An arrhythmia is an abnormal rhythm developing in the heart. This may be either a slow rhythm or a fast rhythm. A myocardial infarction is a localised region of dead muscle due to a loss of blood supply.

Figure 93 50 Hz electric field "pick-up" from capacitive coupling between an electrocardiograph and a power line. Capacitive coupling between the active ("hot") lead of the power line and the lead wires causes current to flow through skin electrode impedances on its way to ground, thus generating a signal voltage.