BME140W15

Lab#6 Report

Lab: C-06

ECG Signal Measurement

Preston Hoang

Due: 3/13/2015 (F)

Signed:______________________________I affirm that I completed this entire document in compliance with the UCI Academic Honesty Policy.

Abstract:

The purpose of this lab is to acquire and analyze ECG signals using an ECG amplifier. After checking the amplifier, leads are formed by attaching electrodes to certain positions in the arms and legs. When results are obtained using one of two leads (lead I), the measured duration for the PR interval, QRS interval, and ST interval is 160 ms, 70 ms, and 240 ms, respectively. In addition, the amplitude of the P, Q, R, and S wave is 0.085 mV, 0.0 mV, 1mV, and 0.285 mV. The frequency of the heart is 88BPM, and the mean QRS vector overall is 45 – 50 degrees. All values are normal except the ST interval. Other results include the derivation of the signal in lead III using information found in leads I and II, and the discussion of increased noise when a person holds their hands tight.

Introduction:

An ECG device measures electrical activity in the heart in periodic waves, which correspond to mechanical events in the heart. This waveform can be broken into three components: P wave (atrial depolarization), QRS wave (atrial re-polarization / ventricular depolarization), and T wave (ventricular re-polarization). By analyzing the duration, amplitude, frequency and the mean QRS vector of these constituent waves, one can better assess a patient’s heart condition. The specific placement of electrodes in the arms and legs determines what lead it is. By knowing the magnitude / direction of the two leads and projecting it in the Einthoven triangle (1D), one can detect the direction of cardiac electrical activity. The ECG measurement device built in lab has a gain of 1000, consisting of an instrumental amplifier and the band-pass filter. This is needed since the amplitude of the ECG signal is small (0.05 – 10mV), and the frequencies are acquired are within a specific range (0.05 – 1000 Hz).

Theoretical Background and Design:

To briefly re-cap on the circuit design, an instrument amplifier (gain of 25) and a band pass filter (gain of 40) are used to create an ECG amplifier with a gain of 1000. The benefits of an instrumental amplifier include having a high CMRR (essential for small signals), excellent precision, and high input impedance. To justify the range of the band-pass filter, ECG signals often include low-frequency noise resulting from respiration and electrode movement (<0.03 Hz), and high-frequency noise resulting from muscle activity (1 – 5000 Hz). Although specific resistor and capacitor values are used in the design of this circuit, they are not discussed in this lab (see previous lab). Here is the overall schematic of the circuit:

Figure 1: Overall schematic of the circuit

Figure 2: Overall breadboard design

For safety measure, two 10kΩ resistors are used for the two electrodes used as a lead (see VA and VB). The reason for this is that it increases the voltage necessary to induce a macro-shock on the subject (> 5mA). The effects of these macro-shocks include unwanted muscle contractions, tissue injury, and in more severe cases, heart fibrillation. Assuming no safety measures are in place, if the electrode gel on skin has resistance of 10 kΩ / cm2, the area of contact is 2 cm2, and the body has a resistance of 400Ω, the shocking voltage would be around 52V (close to unsafe). This is based on the model that the electrode gel resistances (RS) and the body resistances (RB) are in series. Mathematically speaking, the shocking voltage can be calculated using the equation V T=(2 RS+RB )∗I (see Figure 3). By adding two extra resistors (placed in series), the voltage threshold increases enough for practical safety purposes. This is justified based on the following calculation:

Let RE represent the extra resistors in place:

V T=(2 RS+RB+2 RE )∗I=( 2∗100002

+400+2∗10000) (5×10−3 )=152V

Figure 2: The resistances of the gel electrodes and the body

In addition, a third 10kΩ resistor is connected to another electrode placed in ground. Based on Figure 4 and the large resistor values for the input impedance of the ECG, most of the current would be drawn to the 10 kΩ resistor instead of the body. This provides yet another safety mechanism in preventing electric shock.

Figure 4: Circuit model of the ground

Focusing on the electrode placement, the electrodes are typically positioned in the arms and the legs for convenience. In more specific terms, lead I is connected to the right arm (-) and the left arm (+); lead II is connected to the right wrist (-) and the left foot (+); and lead III is connected to the left wrist (-) and left foot (+). In all three of the leads, the ground is connected to the right foot. From there, the cardiac vector can be calculated, which essentially models the direction of the depolarization in time. To calculate the cardiac vector, the magnitude (specifically the magnitude of the QRS complex) and direction of two leads are considered, which is then projected one-dimensionally in an Eithenoven triangle. This is the resultant Einthoven triangle:

Figure 5: Einthoven Triangle

From the middle point on each side, a line is drawn in proportion to the magnitude of the QRS complex. A perpendicular segment is then drawn at the end of this aforementioned line. Repeating the process for the other lead, the intersection between the two lines shows the direction of the QRS complex in relation to the center. Typically, the normal mean range for the electrical axis is between -30 to 90 degrees. Values outside this range typically indicate pathologic hypertrophy on either ventricle.

It is also important to discuss the P wave, QRS wave, and T wave in more detail. The P wave represents atrial depolarization, the QRS wave represents atrial re-polarization / ventricular depolarization, and the T wave represents ventricular re-polarization. One can get the frequency of each heartbeat by measuring how long it takes for each successive wave to occur (i.e. the duration in between two P waves). The value then gets standardized to beats per minute. In pathological cases, the interval between the constituent waves gets delayed significantly or it disappears entirely. One example is atrial defibrillation (extended PR interval) or atrial fibrillation (the absence of a discernible P-wave).

Figure 6: A typical ECG signal

Experimental Methods and Design: It is essential to discuss the steps that were carried out in this lab. As mentioned

before, we connected a 10kΩ resistor in the positive terminal of the two op-amps for safety measures. From there, the circuit is then tested on the NI Elvis Instrument to make sure that the appropriate gain is calculated. The specifications for our validation are ~60 dB at <100 Hz, 57 dB at 100 Hz, and a decrease of 20dB per decade. For the inputs, the electrode is placed in certain positions to form lead I, and is connected to the banana input terminals via adapters and cables. The output of the circuit is then connected to the oscilloscope to view the ECG signal. The input impedance for the oscilloscope is then changed to 1MΩ with a sweeping time of 200 ms / division. The ECG signal is then measured when the user moves his arms, relaxes, and when he holds his fist. The last two are recorded and saved by the oscilloscope website under the “data” menu. These steps are then repeated for lead II.

Results and Discussion:

Figure 7: ECG signal from Lead I when I am relaxed

Figure 8: ECG signal from Lead II when I am relaxed

Based on the ECG readings from lead I (see Figure 7), one can estimate the duration of each constituent waves. To get a rough estimate, the figure was pasted on MS paint. Then the number of pixels for a certain interval is compared to the number of pixels that separate each grid (200 ms / 35 pixels). On average, there are 28, 12, and 42 pixels for the duration of the PR interval, QRS interval, and the ST interval. This equates to 160 ms, 70 ms, and 240 ms for each case respectively. Interpreting these values, the PR and the QRS intervals are within normal range, though both are slightly below average. The ST interval is significantly shorter than normal (350 ms) due to the miniscule ST segment. This may indicate a possible heart condition (myocardial ischemia) or equipment errors within the ECG. To discern the two possibilities, another experiment needs to be run using an ECG measurement that is more precise (found in hospitals).

The amplitude of the ECG signals also needs to be considered as well. Using similar methods from before, one can determine the amplitude of each wave (500 mV/ 35 pixels). On average, there are 6, 0, 70, and 20 pixels for the amplitudes of the P, Q, R, and S intervals respectively. This translates to 0.085V, 0V, 1.0V, and 0.285V for the amplified ECG signal. Given that the gain of the circuit has a gain of 1000, the actual amplitudes for each case are 0.085 mV, 0.0 mV, 1mV, and 0.285 mV. Comparing with the values found in the literature, these amplitudes are all within normal range. Of course, any abnormalities with the amplitude would correspond with various heart problems (e.g. emphysema, pericardial diffusion).

Estimating the heart rate is also possible with the given ECG data in lead I. Given that each successive QRS complex is separated by 120 pixels, this corresponds to 0.685 seconds / beat. Converting this to beats per minute, this comes out to 88 BPM. This elevated range may be due to excessive movement (e.g. walking around to get supplies for the circuit, moving my body to attach the electrodes) and due to elevated stress levels

– my resting heart rate is 55 - 60 BPM. The average resting heart rate for a human adult is 70BPM. Bradycardia is established at <60 BPM, which is associated with either heart complications (e.g. due to old age or issues with the heart tissue) or an efficient heart for well-trained heart. Tradycardia is established at >100 BPM, which is associated with normal response to exercise, stress, anxiety, stimulants, or various heart complications.

Figure 9: ECG signal in lead I with increased noise due to tightened hands

Figure 10: ECG signal in lead II with increased noise due tightened hand

Interestingly enough, another experiment was run where I tightened my hands while measuring the ECG signal. As mentioned before, there appears to be increased noise in both cases when it is recording the data. The heart has to work harder to supply blood for muscle contractions when the hands are tightened. Essentially, more electrical events are being executed when more muscle fibers are being recruited for contraction. More specifically, the cross bridge cycle between myosin and actin (involving the interaction of ADP and P) requires an electrical signal to stimulate the process. Based on these factors, it is important to relax so that an accurate ECG reading is measured without extraneous muscle activity.

It is also important to assess the QRS mean electrical activity of the heart. Using the steps provided in the theoretical background, one can establish the cardiac vector of the electric field. For lead I, the value that will be used to draw the line from the middle (see Figure 5) is based on the amplitude of the Q, R, and S wave. Using the equationPosition=Q−R−S, the value used for lead I is +0.715mV. The values for the amplitude on lead II is 0mV, 1.15mV, and 0mV for Q, R, and S respectively using the MS paint technique. The value used for lead II using the aforementioned equation is +1.15mV. The cardiac vector is then drawn as shown in the net figure:

Figure 10: Einthoven triangle of my ECG

Based on the cardiac vector displayed above, it appears to be oriented at around 45 to 50 degrees. This appears to be within normal range for the mean electrical axis (-30 to 90 degrees). Any left or right axis deviation would indicate pathologic hypertrophy of the ventricles (i.e. more muscles being de-polarized).

Since we only need to use two lines to establish an intersection, the information in Lead III is not necessary. It is possible however draw an Einthoven triangle using any two combinations of three leads. Essentially, the information from Lead III can be derived from the information from Lead I and II. This is governed by the Kirchhoff equation VI-VII+VIII = 0. In other words, the voltage from lead III can be obtained by subtracting lead I to Lead II. This is the predicted voltage data for lead III based on the data obtained from the experiment.

Figure 11: Theoretical ECG reading of lead III

Conclusion:

An ECG measuring device with a gain of 1000 helps to discern the various components off the signal. The electrodes are attached to certain positions (leads), typically the arms and legs for convenience. These leads can be projected in an Einthoven triangle based on the magnitude of the QRS complex, such that the cardiac vector can be calculated. Incidentally, the information from lead III is based off the difference between lead II and I via Kirchhoff’s law. In a typical ECG signal, one can establish the duration and amplitude of each constituent part. In addition, one can also measure the frequency of the beats by measuring the duration of each successive part (i.e. between successive QRS interval). All of this data can be used to accurately assess a patient’s heart condition.