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Hello, and welcome to the next video of the series. In this video, we will talk about the basics of ECG and analog derivation. ECG stands for electrocardiogram, which is an electrical measurement of the activity in the heart. OK, but what is it really measuring?

As we know, the heart is responsible for maintaining adequate blood flow through the body in order to deliver oxygen and nutrients, while removing carbide dioxide and waste. As the oxygenated blood returns from the systemic circuit-- IE, the body-- it must pass through the heart on its way to the pulmonary circuit-- IE, the lungs. This is where carbide dioxide is exchange for oxygen.

Oxygenated blood then returns to the heart and is pumped back out to the systemic circuit. During each heartbeat, the cardiac muscle tissue contracts and a specific sequence in order for the blood to flow in the proper direction, passing from one chamber to the next. The contraction of each segment in the heart produces its own depolarization waveform, like the one shown on the right.

The summation of each of these contractions produces the final resulting waveform, shown at the bottom. This waveform has unique characteristics, which are very identifiable even to non-physicians. By observing the period between repeated segments of the waveform, you can easily determine a person's heart rate.

Furthermore, as each segment of the ECG waveform corresponds to a specific portion of the cardiac cycle, trained cardiologists can use these waveforms to diagnose a range of conditions and diseases affecting the heart's health. The range of information, from heart rate to diagnostic data, is why such a wide variety of health-related applications may require ECG measurements.

Let's take a closer look at the ECG characteristics. ECG is often plotted on a special type of graph paper with a standard xy scale. 10 millimeters on the x-axis represents 0.4 seconds, which equals 25 millimeters per second. On the y-axis, 10 millimeters represents 1 millivolt. As this diagram shows, a typical ECG has a peak-to-peak amplitude of just a few millivolts when measured by an electrode on the skin surface.

To calculate a patient's heart rate, the distance between two R peaks is measured and converted into beats per minute. For example, if the measured distance of one cardiac cycle is 24 millimeters, you would divide 24 millimeters by 25 millimeters per second, then take the reciprocal and multiply it by 60 seconds per minute to get beats per minute.

This plot also illustrates one of the key challenges to measuring ECG-- baseline drift. The baseline of an ECG develops between two measurement electrodes, which can change over time due to things like electrode contact quality, respiration, and patient movement. We can model the connection between the patient and the electrode like an electric circuit. Each layer of the skin provide some impedance, which is typically a complex impedance. The interference between the skin and the electrode itself also provides some complex impedance, plus some DC voltage.

Finally, the electrodes generally have some DC voltage generated from their chemistry. These circuit elements cannot be modeled easily, since they are actually changing constantly when the patient moves or if the electrode contact quality changes. The diagram on the right illustrates how two electrodes may be resting at different DC potentials, VA and VB. The difference between them is the differential offset voltage for that lead, or VD.

Standards for ECG end equipment specified that the ECG must still be measurable in plus or minus 300 millivolts of differential offset. The AC component of the ECG waveform is relatively low and frequency, usually between 0.05 and 40 Hertz. Diagnostic quality ECG applications may require up to 150 Hertz or more to extract additional information from the waveform.

This diagram shows where ECG falls in the frequency spectrum relative to other biopotential measurements. Again, note the magnitude of the ECG is just a few millivolts. Later on in this presentation, we will also discuss the detection and measurement of pacemaker signals. Pacemakers are electronic medical devices used by patients with abnormal heart rates or arrhythmias. These devices initiate the cardiac cycle in a well-controlled manner, such that the heart can maintain a normal level of function.

Other signals often measured in biopotential applications include EMG-- measured in biopotential applications include EMG, electromyogram; EEG, electroencephalogram; and respiration. As we alluded to earlier, there are some common challenges to measuring ECG. These challenges usually come from power line interference, baseline instability from poor electrode connection, muscle shaking, and wandering baseline from the changing DC level of the electrodes.

Some of these challenges can be overcome with a good circuit design, but most often, digital post-processing is required to band-pass filter the ECG waveform from DC drift or high frequency interference. A lead in ECG is the voltage difference between two points on the body. For most leads, each point is measured by an independent electrode, though some use a calculated average as the second point-- but more on this shortly.

You can think of different leads as different views-- information about the heart's electrical activity. The number of leads in an ECG system can vary widely. Simpler ECG systems generally use no more than the first two or three primary leads. This is typical in customer applications and small portable ECG equipment meant for basic diagnostic purposes. On the other hand, high-quality diagnostic systems are more complex and may use up to 15 different leads. The more viewing angles included in the ECG, the more diagnostic information the physician can gather about the patient's heart health.

Next, we will review each of the different ECG lead groups and see how they are measured or calculated. We will cover the three primary leads, three augmented leads, and six chest leads, for a total of 12 standard leads. Let's start with the primary limb leads.

The three primary leads are derived using what is known as-- named after the physiologist who invented the first electrocardiogram, Willem Einthoven. The three main measurement electrodes are placed on the left arm, right arm, and on the left leg. These electrodes are abbreviated as LA, RA, and LL, respectively. From these three primary electrodes, we can derive the three primary limb leads, which are lead I, lead II, and lead III.

Lead I is LA to RA. Lead II is LL to RA. Lead III is LL to LA. This means that any one of the limb leads can be calculated from the other two. Therefore, you really only need to measure two, because the third can always be calculated. In addition to the three primary electrodes, a fourth electrode is commonly used for DC-coupled ECG systems.

This electrode is known as the right-leg drive for our purpose. The common mode voltage of the human body is floating with respect to the ECG measurement system. Without a way to bias the body, the input signals from the electrodes may not be within the acceptable input range of the ADC. Typically, the voltage on each pin must be within the range of the ADC's power supply-- VDD and VSS in the plot on the right.

The right-leg drive electrode drives a voltage to the patient that will cause the DC level of the other electrodes to fall within the supply range of the ECG equipment so they can be measured properly. That covers the first group of ECG leads the primary limb leads. As this table summarizes, you can calculate one of the limb leads if you measure the other two. The formulas for all three are given in the column in the last column on the right.

Next, we will cover the chest leads. This figure illustrates where the electrodes for the chest leads are located on the body-- through V6. V1 and V2 are placed on either side of the sternum between the fourth and fifth rib. The remaining electrodes are slightly lower, and spaced out such that V6 is located under the right armpit. Each of these electrodes is used to provide the physician with different viewing angles of the heart's right and left ventricular activity.

Unlike the primary limb leads, which make a differential measurement between two electrodes, chest leads are single-ended measurements made with respect to the voltage at the center of the Einthoven triangle. This voltage is called the Wilson Central Terminal, or commonly referred to as WCT.

The WCT is calculated as the average voltage of all three limb electrodes, or RA plus LA plus LL divided by 3, as shown in the diagram. The WCT provides the reference that we use as the negative input voltage when measuring the chest leads. Next, we will cover the final group, the augmented leads.

There are remaining leads in the ECG known as augmented leads. These leads provide enhanced vector information, which is used to determine the heart's electrical access within the body and provide physicians with even more information to make a diagnosis. The augmented leads are named augmented vector right, augmented vector left, and augmented vector foot-- or AVR, AVL, and AVF, respectively.

Each augmented lead uses one of the primary electrodes as the positive input, while the average of the remaining two is used for the negative input. For example, AVR is measured as the RA electrode minus the average between LA and LL. Finally, we have defined all 12 standard limb leads. As you can see from this table, most ECG configurations only require that you measure one or two leads, while the remaining leads can be calculated.

The only leads which have to be measured are the chest leads, as there is no way to calculate their values from the other leads. This table summarizes what leads you measure for single-lead, three-lead, six-lead, and 12-lead ECG, which are the most common configurations. Also shown is how many ADC channels are required to measure each one.

Single lead ECG is typically measured using lead I, which uses the two electrodes placed on the arms. Three-lead ECG measures all the limb leads, but only two ADC channels are required, since you can calculate the third from the first two. Six-lead ECG refers to the limb and augmented leads, but still only requires two ADC channels, since all the other leads can be calculated from two limb leads.

Finally, 12-lead ECG refers to measuring the limb, augmented, and chest leads, but requires only eight ECG channels, since two are required for limb and augmented leads and six are required to measure V1 through V6. This concludes analog lead derivation for an ECG circuit. For more information, visit TI.com/medical. Thanks for watching.