How to Measure ECG - Introduction: What is ECG?
ECG measurement is a growing application space both in the traditional hospital environment as well as in wearable and mobile systems. This training series explains the clinical basics of ECG, the physiology behind the signal, and how to model the body with ideal electrical components. Then, we dive into the circuits that are required to measure ECG and learn how TI's ADS129x family of delta-sigma ADCs integrate many of the signal chain requirements to simplify customer designs.
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Hello, and welcome to the first video in this training series on how to measure ECG, a guide to the signals, system blocks, and solutions. ECG is a growing application space. Traditionally, ECG is measured in the hospital environment for patient monitoring. However, we have seen a recent trend in more mobile and wearable ECG systems as well.
This training series will provide an overview of the physiology behind ECG, as well as the key functional blocks and specifications needed to measure it. Later on, you will learn how TI's ADS129x family of delta-sigma ADCs integrate many of the required functional blocks and simplify the ECG signal chain design for customers. Before we get into how ECG is measured, it's important that we understand what the signal characteristics are and where they come from. In this section, we will define what ECG is and explain some of the physiology behind it.
ECG stands for electrocardiogram, which you may know is an electrical measurement of the activity in the heart. OK, but what is it really measuring? The heart is responsible for maintaining adequate blood flow throughout the body, in order to deliver oxygen and nutrients, while removing carbon dioxide and waste. As deoxygenated blood returns from the systemic circuit, i.e. the body, it must pass through the heart on its way to the pulmonary circuit, the lungs. Oxygenated blood then returns to the heart and is pumped back out to the systemic circuit.
During each heartbeat, the cardiac muscle tissue contracts in a specific sequence, in order for blood 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 ones 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 and 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 x, y scale. 10 millimeters on the x-axis represents 0.4 seconds, which translates to 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 by 60 seconds per minute. This results in a heart rate of 62 beats per minute, or BPM. This plot also illustrates one of the key challenges to measuring ECG, baseline drift. The baseline of an EEG waveform is the DC offset voltage that 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 electrodes like an electric circuit. Each layer of the skin provide some impedance, which is typically a complex impedance. The interface 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, such as silver/silver chloride, for example.
These circuit elements cannot be modeled easily, since they are actually constantly changing whenever 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, Vd. Standards for ECG and equipment specify that the ECG must still be measurable in the presence of up to plus or minus 300 millivolts of differential offset.
The AC component of the ECG waveform is relatively low in frequency, usually between 0.05 Hertz 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 only a few millivolts.
Later on in this presentation, we will also discuss the 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 more normal level of function. Other signals often measured in biopotential applications include the electromyogram, or EMG, and the electroencephalogram, EEG, as well as respiration rate.
As we alluded to earlier, there are some common challenges to measuring ECG. These challenges usually come from power line interference, baseline instability due to poor electrode connection, muscle movement, and wandering baseline from changing DC levels in the electrodes. Some of these challenges can be overcome with a good circuit design. But in most cases, digital post-processing is still required to bandpass filter the ECG and remove DC drift or high-frequency interference.
Well, that's all for now. Stay tuned, for additional videos in this series on how to measure ECG.
This video is part of a series
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How to measure ECG: A guide to the signals, system blocks and solutions
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