The electrocardiogram (ECG) is the most accurate method for measuring heart rate (HR) and heart rate variability (HRV). Peer-reviewed journals consider the ECG the "gold standard" for monitoring HR and HRV. Since ECG sensor placement requires skin preparation, more time, disposable supplies, and adjusting or partially removing clothing, clinicians are less likely to use it. The PPG method is simpler and achieves acceptable clinical accuracy. However, PPG sensors are more vulnerable to movement artifacts. Unlike the ECG method, PPG sensors are affected by finger vasoconstriction.
This post covers the Source of the ECG Signal, ECG Sensors, Skin Preparation, ECG Signal, Sampling Rate, Placements, Artifacts, and Tracking Tests.
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The Source of the ECG Signal
The sinoatrial (SA) node initiates each cardiac cycle in a healthy heart by spontaneously depolarizing its autorhythmic fibers. The SA node's firing of 60-100 action potentials per minute usually prevents slower parts of the conduction system and myocardium (heart muscle) from generating competing potentials. Graphic © Alila Medical Media/Shutterstock.com.
The SA node fires an impulse that travels through the atria to the AV node in about 0.03 seconds and causes the AV node to fire. The P wave of the ECG is produced as contractile fibers in the atria depolarize. The P wave culminates in the contraction of the atria (atrial systole). Animation © 2010 Scholarpedia.
ECG Sensors
Three-lead electrode assemblies are sufficient to record the ECG signal. There is no universal color-coding system for ECG electrodes (Lehrer, 2018). ECG sensors can be identical to EMG sensors. Standard lead cables have snap buttons onto which the electrodes are affixed. A Thought Technology Ltd. ECG preamplifier and sensors are shown below.
Clinicians can use dry or pre-gelled electrodes. Pre-gelled disposable ECG electrodes save preparation time and reduce the risk of infection.
Skin Preparation
Prepare the skin by rubbing the area where the electrodes will be applied with an alcohol wipe. Cleaning the skin of oil and dirt helps reduce skin-electrode impedance, which is the opposition to AC flow. You may need to shave the chest and abdomen if body hair prevents satisfactory electrode contact with the skin for men.
ECG Signal
The signal from the SA node rapidly spreads through the atrioventricular (AV) bundle, reaching the top of the septum. Descending right and left bundle branches conduct the action potential over the ventricles about 0.2 seconds after the appearance of the P wave.
Conduction myofibers extend from the bundle branches into the myocardium, depolarizing contractile fibers in the ventricles (lower chambers). Ventricular depolarization generates the QRS complex. The software uses the R-spike (depicted below at 3) to detect a heartbeat and measure the IBI.
The ventricles contract (ventricular systole) soon after the emergence of the QRS complex. Their contraction continues through the S-T segment. Ventricular contractile fiber depolarization generates the T wave about 0.4 seconds following the P wave. The ventricles relax (ventricular diastole) 0.6 seconds after the P wave begins (Tortora & Derrickson, 2021).
Check out the YouTube video 15 Second EKG. Graphic © lotan/Shutterstock.com.
Sampling Rate
The Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996) recommended an ECG sampling rate of at least 250-500 Hz without interpolation. A sampling rate is the number of measurements taken per second. Laborde et al. (2017) advised a minimum 125-Hz rate for research. Where RSA amplitude is low, they suggested a minimum 500-Hz rate.
Placements
Six standard ECG electrode placements can be used. These include the wrist, wrist-to-ankle, forearm, lower torso, and chest (upper chest/xiphoid; heart level). These placements differ in vulnerability to skeletal muscle (EMG) or movement artifact, speed of application, and degree of client comfort.
Wrist
A wrist placement requires electrode straps instead of adhesive electrodes. One strap is used to attach an active electrode to the right wrist, and the other to attach the reference and second active electrode to the left wrist. Although the easiest, most socially comfortable, and quickest ECG electrode placement, it is highly vulnerable to arm EMG and movement artifacts.
Wrist-to-Ankle
Place the active (+) electrodes on the left wrist and ankle and the reference (-) electrode on the right wrist.
The right arm to left leg placement often accentuates the R-spike in individuals with large T-waves and is less invasive than chest or lower torso placements. This placement is more vulnerable to movement artifacts than the chest or lower torso placement (Lehrer, 2018). Graphic © Designua/Shutterstock.com.
Forearm
A forearm placement locates an active electrode on the right forearm and the reference and second
active electrodes on the left forearm. Select an area with minimal or no hair. This placement is more vulnerable to contamination by arm and chest EMG and movement artifacts.
Lower Torso
A lower torso placement suggested by Peper (2010) centers the reference electrode over the angle of the sternum and the active electrodes about 5 centimeters above the navel and 10 centimeters to the left and right of the midline. This placement provides an alternative for clients who are uncomfortable exposing their chests (they can lift their blouse or shirt) and is less vulnerable to arm EMG and movement artifacts.
Chest Placement
A chest placement locates active and reference electrodes over the right and left coracoid processes, respectively, and a second active electrode over the xiphoid process. This placement reduces the risk of arm muscle artifact but exposes the chest area, which can be uncomfortable for female clients (Shaffer & Combatalade, 2013).
An alternative chest placement locates all three electrodes in a row at heart level. This sensor arrangement can detect the largest-amplitude R-spikes (Lehrer, 2018). The Lief Therapeutics sensor uses this placement. Graphic © Lief Therapeutics.
Placement Summary
Wrist or forearm placements offer greater client comfort and quicker application speeds where EMG and movement artifacts don't contaminate your recordings. The lower torso placement may be best for research when these artifacts are present. Sensor placement on the upper chest and abdomen requires client/participant education and written informed consent.
ECG Artifacts
ECG artifacts include 50/60Hz noise, EMG, respiration, movement, DC offset, electromagnetic (EMI), and electrode polarity. These can cause missed or extra heartbeats, profoundly distorting HRV measurements.
Missed or Extra Beats
HRV software determines the interbeat interval (IBI) by detecting adjacent beats and measuring the time between successive R-spikes. Graphic © arka38/Shutterstock.com.
After detecting the first beat, the software starts counting and calculates the first IBI in milliseconds. This process is repeated until the epoch or data collection period ends. Graphic adapted from Dr. Richard Gevirtz.
IBI measurements are the basis of statistical calculations of time-domain (pNN50, RMSSD, and SDNN), frequency-domain (VLF, LF, and HF), and nonlinear measurements.
When distortion prevents software from detecting a heartbeat, this results in a missed beat and a prolonged interbeat interval (IBI) calculation. On the graph below, a missed beat generated the circled IBI (1500 ms).
Conversely, when distortion causes the software to detect an extra beat, this produces an artifactually short interbeat interval (IBI). As emphasized earlier, missed and extra beats also affect PPG recording (Elgendi, 2012).
Line Interference Artifacts
Line interference (50/60Hz) artifacts are the most frequent source of ECG signal contamination. Line interference is less of a problem for the BVP signal, which is based on back-scattered or transmitted infrared light. The primary sources of 50/60Hz artifacts include computers, computer monitors, fluorescent lights, and power outlets. Line interference artifacts look fuzzy because high-frequency fluctuations are superimposed on the signal (Shaffer & Combatalade, 2013).
Clinical Tips to Minimize Line Interference (50/60Hz) Artifacts
1. Use a 50/60Hz notch filter.
2. Place the encoder box 3 feet (1 meter) from electronic equipment.
3. Remove unused sensor cables from the encoder box.
4. Carefully prepare the skin.
5. Apply pre-gelled electrodes to achieve low and balanced skin-electrode impedances (10 Kohms/5 Kohms).
6. Examine the raw signal for artifacts.
EMG Artifacts
Frequencies generated by the depolarization of skeletal muscles overlap with the ECG spectrum and produce EMG artifacts.
The surface EMG ranges from 1-1,000 Hz (Stern, Ray, & Quigley, 2001), while the ECG extends from 0.1-1,000 Hz (Langner & Geselowitz, 1960). Muscle action potentials from large muscle groups travel to ECG sensors via volume conduction (Shaffer & Neblett, 2010).
The contraction of arm muscles can cause the software to "see" many extra beats and calculate shorter IBIs (Shaffer & Combatalade, 2013).
Although EMG artifacts affect ECG recordings, they do not contaminate the BVP signal since the PPG sensor detects pressure wave peaks using infrared light.
Clinical Tips to Minimize EMG Artifacts
1. Use chest or lower rib placements instead of forearm or wrist placements.
2. Instruct your client to sit in a relaxed position and restrict movement, and verify compliance.
3. Examine the raw signal for artifacts.
Movement Artifacts
Client movement can pull the electrode cable so that the electrode partially (or completely) loses contact with the skin. Movement artifacts consist of high-amplitude signal fluctuations that cause the software to "see“ many extra beats and calculate shorter IBIs, as with EMG artifacts.
Below is a BioGraph ® Infiniti ECG display of movement artifact. The ECG (also called EKG) waveform abruptly shifts upward after the sixth heartbeat and then returns to normal. You can enlarge the video by clicking on the bracket icon at the bottom right of the screen. When finished, click on the ESC key.
Clinical Tips to Minimize Movement Artifacts
1. Firmly tape sensor leads to client clothing for strain relief to restrict movement.
2. Use a lower torso placement.
3. Use pre-gelled electrodes to ensure strong skin-electrode contact.
4. Provide instructions to sit in a relaxed position and restrict movement, and confirm compliance.
5. Examine the raw signal for artifacts.
Respiration Artifacts
Respiration artifacts can result from dried gel and inadequate skin preparation.
Clinical Tips to Control Respiration Artifacts
1. Abrade the skin after cleaning with an alcohol wipe.
2. Use pre-gelled electrodes to ensure strong skin-electrode contact.
Direct Current (DC) Offset Artifacts
DC offset artifacts occur when the skin-electrode impedances of the three ECG electrodes differ due to poor skin-electrode contact. The ECG signal may drift up or down, causing extra beats or missed beats.
Clinical Tips to Minimize DC Offset Artifacts
1. Clean the skin using an alcohol wipe.
2. Use fresh electrodes with sufficient gel.
3. Examine the raw signal for artifacts.
Electromagnetic Interference (EMI) Artifacts
Electromagnetic interference (EMI) artifacts are generated by cell phones when they are less than 6 ft (2 m) from ECG sensors or encoder boxes (Lin & Peper, 2009).
Computer monitors and television screens are sources of EMI artifacts. These are also called radiofrequency (RF) artifacts. High-frequency energy expands outward from a monitor like a cone (Montgomery, 2004).
Also, watch out for audiovisual systems and high-voltage equipment like centrifuges, elevators, and X-ray machines (Lehrer, 2018).
Clinical Tips to Minimize Electromagnetic Interference Artifacts
1. Ensure that all cell phones are turned off.
2. Maintain an adequate distance from high-voltage equipment (Lehrer, 2018).
3. Position an encoder box or separate instruments behind or to the monitor's side.
4. Place the client no closer than 2-3 feet from the monitor (Montgomery, 2004).
Polarity Artifacts
Polarity artifacts occur when the active electrodes (yellow and blue for Thought Technology) are misaligned with respect to the heart’s axis. Low-amplitude downward-oriented R-spikes can cause the software to miss beats and lengthen the IBI.
Software packages like J & J Engineering’s Physiolab and Thought Technology Ltd.'s BioGraph Infiniti automatically correct for polarity artifacts (Lehrer, 2018).
Clinical Tips to Minimize Polarity Artifacts
1. Locate active electrodes on the right shoulder and over the xiphoid process (extension of the lower sternum).
2. Laterally adjust the right shoulder active electrode position to correct polarity or increase R-spike amplitude.
3. The best location may require experimentation.
Tracking Test
Using a respirometer, you can determine whether the ECG signal responds to your client's breathing by observing whether instantaneous HR speeds during inhalation and slows during exhalation (gray line) (Nederend et al., 2016).
The BioGraph ® Infiniti display below shows that instantaneous HR (pink) speeds and slows as the abdominal strain gauge (purple) rhythmically expands and contracts. You can enlarge the video by clicking on the bracket icon at the bottom right of the screen. When finished, click on the ESC key.
Dr. Inna Khazan demonstrates ECG and respiration recording, artifacts, and a tracking test © Association for Applied Psychophysiology and Biofeedback. You can enlarge the video by clicking on the bracket icon at the bottom right of the screen. When finished, click on the ESC key.
Quiz
Take a five-question exam on Quiz Maker to test your mastery.
Glossary
artifacts: false signals.
bundle branches: fibers that descend along both sides of the septum (right and left bundle branches) and conduct the action potential over the ventricles about 0.2 seconds after the appearance of the P wave.
conduction myofibers: fibers that extend from the bundle branches into the myocardium, depolarizing contractile fibers in the ventricles.
DC offset artifacts: ECG artifacts that lengthen the IBI when differences in skin-electrode impedance produce signal drift causing the software to miss beats.
diastole: the period when the ventricles or atria relax.
electrocardiogram (ECG): a recording of the heart's electrical activity using an electrocardiograph.
electromagnetic interference (EMI) artifacts: ECG artifacts generated when cell phones transmit an artifactual voltage.
EMG artifacts: ECG artifacts that shorten the IBI when signal contamination by the EMG causes the software to detect nonexistent beats.
epoch: recording period,
extra beats: ECG and PPG artifacts can shorten the IBI when signal distortion causes the software to detect nonexistent beats.
heart rate (HR): the number of heartbeats per minute, also called stroke rate.
HR Max – HR Min: an index of heart rate variability that calculates the difference between the highest and lowest heart rates during each respiratory cycle.
heart rate variability (HRV): beat-to-beat changes in HR, including changes in the RR intervals between consecutive heartbeats.
high-frequency (HF) band: a HRV frequency range from 0.15-0.40 Hz that represents the inhibition and activation of the vagus nerve by breathing (respiratory sinus arrhythmia).
interbeat interval (IBI): the time interval between the peaks of successive heartbeats.
left atrium: the upper chamber of the heart that receives oxygenated blood from the pulmonary veins and pumps it to the left ventricle.
left ventricle: the bottom chamber of the heart that receives oxygenated blood from the left atrium and pumps it through the aorta.
light artifacts: PPG artifact when light leakage increases BVP amplitude.
line interference artifacts: ECG and PPG artifact when 50/60Hz contamination of signals causes the software to detect nonexistent beats and shorten the IBI.
low-frequency (LF) band: a HRV frequency range of 0.04-0.15 Hz that may represent the influence of PNS and baroreflex activity.
missed beats: ECG and PPG artifacts can lengthen the IBI when signal distortion causes the software to overlook a beat and use the next good beat.
movement artifacts: ECG and PPG artifacts can shorten the IBI when signal distortion from movement causes the software to detect nonexistent beats.
P waves: the depolarization of the atria, which precedes atrial contraction. It is the first positive deflection on the ECG tracing.
photoplethysmographic sensor: a photoelectric transducer that transmits and detects infrared light that passes through or is reflected off tissue to measure brief changes in blood volume and detect the pulse wave.
pNN50: the percentage of adjacent NN intervals that differ by more than 50 milliseconds.
polarity artifacts: ECG artifacts produced when reversed electrode placement inverts the direction of the R-spike and causes the software to miss beats and lengthen the IBI.
QRS complex: an ECG structure corresponding to the ventricles' depolarization.
R-spike: the initial upward deflection in the QRS complex of the ECG.
reference electrode: a "ground" ECG electrode that may be placed on the left upper chest, below the palmar aspect of the left elbow, or above the palmar aspect of the left wrist.
respirometer: flexible respiratory sensor.
respiratory sinus arrhythmia (RSA): the respiration-driven heart rhythm that contributes to the high frequency (HF) component of heart rate variability. Inhalation inhibits vagal nerve slowing of the heart (increasing heart rate), while exhalation restores vagal slowing (decreasing heart rate).
RMSSD: the square root of the mean squared difference of adjacent NN intervals.
sampling rate: the number of measurements taken per second.
SDANN: the standard deviation of the average 5-minute NN intervals. The SDANN estimates heart rate changes produced by cycles longer than 5 minutes.
SDNN: the standard deviation of the interbeat interval measured in milliseconds, which predicts morbidity and mortality.
SDRR: the standard deviation of the interbeat interval for all sinus beats measured in milliseconds, which predicts morbidity and mortality.
sinoatrial (SA) node: the heart node that initiates each cardiac cycle through spontaneous depolarization of its autorhythmic fibers.
skin-electrode impedance: the complex opposition to an AC signal measured in Kohms.
S-T segment: an ECG structure that connects the QRS complex and the T wave. Ventricular contraction continues through the S-T segment.
systole: the contraction of the left ventricle.
T wave: ECG structure that represents ventricular repolarization.
tracking tests: checks of whether the biofeedback display mirrors client behavior. BVP amplitude should increase and then decrease as a hand is raised above the heart and then dropped below the heart.
very-low-frequency (VLF): a HRV frequency range of 0.003-0.04 Hz that may represent temperature regulation, plasma renin fluctuations, endothelial, physical activity influences, and possible intrinsic cardiac, and PNS contributions.
xiphoid process: the inferior and smallest segment of the sternum.
References
Aldini, M. (2016). Issues in heart rate variability (HRV) analysis: Motion artifacts & ectopic beats. Blog post: https://www.hrv4training.com/blog/issues-in-heart-rate-variability-hrv-analysis-motion-artifacts-ectopic-beats Andreassi, J. L. (2000). Psychophysiology: Human behavior and physiological response. Lawrence Erlbaum and Associates, Inc.
Berntson, G. G., Quigley, K. S., & Lozano, D. (2007). Cardiovascular psychophysiology. In J. T. Cacioppo, L. G. Tassinary, & G. G. Berntson (Eds.). Handbook of psychophysiology (3rd ed.). Cambridge University Press.
Combatalade, D. (2010). Basics of heart rate variability applied to psychophysiology. Thought Technology Ltd.
Combatalade, D. (2013). CardioPro Infiniti: HRV analysis module for BioGraph Infiniti. Thought Technology Ltd.
Elgendi, M. (2012). On the analysis of fingertip photoplethysmogram signals. Current Cardiology
Reviews, 8, 14-25. https://dx.doi.org/10.2174%2F157340312801215782
Jan, H.-Y., Chen, M.-F., Fu, T.-C., Lin, W.-C., Tsai, C.-L., & Lin, K.-P. (2019). Evaluation of coherence between ECG and PPG derived parameters on heart rate variability and respiration in healthy volunteers with/without controlled breathing. Journal of Medical and Biological Engineering, 39, 783-795. https://doi.org/10.1007/s40846-019-00468-9
Kamath, M. V., & Fallen, E. L. (1993). Power spectral analysis of heart rate variability: A noninvasive signature of cardiac autonomic function. Critical Reviews in Biomedical Engineering, 21(3), 245–311. PMID: 8243093
Lin, I.-M., & Peper, E. (2009). Keep cell phones and PDAs away from EMG Sensors and the human body to prevent electromagnetic interference artifacts and cancer. Biofeedback, 37(3), 14-16.
Laborde, S., Mosley, E., & Thayer, J. F. (2017). Heart rate variability and cardiac vagal tone in psychophysiological research -- Recommendations for experiment planning, data analysis, and data reporting. Frontiers in Psychology, 8, 213. https://doi.org/10.3389/fpsyg.2017.00213
Langner, P. H. Jr., & Geselowitz, D. B. (1960). Characteristics of the frequency spectrum in the normal electrocardiogram and in subjects following myocardial infarction. Circ Res, 8, 577-584. https://doi.org/10.1161/01.RES.8.3.577
Lehrer, P. M. (2018). Personal communication. Montgomery, D. (2004). Introduction to biofeedback. Module 3: Psychophysiological recording. Association for Applied Psychophysiology and Biofeedback. Nederend, I., Jongbloed, M. R. M., de Geus, E. J. C., Blom, N. A., & ten Harkel, A. D. J. (2016). Postnatal cardiac autonomic nervous control in pediatric congenital heart disease. J Cardiovasc Dev Dis, 3(2) 16. https://doi.org/10.3390/jcdd3020016
Shaffer, F., & Combatalade, D. C. (2013). Don't add or miss a beat: A guide to cleaner heart rate variability recordings. Biofeedback, 41(3), 121-130. https://doi.org/10.5298/1081-5937-41.3.04
Shaffer, F., & Neblett, R. (2010, Summer). Practical anatomy and physiology: The skeletal muscle system. Biofeedback, 38(2) 47-51. https://doi.org/10.5298/1081-5937-38.2.47
Stern, R. M., Ray, W. J., & Quigley, K. S. (2001). Psychophysiological recording (2nd ed.). Oxford University Press.
Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996). Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Circulation, 93, 1043-1065. PMID: 8598068
Tortora, G. J., & Derrickson, B. H. (2021). Principles of anatomy and physiology (16th ed.). John Wiley & Sons, Inc.
BCIA Essential Skills
ECG
1. Explain the ECG signal and biofeedback to a client.
2. Explain ECG sensor attachment to a client and obtain permission to monitor her.
3. Explain how to select a placement site and demonstrate how to attach ECG sensors to minimize movement artifacts.
4. Demonstrate skin preparation.
5. Perform a tracking test by asking your client to inhale slowly and then exhale as you watch the change in heart rate.
6. Identify movement artifact in the raw ECG signal and explain how to control movement, and remove this artifact from the raw data.
7. Explain the major measures of heart rate variability, including HR Max - HR Min, pNN50, SDNN, and SDRR.
8. Explain why we train clients to increase power in the low-frequency band of the ECG and how breathing at 5-7 breaths per minute helps them accomplish this.
9. Demonstrate how to instruct a client to utilize a feedback display.
10. Describe strategies to help clients increase their heart rate variability.
11. Demonstrate an HRV biofeedback training session, including record keeping, goal setting, site selection, baseline measurement, display and threshold setting, coaching, and debriefing at the end of the session.
12. Demonstrate selecting and assigning a practice assignment based on training session results.
13. Evaluate and summarize client progress during a training session.
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