Which is better for heart rate monitoring: ECG or PPG? How to measure heart rate variability in most accurately.
The heart is a complex organ. Apart from continuously pumping blood over an entire human lifespan, the heart provides valuable insights into the general state of different processes in the body. Monitoring the heart enables a wide range of mental, emotional, and physical health assessments, like stress management capability or the state of mental and cardiovascular health. Finding new ways to monitor the heart can help detect the early onset of cardiovascular disease, which shows the potential for preventive health.
The contraction activity of the heart muscle relies on electrical impulses and is strongly regulated by the autonomic nervous system (ANS). The ANS controls many body functions through the interchangeable activity of its two branches: sympathetic and parasympathetic. Heart rate variability (HRV) is the most valuable metric for assessing how the autonomic nervous system (ANS) is functioning and how the body is capable of taking on and managing stress. As a measuring unit of the variability between heart beats, HRV directly indicates how the ANS is functioning.
Modern technology and advancements in the field of medical devices offer new methodological possibilities for non-invasive HRV tracking. The most widely used sensor-based technologies are ECG (electrocardiography) and PPG (photoplethysmography). For those who are looking for the best HRV detection technology to include in apps for wearable devices, this article will outline the similarities and differences between the two, as well as point out the advantages and limitations each of these methods, bring to HRV measuring.
ECG for HRV recording
Short for electrocardiography, ECG measures the electrical activity of the heart over a period of time through electrodes that are attached to the skin. ECG essentially records the electrical activity of the heart muscle and how it is affected by different factors like the activity of the ANS. It records signals that happen far from the skin’s surface, which ECG electrodes capture instantly and accurately. This makes ECG a non-invasive method for measuring HRV and it is used as the reference standard diagnostic test by many healthcare providers who assess cardiac health.
The result of an ECG reading is called an electrocardiogram, and its most relevant feature for analyzing HRV is called the QRS complex: a graphical hallmark of the heart’s periodic electrical and contractile activity. Algorithms use the time interval between successive R waves to measure the duration of a heartbeat and calculate HRV values, which make ECG a highly accurate, robust, and sensitive method of measuring HRV. This also means that a substantial reading can be accurately accomplished in a short time frame (e.g. < 5 minutes) as well as during a wider range of activities (e.g. a 24h ECG recorded during habitual daily movements).
PPG for HRV recording
PPG (short for photoplethysmography) is a very simple optical technique for measuring HRV. Unlike ECG, which uses electrodes, PPG applies light reflection to detect the blood volume changes in peripheral arteries which occur with each heartbeat. Recordings are usually measured on parts of the body where blood vessels are close to the skin, like fingertips or earlobes. The recorded pulse waveforms are the basis for measuring HRV through PPG.
Advantages and limitations
Readings obtained through the ECG method have proven to be more accurate and comprehensive than those obtained through PPG. This is mainly because ECG reads the electrical signals produced by the heart activity directly, while PPG provides a more indirect measurement, using light reflection to detect the pulsating activity of the heart. When it comes to ease and accuracy of measuring, PPG can be measured at several positions on the body and offers a wide range of measurement possibilities, while ECG requires more specific anatomical placement of electrodes on the body.
PPG measurements are influenced by the number and proximity of the blood vessels present on the placement spot, which makes the accuracy of PPG change according to the body position. The accuracy of PPG also depends on external factors, like the effect of ambience lighting and in some cases even motion factors. This means that PPG could become less accurate in measuring HRV during exercise and may require additional data processing steps to improve accuracy under challenging conditions.
ECG can produce accurate data faster and in a more direct way than PPG, as it includes electrical readings and does not require settling time. However, ECG also introduces several challenges related to the collection and interpretation of data, as it relies on a physician to be prescribed, performed, and interpreted. This greatly limits access to this tool for diagnosis, care, prevention, or general HRV tracking. An interesting finding is that comparing 5-minute readings from both methods indicated a very high degree of correlation between ECG and PPG as methods for measuring HRV.
ECG and PPG represent two different approaches to measuring HRV in a fast, convenient, and accurate manner. Studies show that PPG is a valuable tool for HRV measuring under ideal conditions and shows great potential in becoming a more practical alternative to ECG for HRV tracking and analyzing. Both methods for measuring HRV offer accessible tools for diagnosing how the ANS is functioning, which can further help early detection of certain conditions and provide insights into the overall health of the body.
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