Electrocardiography records the heart’s electrical activity. This process produces an electrocardiogram (ECG), which is a graph showing the heart’s electrical activity over time, captured via electrodes placed on the skin.
The most clinically used ECG lead system is the standard 12-lead system. This includes six limb leads (I, II, III, aVL, aVR, and aVF) and six chest (or precordial) leads (V1-V6). Limb leads are bipolar; for example, lead I measures the potential difference between the left and right arms, II between the right arm and left leg, and III between the left arm and left leg. Precordial leads measure the potential difference between each chest electrode and Wilson’s central terminal, the average potential of the three limb electrodes.
In many cases, simplified lead systems are sufficient. For HRV analysis, often one lead is adequate. Long-term ECG recorders like Holter monitors can use various setups, from one to seven leads. Clinical Holter monitors typically record limb leads and one or a few precordial leads.
Precordial leads V2-V5 often show a higher QRS complex amplitude compared to other leads, making them preferable for HRV analysis. If precordial leads are unavailable, it’s best to record the ECG in the direction of the heart’s electrical axis, roughly the same direction as limb lead II. Effective ECG waveform analysis requires clear visibility of P and T waves. Therefore, using either lead V2 or II is advantageous, as all ECG waveforms should be clearly visible in these leads. Dubin 2000
A normal heart rhythm is characterized by four primary ECG waveforms:
- P Wave: Represents atrial depolarization.
- QRS Complex: Represents ventricular depolarization.
- T Wave: Represents ventricular repolarization.
- U Wave: Represents papillary muscle repolarization (a small deflection following the T wave).
Figure 1 presents a typical ECG waveform demonstrating these components.
Figure 1: ECG waveforms at leads I, II, and V2.
There are many types of ECG recorders available today, designed for diverse settings such as hospitals, sports clinics, scientific research, and direct consumer use. Their typical applications include wellbeing tracking, heart disease monitoring, and performance and recovery analysis during exercise. The most commonly used ECG recorder types are:
- Holter Monitor: Suitable for long-term recordings ranging from 24 hours to several days. Widely used in hospitals and research. These are portable, support one to five ECG leads, and come in lightweight models that minimally interfere with normal activities. Recorders like Faros and Actiheart 5 are fully compatible with Kubios HRV Scientific.
- Sports ECG Monitor: Some heart rate belt manufacturers, such as Polar and Movesense, offer ECG recording modes. These provide an easy, cost-efficient solution for recording ECG and HRV during sports activities or scientific studies. These recorders are not medical devices and are not intended for clinical use. The Kubios mobile app enables ECG recording with sensors like the Polar H10.
- Event ECG Recorders: Used by both hospitals and consumers to detect occasional arrhythmias. They allow for short, one-lead ECG recordings lasting from 30 seconds to 5 minutes. Recorders such as Alivecor KardiaMobile are compatible with Kubios HRV Scientific.
- Lab ECG Recorders: Often single-lead, these recorders are used for simultaneous recording of various biosignals such as breathing gases, blood pressure, respiratory rate, electromyogram, etc. Biopac recordings can be imported directly into Kubios HRV Scientific.
- 12-Lead ECG Monitor: Employed in hospitals for short-term, resting recordings. Their 12 leads provide clinicians with comprehensive data for diagnosing heart-related diseases like arrhythmias, ischemia, and myocardial infarction.
- Bedside ECG Monitors: These are used in hospitals primarily for heart rate and rhythm monitoring, recording typically one lead ECG.
ECG waveform analysis begins with the detection of wave boundaries. Many ECG analysis software, including Kubios HRV Scientific, can automatically detect these boundaries. However, it is recommended to visually verify and adjust the boundary markers for accuracy.
Typical ECG waveform parameters are outlined in Table 1 and illustrated in Figure 2. The baseline amplitude is defined from the onset point of the P-wave. The amplitudes of each wave are calculated from the baseline to the maximum absolute amplitude of the respective wave.
Table 1: Commonly used ECG-waveform parameters
|Time between P-wave onset and offset
|Time between Q-wave onset and S-wave offset
|Time between Q-wave onset and T-wave offset
|Corrected QT-time, Bazzet’s formula
|Amplitude between the baseline and peak amplitude of P-wave
|Amplitude between the baseline and peak amplitude of Q-wave
|Amplitude between the baseline and peak amplitude of R-wave
|Amplitude between the baseline and peak amplitude of S-wave
|Amplitude between the baseline and peak amplitude of T-wave
Figure 1: ECG waveform parameters.
- Dubin D. Rapid interpretation of EKG’s: an interactive course. Cover Publishing Company, 2000.
- Bittium Faros (Bittium Oyj, Finland, www.bittium.com/medical/bittium-faros)
- Actiheart 5 (CamNtech Inc, UK, www.camntech.com/actiheart-5/)
- Polar H10 (Polar Electro Oy, Finland, www.polar.com/en/sensors/h10-heart-rate-sensor)
- Movesense Medical (Movesense Oy, Finland, www.movesense.com/movesense-medical)
- Alivecor KardiaMobile (Alivecor Inc, USA, www.kardia.com)
- Biopac ECG amplifier (BIOPAC Systems Inc, USA, www.biopac.com/product/ecg-electrocardiogram-amplifier/
- Bazett HC. An analysis of the time‐relations of the electrocardiograms. Annals of Noninvasive Electrocardiography, 2:177-194, 1997.