What Is ECG and How Does It Work?

What Is ECG and How Does It Work?


ECG (electrocardiography) is a method of collecting electrical signals generated by the heart. This allows us to understand the level of physiological arousal that someone is experiencing, but it can also be used to better understand someone’s psychological state.

Below we’ll go through the importance of physiological arousal in emotions, the physiology of the heart, how the activity can be measured, and what parameters are of interest.



Physiology and function of the heart





Before deepening the basics of ECG, let's look at the basics of cardiac physiology and function:

The heart has four rooms. The two upper chambers (left / right atria) are entry points into the heart, while the two lower chambers (left / right ventricles) are contraction chambers that send blood to the body. The circulation is divided into a "loop" through the lungs (pulmonary) and another "loop" through the body (systemic).

The cardiac cycle refers to a complete heart rhythm of its generation at the beginning of the next cycle, comprising several steps of filling and emptying the cavities. The frequency of the cardiac cycle is reflected as a heart rate (beats per minute or bpm).

The heart works automatically - it's self-exciting (it's a unique feature compared to other body muscles that require a nerve stimulus for excitement). The rhythmic contractions of the heart occur spontaneously but are sensitive to nervous or hormonal influences, particularly sympathetic (waking) and parasympathetic (deceleration) activities.

How to measure heart activity?

Heart activity can be recorded in these ways:

 Electrocardiography (ECG, EKG)


ECG records the electrical activity generated by heart muscle depolarizations, which propagate in pulsating electrical waves towards the skin. Although the electricity amount is in fact very small, it can be picked up reliably with ECG electrodes attached to the skin (in microvolts, or uV).

The full ECG setup comprises at least four electrodes which are placed on the chest or at the four extremities according to standard nomenclature (RA = right arm; LA = left arm; RL = right leg; LL = left leg). Of course, variations of this setup exist in order to allow more flexible and less intrusive recordings, for example, by attaching the electrodes to the forearms and legs. ECG electrodes are typically wet sensors, requiring the use of a conductive gel to increase conductivity between skin and electrodes.




Cardiac parameters of interest


Recording heart rate data gives you access to the following parameters that can be interpreted with respect to a participant’s arousal:

Heart Rate (HR). HR reflects the frequency of a complete heartbeat from its generation to the beginning of the next beat within a specific time window. It is typically expressed as bpm. HR can be extracted using ECG and PPG sensors.

Inter-Beat Interval (IBI). The IBI is the time interval between individual beats of the heart, generally measured in units of milliseconds (ms). Typically, the RR-interval is used for the analysis.

Heart Rate Variability (HRV). HRV expresses the natural variation of IBI values from beat to beat. HRV is closely related to emotional arousal: HRV has been found to decrease under conditions of acute time pressure and emotional stress (meaning that the heartbeat is more consistent).

HRV has also been found to be significantly reduced in individuals reporting a greater frequency and duration of daily worry [2], as well as in patients suffering from post-traumatic stress disorder (PTSD) [3]. For IBI and HRV analysis, ECG sensors are recommended as they are more sensitive to certain signal characteristics which PPG sensors cannot pick up.

Why combine ECG with other sensors?

Of course, data based on heart rate alone offers valuable insights into nonconscious arousal in response to emotionally loaded stimulus material. However, data solely based on ECG or PPG data can‘t tell us whether the arousal was due to positive or negative stimulus content.

Why? The change in heart rate is in fact identical. Both positive and negative stimuli can result in an increase in arousal triggering changes in heart rate.

In other words: While ECG/PPG are ideal measures to track emotional arousal, they are not able to reveal emotional valence, the direction of emotion. The true power of ECG/PPG techniques unfolds as these sensors are combined with other data sources such as facial expression analysis, EEG, and eye tracking.

References 


Nickel & Nachreiner (2003). Sensitivity and Diagnostics of the 0.1-Hz Component of Heart Rate Variability as an Indicator of Mental Workload. Human Factors 45 (4): 575–590.

Jönsson (2007). Respiratory sinus arrhythmia as a function of state anxiety in healthy individuals. International Journal of Psycho-physiology 63 (1): 48–54.

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