Biological Psychology Practical Class 6: ECG PDF

6. The activity of the heart (electrocardiography, ECG) The major function of the heart is pumping of the blood (1) through the so-called systemic circuit, in order to deliver oxygen (O 2) and nutrients to tissues in the body and remove carbon dioxide (CO2) and other metabolites, and (2) thr...

6. The activity of the heart (electrocardiography, ECG) The major function of the heart is pumping of the blood (1) through the so-called systemic circuit, in order to deliver oxygen (O 2) and nutrients to tissues in the body and remove carbon dioxide (CO2) and other metabolites, and (2) through the so-called pulmonary circuit in order to exchange O2 and CO2 between the blood and the air in the alveoli of the lung (see also Chapter 5). This pump function is manifested by rhythmic contractions of the heart; direction of blood flow is maintained by valves located in the openings of the internal cavities of the heart and in blood vessels. The internal rhythm of the heart originates from the electrical signal generated by pacemaker cells located in the sinoatrial node (SA). This spontaneous internal rhythm and other features of the activity (e.g. the force of contraction) can be increased or decreased via sympathetic and parasympathetic activation (or the interaction of the two) by the central nervous system. For example, inhalation is accompanied by the slight increase of sympathetic activation leading to the acceleration of pace, whereas exhalation is characterized by increased parasympathetic impact, which decelerates the rhythm (Fig. 6.7). This phenomenon is called respiratory sinus arrhythmia (RSA); it is often used to estimate the impact of the parasympathetic system (via the vagus nerve) on the heart. Also, concentration of O2 and CO2 in the blood impacts the activity of the heart. Fig 6.1. On the left: sources of the major components of the ECG-signal (source: https://www.msdmanuals.com/en-jp/home/multimedia/figure/cvs_ecg_reading). On the right: ECG- signal corresponding to one cardiac period recorded with the Biopac system The contraction of the atrial muscles of the heart is caused by the electrical signal of the SA that is conducted by the electrically coupled muscle cells (syncytium). Concerning the ventricular muscles, the electrical signal generated by the SA is transmitted to the ventricular muscle via internodal fibres, the atrioventricular node, the bundle of His, the right and left bundle branches (aka Tawara branches) and Purkinje fibres. Due to the anatomical structure of this pathway, the ventricular muscle contracts after the atrial contraction only (the delay is about 200 ms), and the contraction starts in the apex of the heart. The contraction (dubbed systole) is evoked by the depolarization signal, whereas the following repolarization signal leads to the relaxation of muscle cells (diastole) (Fig 6.1). The left ventricular systole pushes a volume of blood (called stroke volume) into the aorta; from the aorta, the pressure wave reaches the major and minor arteries of the body. It is still measurable in the fingertips (Fig 6.2) and earlobes using plethysmography. Fig 6.2. Pulse plethysmography measurement with the Biopac system. Source: Biopac Student Lab v4.1.3 software, BIOPAC Systems, Inc. Pulse plethysmography uses near-infrared light beamed through the skin. As hemoglobin of the blood reflects this kind of light particularly well, the amount of near-infrared light sensed by the sensor placed next to the light source can be used to estimate the changes of volume of blood in the capillaries (reflection plethysmography) (Fig 6.3). This method is widely used by smart watches and bracelets recently. Although the measurement conducted with smart devices is not as reliable as the laboratory devices and procedures, it makes possible the continuous recording of HR during everyday activity, physical activity, and sleep. Another possibility is measuring the amount of light that passes through the tissues using a sensor placed on the other side of the body part, typically the earlobe or the distal phalanx of the little finger (transmission plethysmograpy). 2 Fig 6.3. ECG (upper part) and plethysmography recorded in the index finger (lower part). The marker indicates the moment when the hand was raised above the head. Amplitude of the ECG remains unchanged, whereas that of pulse plethysmography decreases due to the decrease of the amount of blood reaching the index finger. This latter change becomes compensated by regulatory reflexes in several seconds. The electrical activity of the heart is measurable with a pair of electrodes everywhere on the surface of the skin, as it is a relatively strong signal (usually several millivolts). Electrodes are placed in the so-called Lead II or modified Lead II pattern (a more sophisticated system is used in medical practice for diagnostic purposes). The measured activity is called electrocardiogram (EKG or ECG). Lead II refers to the placement of the positive electrode on the left ankle, the negative electrode on the right wrist, and the ground electrode on the right ankle (Fig 6.4). Fig 6.4. Measurement of ECG using the Lead II pattern with the Biopac system. Source: Biopac Student Lab v4.1.3 software, BIOPAC Systems, Inc. 3  In the modified Lead II design, the negative and ground electrodes are placed on (or below) the right and left collarbones, respectively, and the positive electrode is placed on the left lower part of the ribcage. In psychophysiological research, the use of the latter method is preferred as it minimizes movement-related artefact and provides an ECG particularly appropriate for the identification of the major components of the electrical signal. Most important components of the ECG are the P wave (depolarization of the two atria), the QRS-complex (depolarization of the two ventricles), and the T-wave (repolarization of the ventricles) (Fig 6.1). It is important the emphasize that there is a delay between the electrical and the respective mechanical events (i.e. muscle contractions); for example, the R-peak precedes the peak of blood pressure by approximately 150 ms. Also, the propagation of the pulse wave from the heart to the extremities takes further time (Fig 6.5). Fig 6.5. Temporal difference between the electrical event initiating the contraction of the ventricles (i.e., the R-peak; upper part) and the peak of the respective pulse wave as assessed on the index finger using plethysmography (lower part). The overall delay (delta T) in this case is approximately 380 ms The most important heart-related variables are the heart rate (HR), i.e., number of contractions per minute (bpm; typically in the 60-80 domain for healthy individuals under resting conditions; Fig 6.6) and the average interval between successive beats, called heart period or interbeat interval (IBI), in milliseconds (typically from 750 to 1000). Please, realize that these two measures are not independent of each other! 4 Fig 6.6. ECG (upper part) and the corresponding HR-values (lower part), based on the beat-to-beat intervals. The average HR for the marked period is 73 bpm (see upper left conner, marked with green). Also, multiple measures of heart rate variability (HRV; i.e., the temporal variation between successive heart beats) are used (e.g. HF, RMSSD, pNN50), typically to estimate the parasympathetic (vagal) influence on the heart (Fig 6.7). Fig 6.7. Periodic changes of heart rate (lower part) due to deep inhalations and exhalations (respiratory sinus arrhythmia, RSA) Unfortunately, there is no appropriate purely ECG-related measure for the estimation of sympathetic influence on the heart, as the HR is the net result of sympathetic and parasympathetic 5 impact. The sympathetic component can be estimated from the pre-ejection period (PeP), i.e., the delay between the electrical signal evoking ventricular contraction and the actual ejection of the blood from the left ventricle to the aorta. However, this indicator cannot be calculated from ECG only. HR can be calculated from both pletysmography and ECG; for the calculation of HRV, however, the use of ECG is recommended. To estimate HR, a sample frequency of 30 Hz is enough; for HRV, 300 Hz or higher is desirable. Fig 6.8. Continuous decrease of HR (recovery) after physical exercise Physical exercise can increase HR (even above 200 bpm for young people); following exercise, it gradually decreases to the resting value (Fig 6.8). Beyond physical activity (the so- called cardiac-somatic coupling), psychological factors, such as preparation for (physical or mental) action, paying attention, being on a novel situation, doing mentally demanding tasks, and emotions, including loneliness, are also able to increase or decrease HR in the absence of actual metabolic demand. The vagal component of HRV is also a widely assessed physiological concomitant of various, mostly stress-related, states and conditions, such as rumination, increased mental effort and workload. Attentional capacity, performance in sustained attention tasks and cognitive control are positively associated with vagal HRV. Finally, the acuity of perception of heartbeats and/or HR, called cardioceptive accuracy, is a frequently assessed variable in various areas of research, such as the development of the self, anxiety and eating disorders, and body objectification. Goal of the class: Demonstration of the basics of recording of cardiac activity (Biopac Student Lab Lesson L05-ECG-1 & Lesson L07 – ECG & Pulse) 6