Resting ECG (electrocardiogram; electrocardiography) refers to the temporary recording of the sum of the electrical activities of all the heart muscle fibers using an electrocardiograph. This ECG is performed with the patient lying down and relaxed. The standard electrocardiogram is a 12-lead ECG, which records 12 leads simultaneously over time. By means of an ECG, heart rate, heart rhythm and position type (electrical axis of the heart) can be determined. Furthermore, the electrical activity of the atria (lat. atrium) and ventricles (lat. ventricles) can be read.
Indications (areas of application)
- Obvious, known, or suspected diseases of the cardiovascular system, such as coronary artery disease (CAD; disease of the coronary arteries), heart failure (cardiac insufficiency), hypertension (high blood pressure) with possible associated cardiac enlargement
- For therapy control (especially with drugs such as tricyclic antidepressants, antipsychotics (neuroleptics), digoxin and others).
- Screening of high-risk patients, sensitive occupational groups (e.g., bus drivers, pilots) and preoperatively.
- Cardiac arrhythmias, e.g.
- Sinus node – sinus tachycardia (> 100 beats per minute), sinus bradycardia (< 60 beats per minute).
- Supraventricular – atrial fibrillation (VHF), atrial flutter (typical and atypical).
- Ventricular – ventricular fibrillation, ventricular flutter, ventricular tachycardia.
- AV blockages:
- AV block I°
- AV block II°
- Mobitz 1 type (Wenckebach periodicity)
- Type Mobitz 2 incl. varying transition
- AV block III
- Intraventricular conduction disorders, e.g., left bundle branch block (LSB; permanent or intermittent), right bundle branch block (RSB; permanent or intermittent, complete or incomplete)
- Cardiac hypertrophy – increase in cardiac muscle mass and heart weight, for example, in hypertension (high blood pressure).
- Myocardial infarction (heart attack)
- Concomitant diseases, e.g. chronic lung disease, acute pericarditis (inflammation of the pericardium), suspected electrolyte imbalances (esp. potassium, calcium).
- Pacemaker therapy
Before the examination
ECG is a non-invasive diagnostic method that does not require any preparation from the patient.
The procedure
Electrocardiography allows the electrical activities of all the heart muscle fibers to be derived and displayed as waveforms in an electrocardiogram (ECG). There is a special stimulation system in the heart in which the electrical excitation is formed, which is then propagated through the conduction system. The excitation is generated in the sinus node, which is located in the right atrium of the heart. The sinus node is also called the pacemaker because it drives the heart at a certain frequency. It is controlled by the sympathetic and parasympathetic nervous systems (vagus nerve), which thus significantly influence the heart rhythm. From the sinus node, the electrical impulse travels via fiber bundles to the AV node (atrioventricular node). This is located at the junction with the ventricles (heart chambers) and regulates the transmission of impulses to the heart chambers. The period of excitation conduction is called the atrioventricular conduction time (AV time). This corresponds to the duration of the PQ time in the ECG. If the sinus node fails, the AV node can take over the function as the primary rhythm generator. The heart rate is then 40-60 beats per minute.If there is a strong temporal delay in the transmission of stimuli by the AV node or it fails, the clinical picture of the so-called AV block occurs. The resting ECG is usually performed while the patient is lying down. The electrical impulses are derived with the help of electrodes (suction electrodes; adhesive electrodes). The electrodes are placed on the arms, legs and chest for this purpose. The derivations denote the measurement of the potential differences created by the cardiac currents. A distinction is made between limb leads, which measure the potential differences between the extremities, and chest leads, which are determined by the electrodes on the thorax.The extremity leads are usually measured according to Einthoven (I, II, III) and Goldberger (aVR, aVL, AVf); the chest wall leads are usually measured according to Wilson (V1-V6; see below). In the 12-lead ECG, the limb leads according to Einthoven (I, II, II) and Goldberger (aVR, aVL, AVf) and the chest wall leads according to Wilson (V1-V6) are recorded simultaneously. An ECG machine amplifies these impulses and either displays them as an ECG curve (electrocardiogram) on a screen or prints them out on a paper strip. The duration of the examination is usually less than a minute.
Electrode position
The Wilson lead is a unipolar chest wall lead that is routinely recorded using 6 electrodes (V1-V6). The electrodes are placed as follows:
| V1 | ICR at the right edge of the sternum (breastbone). |
| V2 | 4th ICR at the left edge of the sternum. |
| V3 | between V2 and V4 on the 5th rib |
| V4 | Intersection of the 5th ICR with the left medioclavicular line. |
| V5 | Same height as V4, on the anterior axillary line (VAL). |
| V6 | Same height as V4, on the midaxillary line (MAL). |
| Elective, e.g., suspected posterior wall infarction, also follow at the level of V4 in addition: | |
| V7 | on the posterior axillary line (HAL) |
| V8 | on the scapular line |
| V9 | on the paravertebral line |
Legend
- ICR – intercostal space
- Medioclavicular line – imaginary line running vertically through the middle of the clavicle (collarbone).
- Axillary line – imaginary lines oriented to the anatomical shape of the armpit (axilla).
- Scapular line – imaginary line running vertically through the inferior angle (angulus inferior) of the shoulder blade (scapula).
- Paravertebral line – imaginary line running vertically through transverse processes (processus transversi) of the spine.
ECG curve
The normal ECG shows characteristic spikes in the curve, which since Einthoven (1990) are named with the letters P, Q, R, S, T and U. The ECG is composed of an atrial portion and a ventricular portion. The electrophysiological processes of the heart can be assigned to different sections of the curve of the electrocardiogram:
| Description | |
| P-wave | Atrial excitation, originating from the sinus node and propagating first through the right atrium, then the left atriumDuration of the P wave: ≤ 100 ms |
| PQ pathway | Horizontal line extending from the end of the P wave to the beginning of the QSR complex; time from the end of atrial excitation to the beginning of ventricular excitationDuration of PQ time: 120-200 ms. |
| QRS complex | Begins with a small, negative Q-spike; the subsequent high R-spike is a reflection of the excitation of most cardiac muscle cells; the negative S-spike indicates the final phase of depolarizationDuration of the QRS complex (QRS duration; QRS time): 110-120 ms. |
| J-point | Transition from S-point to ST-point |
| ST route | All cells of the ventricles (heart chamber) are now polarized; the ECG shows an isoelectric line (= no deflection) extending from the end of the QRS complex to the beginning of the T wave. |
| T wave | Arises from repolarization, i.e., the excitation regression of the ventricles; is usually positive |
| QT duration | Synonyms: QT time, QT interval; corresponds to ventricular systole, depends on heart rate; includes QRS complex, ST segment, and T waveDuration of QT time is strongly frequency dependent: approximately 350-440 ms |
| U-wave | Inconstantly occurring elevation after T-wave; positive shallow rise after T-wave; U-wave is thought to correspond to repolarization of Purkinje cells. |
ECG changes and their possible interpretations are elaborated at the respective clinical picture. Further notes
- The PR interval, which indicates the duration of signal conduction from the atria via the atrioventricular (AV) node into His bundles and Purkinje fibers on the electrocardiogram (ECG), is highly susceptible to genetic disorders. A genome-wide association study describes variants at 202 gene loci that either shorten or lengthen the PR interval:
- Prolongation of the PR interval = delay in conduction of excitation The result may be AV block with bradycardia (heartbeat too slow: < 60 beats per minute), which requires implantation of a pacemaker.
- Shortening of the PR interval. This may result in a preexcitation syndrome – such as Wolff-Parkinson-White syndrome (AV nodal re-entrant tachycardia (AVRT) with preexcitation) – with tachycardia (in this case: sudden onset of regular, usually rapid palpitations; heart rate: 160-250/min).