What the ECG records
Electrocardiography is a method for recording the electrical potentials of the heart, that is, the process of myocardial excitation. It is important to understand that the ECG captures not the contraction itself, but the propagation of excitation through the cardiac conduction system. Therefore, a situation is possible where the mechanical activity of the heart has already ceased (asystole), while electrical activity is still recorded on the strip for some time.
To understand the nature of the waves, one must remember two key properties of atypical cardiomyocytes:
- Automaticity — the ability to spontaneously generate an impulse. This is associated with the absence of a stable resting potential and the presence of slow diastolic depolarization due to constantly open sodium channels.
- Functional syncytium — cardiomyocytes are connected by gap junctions, so the excitation of one cell is immediately transmitted to neighboring cells. When an impulse arises in an ectopic focus, the wave spreads in both directions, which underlies a number of arrhythmias.
Conduction system of the heart
Excitation normally originates and propagates in a strict sequence:
- Sinoatrial (SA) node — the primary pacemaker, located in the right atrium, generates impulses at a rate of about 60–80 beats per minute.
- Internodal tracts — three bundles conducting excitation to the AV node, and Bachmann's bundle, heading toward the left atrium. Therefore, the right atrium is excited before the left.
- Atrioventricular (AV) node — located between the atria and ventricles, the secondary pacemaker (about 40–50 impulses/min). The main function is conduction delay.
- Bundle of His — the tertiary pacemaker (about 30–40 impulses/min), divides into right and left bundle branches; the left branch divides into anterior and posterior fascicles.
- Purkinje fibers — conduct excitation directly to the working myocardium of the ventricles.
The left ventricle is more massive than the right, so more conduction fibers supply it, which is reflected in the amplitude and direction of the vectors on the ECG.
Electrode placement
Limb electrodes are placed according to the traffic light rule in a clockwise direction:
- right arm — red;
- left arm — yellow;
- left leg — green;
- right leg — black (ground).
Skin contact is improved with saline or an antiseptic. Chest leads (V1–V6) are conveniently placed in practice by using the xiphoid process as a landmark:
- V1 — at the right sternal border, slightly above the xiphoid process;
- V2 — symmetrically to the left of the sternum;
- V4 — under the left nipple (in women, the breast tissue is displaced);
- V3 — midway between V2 and V4;
- V5 — on the anterior axillary line at the level of V4;
- V6 — on the midaxillary line at the same level.
Chest leads provide a three-dimensional view of the heart: V1, V2 "look" at the right ventricle, V5, V6 at the left.
Elements of the cardiogram
Four basic concepts are distinguished on the ECG:
- Isoelectric line — a horizontal segment with no deviations, recorded when the sum of vectors is zero.
- Wave (deflection) — any deviation from the isoelectric line upward or downward. A wave directed toward the electrode is positive, away from the electrode is negative.
- Segment — the stretch of the isoelectric line between two waves.
- Interval — a wave together with the adjacent segment up to the beginning of the next wave.
Formation of waves and complexes
Each ECG element corresponds to a specific stage of excitation propagation:
- P wave — atrial depolarization. Consists of two merging waves (right, then left atrium). Normally positive.
- PQ segment — conduction through the AV node and the bundle of His (these structures themselves are "not visible" on the ECG).
- PQ interval — the total time from the start of atrial excitation to the start of ventricular excitation.
- QRS complex — ventricular depolarization. The Q wave reflects the excitation of myocardial areas near the bundle of His in the opposite direction (negative); R is the main positive wave, corresponding to the excitation of the bulk of the myocardium; S is the late excitation of the basal regions.
- ST segment — the period when the heart is completely encompassed by excitation; depolarization and repolarization balance each other out.
- T wave — ventricular repolarization.
- U wave — inconstant, recorded in children and during electrolyte disturbances (hypo- and hyperkalemia). It has diagnostic significance, but its exact origin is unknown.
Atrial repolarization is not visible on a standard ECG — it is masked by the QRS complex.
Recording speed and element duration
There are two standard paper speeds for recording: 25 mm/s and 50 mm/s. On graph paper, small squares (1 mm) and large squares (5 mm; a large 5×5 mm square corresponds to 1 cm) are distinguished.
Duration of one small square:
- at 25 mm/s — 0.04 s;
- at 50 mm/s — 0.08 s.
Normal values of the main elements:
- P wave — up to 0.1 s;
- QRS complex — up to 0.1 s;
- PQ segment — about 0.08 s;
- PQ interval — 0.12–0.2 s (average 0.18 s);
- QT interval — approximately equal to twice the PQ.
Prolongation of waves and intervals reflects a slowdown in the respective conduction and depolarization processes, which is directly related to heart rate and the risk of rhythm disturbances.
Key points
- The ECG records the electrical excitation of the myocardium, not the contraction itself.
- Conduction system: SA node → internodal tracts and Bachmann's bundle → AV node → bundle of His → bundle branches → Purkinje fibers.
- SA node is the primary pacemaker (60–80/min), AV node is the secondary (40–50/min), bundle of His is the tertiary (30–40/min).
- Limb electrodes are placed according to the traffic light rule in a clockwise direction; the landmark for chest leads is the xiphoid process.
- V1–V2 reflect the right ventricle, V5–V6 the left.
- The wave is positive if the excitation vector is directed toward the electrode, and negative if it is directed away from it.
- P represents atrial depolarization, QRS ventricular depolarization, and T ventricular repolarization; the U wave is associated with electrolyte disturbances.
- At 25 mm/s, one small grid square = 0.04 s; normal QRS ≤ 0.1 s, PQ — 0.12–0.2 s.