Atrial complex and repolarization waves
The graphic ECG curve begins with the P wave, which reflects the depolarization (excitation) of both atria. The resulting wave is formed by the sum of electrical vectors: its first part corresponds to the excitation of the right atrium, and the second to the left.
Normative parameters of the P wave:
- Amplitude (height): 0.5–2.5 mm.
- Duration (width): 0.07–0.1 s.
- Polarity: most clearly expressed in lead II. In lead aVR, it is always negative. In leads III, aVL, and aVF, the wave is variable (it can be positive, isoelectric, or negative, depending on the electrical axis of the heart). In chest lead V1, it is often biphasic with equal phases.
The T wave is responsible for the recovery of the ventricular myocardium (repolarization). It follows the ventricular complex after a short pause. Normally, its amplitude is 2–6 mm. The T wave is usually positive in leads I, II, aVL, aVF, and V2–V6, but always negative in aVR. In standard lead III, the T wave can be negative, however, if it becomes positive on inspiration, this is an absolute variant of normal.
Sometimes, a small positive U wave is recorded after the T wave. It is believed to reflect the final stage of repolarization of the cardiac conduction system, specifically the Purkinje fibers.
Ventricular QRS complex
The QRS complex is a high-amplitude ECG element following the P wave. It reflects the depolarization (excitation) of the ventricular myocardium. Normally, all waves of this complex should be clean, without notches and slurring (small notches are allowed only in standard lead III and aVF). The normal duration of the complex is 0.06–0.1 s.
A clear letter nomenclature system is used to describe the QRS shape:
- Capital letters (Q, R, S) denote amplitude-dominant waves.
- Lowercase letters (q, r, s) are used for low-amplitude waves.
Wave identification rules:
- R: any upward-directed (positive) wave. The main wave of the complex.
- Q: a negative wave that strictly precedes the first R wave.
- S: a negative wave immediately following the R wave.
- R' (R-prime): a second positive wave appearing after the S wave (e.g., in notched rSR' complexes).
- QS: a situation where only a single negative wave is present on the ECG, and R is completely absent.
The voltage of the complex is calculated as the sum of the absolute values of the R wave and the largest negative wave (Q or S). Normally, this parameter ranges from 5 to 20 mm. In chest leads, it is important to determine the transition zone — the lead where the amplitudes of R and S are equal. With normal heart anatomy and function, it is located in lead V3.
Intervals and segments
In addition to waves, the ECG features time periods: intervals (including waves and segments) and segments (distances between waves).
- P-Q interval: measured from the beginning of the P wave to the beginning of the Q wave. Reflects atrioventricular conduction time. Normally ranges from 0.12 to 0.20 s.
- ST segment: corresponds to the phase of complete ventricular excitation (slow repolarization). Normally located almost on the baseline and has a slightly upward slope. Minimal deviations are allowed: in limb leads — downward up to 0.5 mm and upward up to 1 mm; in right chest leads (V1-V2) — elevation by 1–2 mm; in left chest leads (V5-V6) — elevation up to 1 mm or depression up to 0.5 mm.
- Q-T interval: electrical systole of the ventricles (includes depolarization and repolarization, measured from the beginning of the QRS to the end of the T wave). Most accurately assessed in standard lead II. Duration depends on heart rate: the higher the heart rate, the shorter the interval. Prolongation above 480–500 ms is a criterion for pathology.
- R-R interval: reflects the total duration of the cardiac cycle. With a regular sinus rhythm, it must be constant; the allowable variability (spread) does not exceed 0.1 s.
Briefly: answers to topical questions
How to calculate the electrical axis of the heart (EAH) from the QRS waves?
The electrical axis of the heart (EAH) is determined geometrically or visually based on the QRS waves. Sources distinguish the following methods:
- Geometric (calculated) — plotting a vector in Einthoven's triangle using the amplitude of QRS waves in two standard leads.
- Visual (estimated) — based on the ratio of wave amplitudes in standard leads:
- +QRS (R > S) — the axis points toward the lead;
- ±QRS (R = S) — the axis is perpendicular to the lead;
- –QRS (R < S) — the axis is directed away from the lead.
What is the formula for calculating the corrected QT interval (QTc)?
The corrected QT interval is calculated using Bazett's or Fridericia's formulas (the mathematical expressions themselves are not provided in the sources). The choice of method depends on the heart rate (HR):
- Bazett's formula — used at a heart rate of 60 to 100 beats per minute.
- Fridericia's formula — used in tachycardia or bradycardia.
What are the criteria for a pathological Q wave?
The main sign of a pathological Q wave described in the sources is that it is wide and deep (exact numerical values are not provided in the sources).
The formation of such a wave has the following characteristics:
- Appearance — the Q wave is described as deep and wide.
- Significance — in transmural infarction, it reflects the loss of electrical activity of the tissue.
- Diagnostics — the appearance of a deep and wide Q wave is an ECG criterion for transmural myocardial infarction; in non-transmural infarction, a pathological Q wave is absent.
QRS letter nomenclature: the R wave always "grows" upward (positive), while the Q and S waves always "point" downward (negative). At the same time, Q is always strictly before R, and S is after.
Class and exam questions
Why is the T wave always negative in lead aVR?
This is a physiological norm caused by the ventricular repolarization vector. In lead aVR, the T wave, like the P wave, should normally be directed downward.
What should be done if the T wave is negative in lead III?
It is necessary to record an ECG on a deep breath. If the negative T wave becomes positive in this case, this is regarded as a variant of normal.
What does ST-segment displacement indicate?
Normally, the ST segment is on the baseline. However, slight elevation (up to 2 mm in V1-V2) or depression (up to 0.5 mm) is permissible. Significant deviations require clinical evaluation.
How to determine rhythm regularity from an ECG?
It is necessary to measure the R-R intervals between several complexes. If the spread of their duration does not exceed 0.1 seconds, the rhythm is considered regular.
What else to study on this topic
- Dependence of P wave morphology on the position of the electrical axis of the heart.
- Evaluation of normative voltage in chest and standard leads.
- Influence of heart rate on Q-T interval duration.
- Causes of physiological ST-segment elevation in the right chest leads.
Master the entire topic in the "Propaedeutics" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Elements of a Normal ECG" and 3,152 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 113 video lessons on the subject
Other topics in the "Cardiovascular system" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.