Extra-cardiac factors and electrical axis
The shape and direction of the QRS complex are significantly influenced by the patient's constitutional features. This is related to body type and the level of diaphragmatic excursion. When interpreting an ECG, the following patterns must be taken into account:
- Left axis deviation (LAD): Typical for individuals with a hypersthenic body type. In such patients, the diaphragm is positioned high, which gives the heart a more horizontal position.
- Right axis deviation (RAD): Characteristic of tall, thin patients (asthenics). A low-positioned diaphragm causes a vertical position of the heart in the thoracic cavity.
Assessment of the electrical axis of the heart (EAH)
Ventricular depolarization forms a resultant vector that we assess as the electrical axis. To accurately determine the EAH, it is necessary to analyze the amplitude ratio of all complex components—the Q, R, and S waves—strictly across all 12 standard leads.
A comprehensive amplitude analysis allows the physician to identify:
- True shift of the electrical axis in the frontal plane.
- Possible rotations and turns of the heart around other axes (specifically, around the longitudinal axis).
Morphological analysis of waves
Each wave within the complex requires targeted visual and quantitative assessment. Changes in their morphology are key markers of myocardial structural remodeling.
- Q wave: Its amplitude and total duration (width) are evaluated. This is critically important for diagnosing scar changes in the heart muscle.
- R wave: The physician analyzes the wave's amplitude, records the presence of any potential notching, and also notes the appearance of any additional waves (designated as R' or r').
- S wave: Evaluated for amplitude and the presence of widening. In addition, identifying slurring or notching along its course has diagnostic significance.
Limitations of resting ECG
Despite the high clinical significance of QRS complex analysis, a standard resting ECG has strict limitations.
The method demonstrates extremely low diagnostic value in the diagnosis of coronary artery disease if the test is performed outside of a pain episode. Even if the patient has significant stenosing coronary artery disease, the resting tracing may remain normal. The only exception to this rule is acute coronary pathology — such changes are clearly visualized on the ECG even at rest.
Briefly: answers to topical questions
What is the normal duration (width) of the QRS complex in seconds in a healthy person?
The normal duration (width) of the QRS complex in a healthy individual ranges from 0.06 to 0.1 seconds. When evaluating this complex, the following normative characteristics are considered:
- Wave morphology — normally, they should be clean, without splitting or notching (with the exception of minor notches permissible in standard lead III and aVF).
- Amplitude voltage — the average value is calculated as the sum of the absolute values of the positive R wave and the largest negative wave (Q or S) and ranges from 5 to 20 mm. An amplitude of less than 5 mm in all leads indicates low-voltage ECG.
In which standard and chest leads is the recording of a physiological Q wave normally permitted?
Normally, the registration of a physiological Q wave is permitted in the following leads:
- Standard leads — in leads II and III: depth no more than 3 mm, width no more than 0.03 s.
- Augmented leads — in aVL and aVF: depth up to 4 mm; in aVR the Q wave can be very deep, up to 8 mm, and is the equivalent of the R wave in other leads.
- Left chest leads — in V5 and V6: depth 3 mm.
The Q wave is optional and may be absent.
To remember the relationship between body type and the axis, use the correspondence rule: "Broad person — broad (horizontal) axis and left axis deviation; tall, slender person — vertical axis and right axis deviation."
Class and exam questions
What does the QRS complex reflect on an electrocardiogram?
It fully reflects the process of electrical depolarization (excitation) of the heart ventricles.
Why is right axis deviation often recorded in asthenics?
In tall and thin individuals, the diaphragm is positioned low. This causes the heart to assume a more vertical position, which appears on the ECG as a right axis deviation.
What should be paid attention to when analyzing the Q wave?
First and foremost, its amplitude and duration must be assessed, as deviations in these parameters are important for diagnosing myocardial scarring.
Can severe ischemia be ruled out based on a resting ECG?
No. Outside of an episode, a resting ECG has low diagnostic value, and even severe coronary artery stenosis may not produce changes on the tracing (unless acute coronary pathology is present).
What else to study on this topic
- Criteria for a pathological Q wave in scarring
- Causes and significance of the appearance of additional R waves (R', r')
- Diagnostics of heart rotations around the longitudinal axis
- Signs of acute coronary pathology on a resting ECG
- Assessment of S wave notching and slurring
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 the "QRS complex" topic and 3,152 other subject pages
- 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.