The QRS complex is a name for the combination of three of the graphical deflections seen
on a typical electrocardiogram. It is usually the central and most visually obvious part
of the tracing. It corresponds to the depolarization of the right and left ventricles of the human
heart. In adults, it normally lasts 0.06–0.10 s; in children and during physical activity,
it may be shorter. Typically an ECG has five deflections, arbitrarily
named "P" to "T" waves. The Q, R, and S waves occur in rapid succession, do not all appear
in all leads, and reflect a single event, and thus are usually considered together.
A Q wave is any downward deflection after the P wave. An R wave follows as an upward
deflection, and the S wave is any downward deflection after the R wave. The T wave follows
the S wave, and in some cases an additional U wave follows the T wave.
Etiology The His/Purkinje specialized muscle cells
coordinate the depolarization of both ventricles nearly simultaneously; if they are working
efficiently, the QRS complex is 80 to 120 ms in duration. This is represented by three
small squares or less at the standard paper speed of 25mm/s. Any abnormality of conduction
takes longer and causes "widened" QRS complexes. In bundle branch block, there can be an abnormal
second upward deflection within the QRS complex, in this case such a second upward deflection
is referred to as R'. This would be described as an RSR' pattern.
Ventricles contain more muscle mass than the atria, therefore the QRS complex is considerably
larger than the P wave. The QRS complex is often used to determine the axis of the electrocardiogram,
although it is also possible to determine a separate P wave axis.
The duration, amplitude, and morphology of the QRS complex are useful in diagnosing cardiac
arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte
derangements, and other disease states. Parameters
The QRS complex is also included in estimating the QT interval.
Q wave Normal Q waves, when present, represent depolarization
of the interventricular septum. For this reason, they are referred to as septal Q waves and
can be appreciated in the lateral leads I, aVL, V5 and V6.
Pathologic Q waves occur when the electrical signal passes through stunned or scarred myocardium;
as such, they are usually markers of previous myocardial infarcations, with subsequent fibrosis.
A pathologic Q wave is defined as having a deflection amplitude of 25% or more of the
subsequent R wave, or being > 0.04 s in width and > 2 mm in amplitude. However, diagnosis
requires the presence of this pattern in more than one corresponding lead.
R wave progression Looking at the precordial leads, the r wave
usually progresses from showing a rS-type complex in V1 with an increasing R and a decreasing
S wave when moving towards the left side. There is usually an qR-type of complex in
V5 and V6 with the R-wave amplitude usually taller in V5 than in V6. It is normal to have
a narrow QS and rSr' patterns in V1, and so is also the case for qRs and R patterns in
V5 and V6. The transition zone is where the QRS complex changes from predominately negative
to predominately positive, and this usually occurs at V3 or V4. It is normal to have the
transition zone at V2, and at V5. In biomedical engineering, the maximum amplitude in the
R wave is usually called "R peak amplitude", or just "R peak". Accurate R peak detection
is essential in signal processing equipment for heart rate measurement and it is the main
feature used for arrhythmia detection. The definition of poor R wave progression
varies in the literature, but a common one is when the R wave is less than 2–4 mm
in leads V3 or V4 and/or there is presence of a reversed R wave progression, which is
defined as R in V4 < R in V3 or R in V3 < R in V2 or R in V2 < R in V1, or any combination
of these. Poor R wave progression is commonly attributed to anterior myocardial infarction,
but it may also be caused by left bundle branch block, Wolff–Parkinson–White syndrome,
right and left ventricular hypertrophy as well as by faulty ECG recording technique.
J-point The point at which the QRS complex meets the
ST segment is known as the J-point. The J-point is easy to identify when the ST segment is
horizontal and forms a sharp angle with the last part of the QRS complex. However, when
the ST segment is sloped or the QRS complex is wide, the two features do not form a sharp
angle and the location of the J-point is less clear. There is no consensus on the precise
location of the J-point in these circumstances. Two possible definitions are:
The "first point of inflection of the upstroke of the S wave"
The point at which the ECG trace becomes more horizontal than vertical.
Monomorphic or polymorphic Monomorphic refers to all QRS waves in a single
lead being similar in shape. Polymorphic means that the QRS change from complex to complex.
These terms are used in the description of ventricular tachycardia.
Terminology
Not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any
combination of these waves can be referred to as a QRS complex. However, correct interpretation
of difficult ECGs requires exact labeling of the various waves. Some authors use lowercase
and capital letters, depending on the relative size of each wave. For example, an Rs complex
would be positively deflected, while an rS complex would be negatively deflected. If
both complexes were labeled RS, it would be impossible to appreciate this distinction
without viewing the actual ECG. Algorithms
A common algorithm used for QRS complex detection is the Pan-Tompkins algorithm; another is
based on the Hilbert transform. Numerous other algorithms have been proposed and investigated.
See also Electrophysiology
References