An electrocardiogram (ECG) is a simple, non-invasive test that records the electrical activity of the heart over time. The tracing is composed of various waves and segments, each representing a specific phase of the cardiac cycle. The QRS complex is a prominent feature, appearing as a rapid, sharp deflection that reflects the electrical activation of the heart’s lower chambers (ventricles). Analyzing the duration of this complex is an important measure for assessing the health of the heart’s electrical conduction system. An abnormal QRS duration provides insight into potential problems with the heart’s ability to coordinate its pumping action.
The Electrical Basis of the QRS Complex
The QRS complex is the physical manifestation of ventricular depolarization, the electrical activation that immediately precedes the heart’s pumping action. This event involves the rapid spread of an electrical impulse through the ventricles, the two large chambers responsible for pushing blood out to the body and lungs. The electrical signal travels down a specialized, high-speed wiring system known as the His-Purkinje network.
This dedicated system ensures the entire ventricular muscle mass contracts nearly simultaneously. This synchronized activation allows for an efficient and powerful squeeze. The QRS duration reflects the time it takes for this electrical wave to complete its journey across the ventricles. Disruption to this pathway forces the signal to take a longer, slower route through the muscle tissue, resulting in a wider QRS complex on the ECG.
Measuring and Defining Normal Duration
The QRS duration is measured horizontally on the ECG strip, representing the time from the beginning of the Q wave to the end of the S wave. Each small box on standard ECG paper represents 40 milliseconds (ms), allowing for precise measurement of the complex’s width. In a healthy adult, the normal QRS duration falls between 80 ms and 100 ms.
While a measurement up to 110 ms is sometimes seen, a QRS duration of 120 ms or more is considered prolonged or “wide.” Conversely, a complex shorter than 80 ms is considered a shortened QRS duration. These cutoffs define whether the electrical activation of the ventricles is happening faster or slower than the expected pace.
Medical Significance of Prolonged QRS
A prolonged QRS duration (120 ms or greater) signifies a delay in the electrical signal’s passage through the ventricles. This delay suggests the impulse is not traveling along the fast, specialized His-Purkinje system, but is forced to move slowly through the ordinary ventricular muscle. The most common cause of a significantly wide QRS is a Bundle Branch Block (BBB), where one of the main branches of the conduction system is partially or fully blocked.
In a Left Bundle Branch Block (LBBB), the impulse must travel to the left ventricle via the right ventricle and the septum, causing a profound delay in activation. A Right Bundle Branch Block (RBBB) similarly delays the activation of the right ventricle. This lack of coordination between the two ventricles reduces the heart’s pumping efficiency, leading to mechanical dyssynchrony.
Other Causes of Prolonged QRS
Other causes include rhythms that originate outside the normal conduction pathway, such as Ventricular Rhythms or Ectopy. When an electrical impulse starts in the ventricle itself, it must spread slowly through the muscle tissue. Conditions that increase the mass of the heart muscle, such as Left Ventricular Hypertrophy (LVH), also increase the time required for depolarization. The increased electrical mass means the signal needs more time to activate all the tissue. A prolonged QRS duration is an indicator of intraventricular conduction delay and is associated with an increased risk of heart failure and sudden cardiac death.
Causes of Shortened QRS
A shortened QRS duration, typically measured as less than 80 ms, indicates that the electrical activation of the ventricles is happening faster than normal. This accelerated conduction is less common than a prolonged duration but can still be medically significant. The primary mechanism involves an electrical pathway that bypasses the natural delay built into the atrioventricular (AV) node.
The most well-known condition causing a short QRS is Wolff-Parkinson-White (WPW) syndrome, a pre-excitation disorder where an accessory pathway connects the atria and ventricles. This pathway allows the electrical impulse to reach the ventricles early, resulting in a short QRS with a characteristic slurring at the beginning, known as a delta wave. Other causes include certain channelopathies, which are genetic disorders affecting the heart’s ion channels, such as some forms of Short QT syndrome. While a short QRS might seem beneficial, these accelerated pathways can lead to extremely rapid heart rates and dangerous arrhythmias.