An electrocardiogram (ECG) records the electrical activity of the heart via electrodes on the skin. The ECG displays these signals as waves and complexes. The QRS complex is a key part of the heart’s electrical cycle, providing information about heart function.
Understanding the QRS Complex
The QRS complex appears as a rapid spike on an ECG tracing. Physiologically, it represents the depolarization of the heart’s ventricles. This electrical event triggers the ventricles to contract. The QRS complex is considerably larger than other waves on an ECG because the ventricles contain more muscle mass than the atria.
The complex is typically composed of three main deflections: the Q wave, the R wave, and the S wave. The Q wave is the first downward deflection immediately following the P wave, which represents atrial depolarization. The R wave is the subsequent upward deflection. The S wave appears as a downward deflection. While often referred to as the “QRS complex,” not all three waves are always present in every lead of an ECG, but they collectively signify ventricular electrical activity.
Normal QRS Complex Measurements
The duration of the QRS complex indicates the time it takes for the electrical impulse to spread through the ventricles. A normal QRS complex duration generally ranges from 60 to 100 milliseconds (ms), which is equivalent to 0.06 to 0.10 seconds. This specific range suggests efficient and synchronized depolarization of the ventricular muscle. Some sources indicate a normal range up to 110 ms or even 120 ms.
To measure the QRS complex on an ECG strip, a healthcare professional identifies the beginning of the Q wave (or the first deflection if a Q wave is absent) and measures to the end of the S wave (or the last deflection before the T wave). Each small square on the paper typically represents 1 millimeter (mm) and corresponds to 0.04 seconds (40 ms) horizontally. A larger square consists of five small squares, representing 0.20 seconds (200 ms). Therefore, a normal QRS duration of 0.06 to 0.10 seconds would span approximately 1.5 to 2.5 small squares on the ECG paper.
What Abnormal QRS Measurements Indicate
Measurements of the QRS complex that fall outside the normal range can suggest underlying heart conditions. A QRS complex that is too wide, greater than 100 ms, indicates a delay in the electrical conduction through the ventricles. Such widening can be associated with various issues, including bundle branch blocks, where electrical signals are delayed or blocked. Ventricular hypertrophy, an enlargement of the heart’s chambers, or certain types of arrhythmias originating in the ventricles, can also lead to a widened QRS complex.
Conversely, a QRS complex that is too narrow indicates that the electrical signal primarily originates from above the ventricles, such as in the atria. Narrow QRS complexes are typically considered 80 ms or shorter. While a narrow QRS is often associated with normal heart rhythms, specific conditions like certain supraventricular tachycardias can present with a narrow QRS. A healthcare professional is necessary for accurate diagnosis and interpretation of any abnormal QRS measurements.