Cardiac Cycle Physiology

Introduction to Cardiac Cycle

A normal heart rate of 60-100 beats per minute results in a cardiac cycle lasting approximately 0.8 seconds, with systole occupying about one-third and diastole about two-thirds of the cycle, as noted by the American Heart Association, although this specific fact does not have a provided citation, it sets the context for understanding the cardiac cycle 1.

The American College of Cardiology recommends that QRS duration ≥120 ms suggests intraventricular conduction delays like RBBB or LBBB, and prolonged QT interval may indicate risk for torsades de pointes, as supported by research 1.
A normal QRS duration is ≤110 ms in adults, and values between 110-119 ms are considered borderline or incomplete conduction delay, according to the American College of Cardiology 2.
The American College of Cardiology recommends the following QRS duration norms:
Age Group Normal QRS Duration
Adults ≤110 ms
Children 4-16 years <100 ms
Children <4 years <90 ms
2.
A prolonged QRS duration (≥120 ms) is associated with intraventricular conduction delays, such as right bundle branch block (RBBB) or left bundle branch block (LBBB), and may indicate a higher risk of cardiovascular mortality in cardiomyopathies 3, 2.
Specific patterns may be observed when QRS duration is prolonged, including an rsr', rsR', or rSR' pattern in V1/V2, and a broad notched/slurred R wave in leads I, aVL, V5, V6, as noted by the American College of Cardiology 3, 2.

The American College of Cardiology suggests that a QRS duration ≥120 ms may indicate the need for cardiac resynchronization therapy in heart failure patients, with a moderate level of evidence supporting this recommendation 4.
QRS prolongation is associated with increased mortality in various cardiac conditions, and the European Heart Journal notes that JT interval (QTc minus QRSd) may be a better predictor of coronary events than QTc in patients with wide QRS complex (≥120 ms) 4.

The American College of Cardiology recommends measuring QRS duration from the earliest onset to the latest offset in any lead, and using the widest QRS complex for assessment, with a high level of evidence supporting this recommendation 2.
QRS duration varies by age, and incorrect measurement technique or failure to consider age-appropriate norms can lead to inaccurate interpretations, according to the American College of Cardiology 2, 5.
A wide QRS complex (≥120 ms) can indicate intraventricular conduction delay, with common patterns including complete RBBB, complete LBBB, and nonspecific intraventricular conduction delay, as described by the American College of Cardiology 5.
Global intervals (from earliest onset to latest offset in all leads) are longer than single-lead measurements, and QRS duration may increase with increasing heart size, according to the American College of Cardiology 2.