Registered Cardiovascular Invasive Specialist (RCIS) Practice Test

What happens to cardiac muscle fiber contraction when heart rate slows?

1. Increases

2. Decreases

3. Remains constant

4. Varies with preload

The correct answer is: Increases

When the heart rate slows, the contraction of cardiac muscle fibers generally increases. This is primarily due to the longer duration of each cardiac cycle, which allows for enhanced filling of the heart's chambers during diastole. As the heart rate decreases, the time available for ventricular filling increases, leading to a greater volume of blood entering the ventricles. This increase in preload (the initial stretching of the cardiac muscle fibers due to the volume of blood) ultimately enhances the force of contraction due to the Frank-Starling mechanism, where the strength of cardiac muscle contraction is directly related to its degree of stretch before contraction. Additionally, with a slower heart rate, the heart has more time to effectively engage in both the systolic and diastolic phases, allowing for better myocardial oxygenation and metabolic conditions. Consequently, a lower heart rate can facilitate stronger contractions despite reduced frequency, aligning with the physiological relationship that increased preload results in increased stroke volume and, in many cases, increased contractility of the cardiac muscle fibers.