What does heart rate variability (HRV) indicate in terms of cardiac health?

Heart rate variability (HRV) is a key indicator of the heart's ability to adapt to stress and the balance between the sympathetic and parasympathetic branches of the autonomic nervous system. A higher HRV usually signifies a healthy heart and a well-functioning autonomic nervous system, reflecting the body's ability to respond effectively to various stressors, such as physical activity, emotional stress, and other stimuli. Conversely, a low HRV can be associated with stress, fatigue, and an increased risk for cardiovascular issues.

In contrast, the other options do not accurately capture what HRV measures. The stability of the heart's rhythm relates more to arrhythmias or conduction disturbances rather than HRV. Likewise, while overall blood volume and heart muscle thickness are important aspects of cardiac health, they are assessed by different methods and are not directly related to HRV. Thus, HRV is best understood as a measure of the heart's adaptability and autonomic regulation rather than simply a reflection of rhythm stability, blood volume, or muscle thickness.