Final answer:
The cardiac index begins to decline at an upper heart rate limit of approximately 160 bpm due to reduced ventricular filling time. As heart rate increases, stroke volume decreases, and beyond 160 bpm, the heart rate can no longer compensate, leading to a decrease in cardiac output.
Step-by-step explanation:
The upper limit of heart rate at which cardiac index starts to decline is approximately 160 beats per minute (bpm). This phenomenon can mainly be attributed to insufficient ventricular filling time during diastole. As heart rate (HR) increases, there is less time for the ventricles to fill, leading to a decreased stroke volume (SV). Initially, an increase in HR corresponds to an increase in cardiac output (CO), which stabilizes despite further increases in HR up to around 160 bpm. Beyond this point, the SV decreases more rapidly than the HR can compensate, leading to a downward trend in CO.
Factors contributing to the decrease in cardiac index at high HR include the decreased duration of diastole, lessened ventricular filling, and a decrease in the efficiency of cardiac muscle contractions. This is particularly relevant during high-intensity aerobic exercises, where monitoring HR is important to maximize cardiovascular benefits without overburdening the heart. The target HR range is typically 120-160 bpm, which is particularly important as age-related changes can impact maximum HR and overall cardiovascular health.
Cardiac index declines beyond an HR of approximately 160 bpm due to the reduced time for ventricular filling caused by shorter diastolic phases, ultimately impacting SV and CO.